Terminology and Weather Symbols

Cold front -The leading edge of a relatively colder air mass which separates two air masses in which the gradients of temperature and moisture are maximized. In the northern hemisphere winds ahead of the front will be southwest and shift into the northwest with frontal passage.

Frontogenesis -The formation of a front occurs when two adjacent air masses with different densities and temperatures meet and strengthen the discontinuity between the air masses. It occurs most frequently over continental land areas such as over the Eastern US when the air mass moves out over the ocean. It is the opposite of frontolysis.

Frontolysis -The weakening or dissipation of a front occurs when two adjacent air masses lose contrasting properties such as the density and temperature. It is the opposite of frontogenesis.

Occluded front - The union of two fronts, formed as a cold front overtakes a warm front or quasi-stationary front refers to a cold front occlusion. When a warm front overtakes a cold front or quasi-stationary front the process is termed a warm front occlusion. These processes lead to the dissipation of the front in which there is no gradient in temperature and moisture.

Ridge - an elongated area of relatively high pressure that is typically associated with a anti-cyclonic wind shift.

Stationary front - A front that has not moved appreciably from its previous analyzed position.

Trough - An elongated area of relatively low pressure that is typically associated with a cyclonic wind shift.

Warm front - The leading edge of a relatively warmer surface air mass which separates two distinctly different air masses. The gradients of temperature and moisture are maximized in the frontal zone. Ahead of a typical warm front in the northern hemisphere, winds are from the southeast and behind the front winds will shift to the southwest.

LOW & HIGH PRESSURE SYSTEMS AND MISCELLANEOUS TERMS USED

Low pressure with a number such as 99 means 999 mb and with 03 means 1003 mb. High pressure with a number such as 25 means 1025 mb.

Extratropical low - A low pressure center which refers to a migratory frontal cyclone of middle and higher latitudes. Tropical cyclones occasionally evolve into extratropical lows losing tropical characteristics and become associated with frontal discontinuity.

Low pressure - An area of low pressure identified with counterclockwise circulation in the northern hemisphere and clockwise in the southern hemisphere. Also, defined as a cyclone.

High pressure - An area of higher pressure identified with a clockwise circulation in the northern hemisphere and a counterclockwise circulation in the southern hemisphere. Also, defined as an anticyclone.

New - The term "NEW" may be used in lieu of a forecast track position of a high or low pressure center when the center is expected to form by a specific time. For example, a surface analysis may depict a 24-hour position of a new low pressure center with an "X" at the 24-hour position followed by the term "NEW", the date and time in UTC which indicates the low is expected to form by 24 hours.

Rapidly intensifying - Indicates an expected rapid intensification of a cyclone with surface pressure expected to fall by at least 24 millibar (mb) within 24 hours.

Squall - A sudden wind increase characterized by a duration of minutes and followed by a sudden decrease in winds.

Fog -Over the marine environment the term fog refers to visibility greater than or equal to 1/2 NM and less than 3 NM. Fog is the visible aggregate of minute water droplets suspended in the atmosphere near the surface.

Dense fog -Over the marine environment the term dense fog refers to visibility less than 1/2 NM. Fog is the visible aggregate of minute water droplets suspended in the atmosphere near the surface. Usually dense fog occurs when air that is lying over a warmer surface such as the Gulf Stream is advected across a colder water surface and the lower layer of the air mass is cooled below its dew point.

Sea fog - Common advection fog caused by transport of moist air over a cold body of water.
FREEZING SPRAY

CONVENTIONS USED WITH WARNINGS FOR EXTRATROPICAL SYSTEMS

Complex gale/storm -An area in which gale/storm force winds are forecast or are occurring, but in which more than one center is the generating these winds.

Developing Gale -Refers to an extratropical low or an area in which gale force winds of 34 knots (39 mph) to 47 knots (54 mph) are "expected" by a certain time period. On surface analysis charts, a "DEVELOPING GALE" label indicates gale force winds within the next 24 hours. When the label is used on the 48 hour surface forecast and 96 hour surface forecast charts, gale force winds are expected to develop by 72 hours and 120 hours, respectively.

Developing Storm -Refers to an extratropical low or an area in which storm force winds of 48 knots (55 mph) to 63 knots (73 mph) are "expected" by a certain time period. On surface analysis charts, a "DEVELOPING STORM" label indicates storm force winds forecast within the next 24 hours. When the label is used on the 48 hour surface and 96 hour surface charts, storm force winds are expected to develop by 72 hours and 120 hours, respectively.

Developing Hurricane Force -Refers to an extratropical low or an area in which hurricane force winds of 64 knots (74 mph) or higher are "expected" by a certain time period. On surface analysis charts, a "DEVELOPING HURRICANE FORCE" label indicates hurricane force winds forecast within the next 24 hours. When the label is used on the 48 hour surface and 96 hour surface charts, hurricane force winds are expected to develop by 72 hours and 120 hours, respectively.

Gale - Refers to an extratropical low or an area of sustained surface winds (averaged over a ten minute period, momentary gusts may be higher) of 34 knots (39 mph) to 47 knots (54 mph).

Storm - Refers to a extratropical low or an area of sustained winds (averaged over a ten minute period, momentary gusts may be higher) of 48 knots (55 mph) to 63 knots (73 mph).

Hurricane Force - Refers to a extratropical low or an area of sustained winds (averaged over a ten minute period, momentary gusts may be higher) in excess of 64 knots or higher(74 mph).

Small Craft Advisory - Refers to areas within the coastal waters with sustained winds of 18 knots (21 mph) to 33 knots (38 mph).

Heavy Freezing Spray -Spray in which supercooled water droplets freeze upon contact with exposed objects below the freezing point of water at the rate of greater than 2 cm/hr. It usually develops in areas with winds of at least 25knots.

CONVENTIONS USED WITH WARNINGS FOR TROPICAL SYSTEMS

Hurricane - A tropical cyclone with closed contours, a strong and very pronounced circulation, and one minute maximum sustained surface winds 64 knots (74 mph) or greater. A system is called a hurricane over the North Atlantic, Gulf of Mexico, North Pacific E of the dateline, and the South Pacific E of 160E.

Intertropical Convergence Zone - (ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.

Tropical cyclone - A non-frontal, warm-core, low pressure system of synoptic scale, developing over tropical or subtropical waters with definite organized convection (thunderstorms) and a well defined surface wind circulation.

Tropical depression - A tropical cyclone with one or more closed isobars and a one minute max sustained surface wind of less than 34 knots (39 mph).

Tropical storm - A tropical cyclone with closed isobars and a one minute max sustained surface wind of 34 knots (39 mph) to 63 knots (73 mph).

Typhoon - Same as a hurricane with exception of geographical area. A tropical cyclone with closed contours, a strong and very pronounced circulation, and one minute maximum sustained surface winds of 64 knots (74 mph) or greater. A system is defined as a typhoon over the North Pacific W of the dateline.

NOTE: It can be difficult to determine the central pressures of tropical depressions, tropical storms, and hurricanes/typhoons and at times no estimates or measurements is provided by a hurricane or typhoon specialist. An estimate of central pressure may be provided over the Atlantic. Otherwise an XXX is used in place of actual or estimated pressures associated with these systems and an XX is used for forecast central pressure.

SEAS
Combined seas -The combination of both wind waves and swell which is generally referred to as "seas".

Primary swell direction - Prevailing direction of swell propagation.

Significant wave height - The average height (trough to crest) of the 1/3rd highest waves. An experienced observer will most frequently report the highest 1/3rd of the waves observed. The generation of waves on water results not in a single wave height but in a spectrum of waves distributed from the smallest capillary waves to larger waves. Within this spectrum there is a finite possibility of each of the wave heights to occur with the largest waves being the least likely. The wave height most commonly observed and forecast is the significant wave height. This is defined as the average of the one third highest waves. The random nature of waves implies that individual waves can be substantially higher than the significant wave height. In fact, observations and theory show that the highest individual waves in a typical storm with typical duration to be approximately two times the significant wave height. Some reported rogue waves are well within this factor of two envelope. Waves higher than roughly twice the significant wave height fall into the category of extreme or rogue waves.
Swell - Wind waves that have moved out of their fetch or wind generation area. Waves generated by swell exhibit a regular and longer period than wind waves.

MISCELLANEOUS TERMINOLOGY
Coastal Waters - Includes the area from a line approximating the mean high water along the mainland or island as far out as sixty nautical miles including the bays, harbors and sounds.

High Seas - That portion of the Atlantic and Pacific oceans which extends off the Western and Eastern US coasts and extends to 35W in the Atlantic ocean and to 160E in the Pacific Ocean. The area includes both the coastal and offshore waters.

Offshore waters - That portion of oceans, gulfs, and seas beyond coastal waters extending to a specified distance from the coastline, to a specified depth contour, or covering an area defined by a specific latitude and longitude points.

Summer Triangle, Vega, Deneb, and Altair

The Summer Triangle is an an imaginary triangle drawn on the northern hemisphere's celestial sphere, with its defining vertices at Altair, Deneb, and Vega. This triangle connects the constellations of Aquila, Cygnus, and Lyra.

Near midnight the Summer Triangle lies virtually overhead at mid-northern latitudes during the summer months, but can also be seen during spring in the early morning. In the autumn the summer triangle is visible in the evening well until November. From the southern hemisphere it appears upside down and low in the sky during the winter months.

Name / Constellation

Vega / Lyra

Deneb / Cygnus

Altair / Aquila

Altair is the brightest star in the constellation Aquila and the twelfth brightest star in the night sky with an apparent visual magnitude of 0.77 .

Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A white supergiant, Deneb is also one of the most luminous stars known.

Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus. It is a relatively nearby star at only 25 light-years from Earth, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood.

Declination

In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator. Points north of the celestial equator have positive declinations, while those to the south have negative declinations.

An object on the celestial equator has a dec of 0°.
An object at the celestial north pole has a dec of + 90°.
An object at the celestial south pole has a dec of - 90°.

The sign is customarily included even if it is positive. Any unit of angle can be used for declination, but it is often expressed in degrees, minutes, and seconds of arc. If instead of measuring from and along the equator the angles are measured from and along the horizon, the angles are called azimuth and altitude.

Stars
Because a star lies in a nearly constant direction as viewed from earth, its declination is approximately constant from year to year. However, both the right ascension and declination do change gradually due to the effects of precession of the equinoxes and proper motion.

Sun
The declination of the Sun (δ) is the angle between the rays of the sun and the plane of the earth's equator. Since the angle between the earth axis and the plane of the earth orbit is nearly constant, δ varies with the seasons and its period is one year, that is the time needed by the earth to complete its revolution around the sun.

When the projection of the earth axis on the plane of the earth orbit is on the same line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is maximum and its value is 23°27'. This happens at the solstices. Therefore δ = +23°27' at the northern hemisphere summer solstice and δ = -23°27' at the northern hemisphere winter solstice. Due to the changes in the tilt of the Earth's axis, the angle between the rays of the sun and the plane of the earth equator is slightly decreasing.

Moon
Declination of the Moon is computed by adding Sun's declination (which is called Declination of Place while computing declination of other planets and Moon) to Moon's latitude. Sun's declination (± 23.44°) is much larger in magnitude than Moon's latitude (± 5.14°). The Moon's declination can be said to have an annual cycle synchronous with that of the Sun starting with the vernal equinox.

Moon's latitude is a function of the difference between True Moon and its ascending node. Since lunar nodes make one revolution in nearly 19 years, lunar latitude has an approximately 19 year long cycle. Lunar latitude is equal to inverse sine of the product of sine of maximum lunar latitude and sine of difference between Moon and its node.

For greater accuracy, reduced latitude is used instead of Moon's true latitude, which is obtained by multiplying lunar latitude with a multiplier having a maximum value of 1 for tropical Moon at 180° and 0.91745 for tropical Moon at 0°. This is caused by a third cycle in lunar declination which has a period of one lunar month and a maximum range of ± 0.425°. Summing all three components gives a range of maximum declination from +28°35' to +18°18' and the minimum from -18°18' to -28°35' for lunar declination.

Zenith

In general terms, the zenith is the direction pointing directly above a particular location. The concept of above is more specifically defined in astronomy, geophysics and related sciences as the vertical direction opposite to the force of gravity at a given location. The opposite direction, the direction of the gravitational force is called the nadir. The term zenith is also used to represent the highest point reached by a celestial body during its apparent orbit around a given point of observation. This sense of the word is used to describe the location of the Sun, but it is only technically accurate for one latitude at a time and impossible for latitudes outside the tropics.

Strictly speaking, the zenith is only approximately contained in the local meridian plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field. The two coincide only for a perfectly rotationally symmetric body. On Earth, the axis of rotation is not fixed with respect to the planet so that the local vertical direction, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).

Nadir
The nadir is the direction pointing directly below a particular location (perpendicular, orthogonal). Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e.g., meteorology), the nadir at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The direction opposite of the nadir is the zenith.

Nadir also refers to a downward-facing viewing angle of an orbiting satellite, such as is employed during remote sensing of the atmosphere, as well as when an astronaut faces the Earth while performing an EVA.
Extra-vehicular activity (EVA) is work done by an astronaut away from the Earth, and outside of a spacecraft.

Circumpolar Stars

A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets (it never disappears below the horizon), due to its proximity to one of the celestial poles. Circumpolar stars are visible for the entire night on every night of the year, and would be continuously visible throughout the day were they not overwhelmed by the Sun's glare.

As Earth spins daily on its axis, the stars appear to rotate in circular paths around one of the celestial poles (the north celestial pole for observers in the northern hemisphere, or the south celestial pole for observers in the southern hemisphere). Stars far from a celestial pole appear to rotate in large circles, stars located very close to a celestial pole rotate in small circles and hardly seem to engage in any diurnal motion at all. Depending on the observer's latitude on Earth, some stars, the circumpolar ones, are close enough to the celestial pole to remain continuously above the horizon, while other stars dip below the horizon for some portion of their daily circular path, and others remain permanently below the horizon.

The circumpolar stars appear to lie within a circle that is centered at the celestial pole and tangential to the horizon. At the Earth's North Pole, the north celestial pole is directly overhead, and all stars that are visible at all (that is, all stars in the northern celestial hemisphere) are circumpolar. As one travels south, the north celestial pole moves towards the northern horizon. More and more stars that are at a distance from it begin to disappear below the horizon for some portion of their daily orbit, and the circle containing the remaining circumpolar stars becomes increasingly small. At the Earth's equator this circle vanishes to a single point, the celestial pole itself, which lies on the horizon, and there are no circumpolar stars at all.

As one travels south of the equator the opposite happens. The south celestial pole appears increasingly high in the sky, and all the stars lying within an increasingly large circle centred on that pole become circumpolar about it. This continues until one reaches the Earth's South Pole where, once again, all visible stars are circumpolar.

The north celestial pole is located very close to the North Star (Polaris), so, from the northern hemisphere all circumpolar stars appear to rotate around Polaris. Polaris itself remains almost stationary, always at the north (the azimuth is 0°), and always at the same altitude (angle from the horizon), equal to the latitude of the point of observation on Earth.

The circumstances making a star circumpolar is solely dependant on the observer's hemisphere and their latitude. As the altitude of either the north celestial pole or south celestial pole is the same as the observer's latitude, any star whose position from the pole is less than the latitude, will be circumpolar and will never set below the horizon. If the observer latitude is 45°N and is facing north, then any star will become circumpolar if it lies less than 45° from the north celestial pole. If the observer's latitude is 35°S and is facing south, then these stars are circumpolar within 35° of the south celestial pole. Stars on the celestial equator will not be circumpolar when seen from any latitude in either hemisphere of the Earth.

This is easily calculate if some star will be circumpolar (or not) at the observer's latitude by just knowing the star's declination. Some stars within the far northern constellations, such as Cassiopeia, Cepheus, Ursa Major, and Ursa Minor, roughly north of the Tropic of Cancer (+23½°N), will be circumpolar stars that never rise or set.

Some Stars within the far southern constellations, such as Crux, Musca, and Hydrus, roughly south of the Tropic of Capricorn (-23½°S), will also be circumpolar stars.
Stars (and constellations) that are circumpolar in one hemisphere are always invisible in the high latitudes of the opposite hemisphere, and these never rise above the horizon. For example, the southern circumpolar star Acrux is invisible from most of the Continental United States, likewise, the seven stars of the northern circumpolar Big Dipper asterism are invisible from most of the Patagonia region of South America.

Celestial Poles

The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, intersects the imaginary rotating sphere of stars called the celestial sphere. The north and south celestial poles appear directly overhead to an observer at the Earth's North Pole and South Pole.

At night the stars appear to drift overhead from east to west, completing a full circuit around the sky in 24 (sidereal) hours. Of course, exactly the same motion occurs during the day, except that the stars are not visible due to the sun's glare. This apparent motion is due to the spinning of the Earth on its axis. As the Earth spins, the celestial poles remain fixed in the sky, and all other points seem to rotate around them.

The celestial poles are also the poles of the celestial equatorial coordinate system, meaning they have declinations of + 90 degrees and - 90 degrees for the north and south celestial poles. The celestial poles do not remain permanently fixed against the background of the stars. Because of a phenomenon known as the precession of the equinoxes, the poles trace out circles on the celestial sphere. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths. Over very long periods the positions of the stars themselves change, due to the stars proper motions.

A planet's celestial poles are the points in the sky where the projection of the planet's axis of rotation intersects the celestial sphere. These points vary because different planets axes are oriented differently and the apparent positions of the stars also change slightly due to parallax effects.

Polaris

The north celestial pole has nearly the same coordinates as the star Polaris (also called the "pole star"). This makes Polaris useful for navigation in the northern hemisphere. Not only is it always above the north point of the horizon, but its altitude angle is nearly equal to the observer's geographic latitude. Polaris can only be seen from in the northern hemisphere.

To find Polaris, face north and locate the Big Dipper (Plough) and Little Dipper constellations. Looking at the "cup" part of the Big Dipper, imagine that the two stars at the outside edge of the cup form a line pointing upward out of the cup. This line points directly at the star at the tip of the Little Dipper's handle. That star is Polaris, the North Star.

Southern Cross

The south celestial pole is visible only from the southern hemisphere. It lies in the dim constellation Octans, the Octant. Sigma Octantis is identified as the south pole star, over a degree away from the pole, but with a magnitude of 5.5 it is barely visible on a clear night. The south celestial pole can be located from the Southern Cross (Crux) and its two "pointer" stars Centauri and Centauri. Very few bright stars of importance lie between Crux and the pole itself, although the constellation Musca is fairly easily recognised. Canopus (the second brightest star in the sky) and Achernar. Make a large equilateral triangle using these stars for two of the corners. The third imaginary corner will be the south celestial pole.

For a moonless and cloudless night you can use two faint clouds in the southern sky. These are marked in astronomy books as Large and Small Magellanic Clouds. These clouds are actually galaxies close to our own Milky Way. They make an equilateral triangle, the third point of which is the south celestial pole.

What kind of Sextant should I get

The first choice to make is between plastic or metal construction. Today's low cost metal sextants offer better accuracy and are easier to use. This will help you when first starting, and satisfy the professional's demands. Plastic models are perfect if your budget is restricted. They are also acceptable to some professionals who don't mind making frequent adjustments.

New or Used

Older sextants tend to have smaller mirrors and scopes which make them harder to use. Spare parts and maintenance are also more uncertain. Avoid discontinued models, and those out of date. Purchase only from someone you know and trust, or a reputable dealer. You will find that today's low cost metal sextants are very competitive with expensive used ones.

Accuracy

For all practical purposes, metal sextants are error free when compared to the many uncontrollable errors which may exist from such things as refraction, oblateness of the earth, and data tabulation. Generally, a minute of arc (one mile) is about the best anyone can hope to achieve. For these reasons, undue emphasis should not be placed on extreme accuracy guarantees. Plastic sextants can have errors in excess of 5 minutes, even when care is exercised. Although this is sufficient to make landfalls, precision navigation is difficult.

Mirror Size

The size of the mirrors on sextants generally vary directly with the quality of the instrument. Large index and horizon mirrors are desirable because larger mirrors allow more movement of the sextant while taking a sight, and lessen the possibility of losing the image as the body is brought down to the horizon.

Weight

Sextants are available with their major metal parts made of either aluminum, bronze or brass. The alloys of these metals are well suitable for use at sea. Some people feel that the heavier weight of a bronze sextant provides greater steadiness and more accurate readings, especially if it is windy. Others find that the lightweight models are less tiring to their wrist and arm and that the reduced fatigue gives better results. As the observer develops proficiency and speed in sight taking, fatigue becomes less of a factor. Lightweight plastic models can be difficult to use facing into a stiff wind because they tend to "flutter".

Scopes

A 3.5 x 40 scope is a good choice for stars. The large objective 40mm lens admits a great deal of light. The 3.5 power magnification helps you find and maintain stars in view in both calm or pitching seaways. A 7x35 monocular having greater magnification is well suited for sun sights, or the greater heights of eye associated with large ships.The increased magnification allows the sun's diameter to appear larger, and better defines a more distant horizon. This helps the navigator determine the point of tangency of the sun's limb and the horizon. The increased magnification makes finding and holding sights more difficult on a moving deck. A Sight Tube of zero magnification affords a wider field of view for rough weather, horizontal angles, and finding stars. If your sextant is to have only one scope, a 3.5x would be the your best choice for yacht sized vessels.

Traditional or Whole Horizon Mirror

Many sextants have an option of either the traditional (half-silvered) horizon mirror or what is called a "whole horizon mirror". With the traditional mirror, the horizon glass is divided vertically into two halves producing a "split image." The half nearest the frame is a silvered mirror and the other half is clear glass. In some cases this clear glass is eliminated. A later development in sextant technology is the whole horizon mirror. Using specially coated optics, the whole horizon mirror superimposes both the horizon and the celestial body on the entire mirror with no split image. This greatly simplifies "bringing down" the celestial body and makes it easier to hold the body in view. A draw back to this system is a very slight reduction in light transmission and reflection which may affect marginally lighted observations. Some feel these two aspects are a "trade off" that is, one can more quickly take observations with the whole horizon mirror, and be finished before marginal conditions occur. In general, people on stable platforms such as large ships tend to favor the traditional horizon mirror while those on yachts tend to favor the whole horizon mirror.

Illumination

Sextant lighting is the least needed feature on a sextant, since a flashlight should normally be available in any event for recording observations.

What's In It For Me?
It's tempting to think those solemn warnings only applied in the days before we could carry a GPS receiver in every pocket, but even the least paranoid among us knows that's not true: the GPS system itself can be intentionally switched off or degraded very quickly. It can be physically damaged; it can possibly be sabotaged by hackers. None of those things is likely to happen. More likely are on-the-water problems: GPS receivers can be fried by lightning, dropped overboard, crushed and damaged. Batteries do run out or get soaked. Well, as any offshore sailor knows, you just carry a back-up handheld. Or two, or three. Plus batteries and waterproof bags.

With all that, the chances of being far offshore without the ability to find a way home have diminished to a point where the demand for sextants has decreased markedly over the past 10 years. It's not what it was, but the demand for sextants has steadied. It's sailors rather than professional navigators who are interested in sextants now. There's a reasonable and steady demand. It's not so much that people need to have them, it's that they want to use them to navigate or just learn something new. Alot of today's interest in sextants stems from tradition.

There's more to it than that. Reading the sky not only tells us where we are, it connects us with all those who for centuries made their way over the world's waters with nothing more than simple instruments and tables, and the wisdom handed down to them on how to use them. Those skills, in turn, connect us in a practical way to the relationship of the earth to the sun, moon, and stars, a relationship that fewer and fewer sailors understand, to their detriment. The challenge of reading data on a GPS receiver pales next to the challenge of navigating by sextant, and for many people the resultant levels of satisfaction are proportional.

Selecting a Sextant

The clearer you are about your intended uses for the sextant, the better your chance of finding a place along the price range where your standards of value can be met. You can spend anywhere from $19.95 up to $3,000 for a serviceable sextant. The cheapest are made of cardboard (a German-made kit and plastic. While the cardboard kit is something of a novelty item with limited navigational potential (and no waterproofing) some plastic sextants are viable instruments for celestial navigation.

Still, most salts say plastic sextants aren't reliable enough for "real" navigation and that they should be limited to practice and/or lifeboat duty. Plastic sextants can yield results that are very close to those received from metal sextants: While most metal sextants can be shown to yield accuracy within a nautical mile the limitations built into plastic sextants give them a margin of error of five miles at best.

Instrument accuracy is really the least of your worries. Virtually all new sextants have negligible instrument or uncorrectable error. They also come with instructions for removing index (correctable) error. At that point you can bank on your new sextant as being virtually error free. Data reduction, refraction, and the oblateness of the earth are all more likely to be sources of inaccuracy than the sextant itself.

One of the biggest development in sextants over the past 20 years has been an across-the-board improvement of the optics involved. Mirrors have gotten bigger, coatings have been hardened, scopes made more versatile. Whether the sextant you buy comes from Germany, Japan, or China, you're likely to find that the optics are first-class.

Most navigators like a sextant with some "heft." Mass helps to steady the instrument. If, however, extending the sextant strains your arms and you're rushing your sights to be free of its weight, then you've got too much heft. Modern sextants range from 2 to just over 4 pounds. Finding one that's got substance enough to be steady but is light enough not to be taxing is a definite quest. Comfort in the grip of the handle is also important. Beware an overlycocked wrist and a sextant whose weight is not virtually centered around the handle.

Some top of the line sextants are made with brass armatures, but it's more common today to see framesmade of aluminum. This raises the topic of maintenance and the spectre of dissimilar metals. Even though you will obviously do your best to keep your sextant dry, smallboat use makes it likely that sooner or later it will get wet. Rinsing with fresh water and patting the sextant dry with a clean cloth is generally all that's required, but pay particular attention to those spots (like the stainless steel screws holding the mirror frame to the aluminum arm) where dissimilar metals are in contact.

Why should I bother with Celestial Navigation

GPS is great, and with world-wide coverage that helps get accurate positioning of vessels. But do you need a backup. The answer is yes, and that would be celestial navigation, its great to learn and it can be a interesting hobby.

Instances of electronic failure, total electric failure, and flooding are often documented. Even battery powered handhelds can be rendered inoperable in these ways. Batteries can run down, spares can be lost. The GPS system itself is not guaranteed.

GPS will track your boat and steer your boat. Some say it will even take your boat across the ocean for you. Without establishing a discipline, one's navigational skills will be lost. The key to navigation is the time-proven DR track. It should be maintained and updated with fixes, whether electronic or celestial. This gives information about the current and leeway, steering and compass errors.

Who can contemplate an 18th century brass and ebony sextant and not wonder what it was like to peer through it at the heavens, and bring an evening star down to a twilight horizon from the deck of a tall ship? To sense the approval of those who witness this magic-like prowess. To triumph at a land-fall well predicted? To know that they can navigate any ocean with no help from anyone?

Fun is doing something that is both easy and difficult. Easy to get started with, but having enough challenage that mastery does not come easy.

What could be easier than reckoning the longitude by simply observing the time of sunrise or sunset, or steering by a star? Almost as easy is the finding of latitude at noon. But how about identifying the navigational stars? Recognizing the planets? Accounting for the parallax of the moon? More experienced celestial navigators can use an unknown star shot through a hole in the overcast, shoot planets in broad daylight, predict sunrise underway, and calculate great circle distances.

Being familiar with the night sky is like having a giant roadmap overhead. One star may lead to another and before you know it you can identify 12 - 20 navigational stars. It is really cool to beable to take a sight of Sun, Moon, Stars, or Planets and get your location. People enjoy using sextants, even from a backyard with an artificial horizon. It's one of the few nautical activities that can be done without a boat, or even being on the water.

Right Ascension (RA)

Right ascension (RA) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.

RA is the celestial equivalent of terrestrial longitude. Both RA and longitude measure an east-west angle along the equator, and both measure from a zero point on the equator. For longitude, the zero point is the Prime Meridian, for RA, the zero point is known as the First Point of Aries, which is the place in the sky where the Sun crosses the celestial equator at the March equinox.

RA is measured eastward from the March equinox. Any units of angular measure can be used for RA, but it is most of the time it is measured in hours, minutes, and seconds, with 24 hours being a full circle. The reason for this is that the earth rotates at an approximately constant rate. Since a complete circle has 360 degrees, an hour of right ascension is equal to 1/24 of this, or 15 degrees of arc, a single minute of right ascension is equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc. Sidereal Hour Angle, used in celestial navigation, is similar to RA, but increases westward rather than eastward. Don't confuse SHA with the concept of hour angle as it is usually used in astronomy, which is how far west an object is from your local meridian.

RA can be used to determine a star's location and to determine how long it will take for a star to reach a certain point in the sky. For example, if a star with RA = 01:30:00 is at a location's meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later.

The concept of right ascension has been known at least as far back as Hipparchos who measured stars in equatorial coordinates in the 2nd century BCE. But Hipparchos and his successors made their star catalogs in ecliptical coordinates, and the use of RA was limited to special cases.

With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do that is to use an equatorial mount for the telescope, which allows the telescope to rotate at the same rate as the earth. As the equatorial mount became adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles.

Diurnal Motion

Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for one complete rotation is 23 hours, 56 minutes and 4 seconds (1 sidereal day).

Direction of the motion in the Northern hemisphere:
looking to the north, below the North Star: left-right, West-East
looking to the north, above the North Star: right-left, East-West
looking to the south: left-right, East-West

Northern circumpolar stars move counterclockwise around the North Star.

At the North Pole, North, East and West are not applicable, the motion is simply left-right, or looking vertically upward, counterclockwise around the zenith.

For the southern hemisphere, interchange North / South and left / right, and replace North Star by southern celestial pole. The circumpolar stars move clockwise around it. East / West are not interchanged.

At the equator both celestial poles are at the horizon and motion is counterclockwise (to the left) around the North Star and clockwise (to the right) around the southern celestial pole. All motion is from East to West, except for the two stationary points.

The daily path of an object on the celestial sphere, including the possible part below the horizon, has a length proportional to the cosine of the declination. The speed of the diurnal motion of a celestial object is this cosine times 15° / hr = 15' / min = 15" / s.

Sideral Time

Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars. They differ by the precession of the vernal equinox in right ascension relative to the stars.

Earth's sidereal day also differs from its rotation period relative to the background stars by the amount of precession in right ascension during one day. Sideral time means to measure time relative to the position of the stars.

Sidereal time is defined as the hour angle of the vernal equinox. When the meridian of the vernal equinox is directly overhead, local sidereal time is 00:00. Greenwich Sidereal Time is the hour angle of the vernal equinox at the prime meridian at Greenwich, England, local values differ according to longitude. When one moves eastward 15° in longitude, sidereal time is larger by one hour (note that it wraps around at 24 hours). Unlike computing local solar time, differences are counted to the accuracy of measurement, not just in whole hours.

Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observed at a given time. Objects are located in the night sky using right ascension and declination relative to the celestial equator, and when sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, or at which time it is best placed for observation.

Solar time is measured by the apparent diurnal motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south or north depending on the observer's latitude and the season). The average time taken for the sun to return to its highest point is 24 hours.

During the time needed by the Earth to complete a rotation around its axis (a sidereal day), the Earth moves a short distance (approximately 1°) along its orbit around the sun. After a sidereal day, the Earth still needs to rotate a small extra angular distance before the sun reaches its highest point. A solar day is, nearly 4 minutes longer than a sidereal day.

The stars, are so far away that the Earth's movement along its orbit makes a generally negligible difference to their apparent direction and so they return to their highest point in a sidereal day. A sidereal day is almost 4 minutes shorter than a mean solar day. Another way to see this difference is to notice that, relative to the stars, the Sun appears to move around the Earth once per year. Which means, there is one less solar day per year than there are sidereal days.

Apparent Time

Solar times are measures of the apparent position of the Sun on the celestial sphere. They are not actually the physical time, but they are hour angles, angles expressed in time units. They are also local times in the sense that they depend on the longitude of the observer.

Apparent solar time or true solar time is the hour angle of the Sun. It is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. Note that the solar day starts at noon, so apparent solar time 00:00 means noon and 12:00 means midnight.

The length of a solar day varies throughout the year for two reasons. First, Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun (perihelion) and slower when it is farthest from the Sun (aphelion) Second, due to Earth's axial tilt, the Sun moves along a great circle (the ecliptic) that is tilted to Earth's celestial equator. When the Sun crosses the equator at both equinoxes, it is moving at an angle to it, so the projection of this tilted motion onto the equator is slower than its mean motion, when the Sun is farthest from the equator at both solstices, it moves parallel to it and closer to the polar axis than the equator, so the projection of this parallel motion onto the equator is faster than its mean motion Consequently, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21). These dates are shifted from those of the equinoxes and solstices by the fast / slow Sun at Earth's perihelion / aphelion.

Mean solar time
Mean solar time is the hour angle of the mean Sun. As the mean Sun is a mathematical construction only and cannot be physically observed, the mean solar time is computed from an artificial clock time adjusted via observations of the diurnal rotation of the fixed stars to agree with average apparent solar time. Though the amount of daylight varies significantly, the length of a mean solar day does not change on a seasonal basis. The length of a mean solar day increases at a rate of approximately 1.4 milliseconds each century. An apparent solar day can differ from a mean solar day by as much as 22 seconds shorter to 29 seconds longer. Because many of these long or short days occur in succession, the difference builds up to as much as nearly 17 minutes early or a little over 14 minutes late. Since these periods are cyclical, they do not accumulate from year to year. The difference between apparent solar time and mean solar time is called the equation of time. The mean solar day also starts at noon, with 00:00 meaning noon and 12:00 meaning midnight. The civil time is defined as mean solar time minus 12 hours.

The length of the mean solar day is increasing due to the tidal acceleration of the Moon by Earth, and the corresponding deceleration of the Earth by the Moon.

The mean Sun is defined as follows. First, consider a fictitious Sun that moves along the ecliptic at a constant speed and occupies the same position as the real Sun when Earth passes through the perihelion and also when it passes through the aphelion. Then, the mean sun is a second fictive Sun that moves along the celestial equator at constant speed and passing through the vernal point simultaneously with the first fictive sun.

In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric and coaxial with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected from their corresponding geographic equivalents are the celestial equator and the celestial poles. The celestial sphere projection is a very practical tool for positional astronomy.

In astronomy, the hour angle is one of the coordinates used in the equatorial coordinate system for describing the position of a point on the celestial sphere. The hour angle of a point is the angle between the half plane determined by the Earth axis and the zenith (half of the meridian plane) and the half plane determined by the Earth axis and the given point. The angle is taken with minus sign if the point is eastward of the meridian plane and with the plus sign if the point is westward of the meridian plane.

The hour angle is usually expressed in time units, with 24 hours corresponding to 360 degrees. The hour angle must be paired with the declination in order to fully specify the position of a point on the celestial sphere as seen by the observer at a given time.

Equation of Time

The equation of time is the difference at any moment deduced from the current position of the Sun and time as read from a regulated clock set to the local mean time. The equation of time varies over the course of a year, in way that is almost exactly reproduced from one year to the next. It can be ahead (fast) by as much as 16 minutes 33 seconds (around November 3) or fall behind by as much as 14 minutes 6 seconds (around February 12). It is caused by irregularity in the path of the Sun across the sky, due to a combination of the obliquity of the Earth's rotation axis and the eccentricity of its orbit. The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth.

The equation of time was used historically to set clocks. One of two common land based ways to set clocks was by observing the passage of the sun across the local meridian at noon. The moment the sun passed overhead, the clock was set to noon, offset by the number of minutes given by the equation of time for that date. The second method did not use the equation of time, it used stellar observations to give sidereal time, in combination with the relation between sidereal time and solar time.The equation of time values for each day of the year, compiled by astronomical observatories, were listed in almanacs and ephemerides.

Other planets will have an equation of time too. On Mars the difference between sundial time and clock time can be as much as 50 minutes, due to the considerably greater eccentricity of its orbit.

The Earth revolves around the Sun. As such it appears that the Sun makes one rotation around the Earth in one year. If the Earth orbited the Sun with a constant speed, in a circular orbit in a plane perpendicular to the Earth's axis, then the Sun would culminate every day at exactly the same time, and be a perfect time keeper, except for the very small effect of its slowing rotation. But the orbit of the Earth is an ellipse, and its speed varies between 30.287 and 29.291 km/s, according to Kepler's laws of planetary motion, and its angular speed also varies, and the Sun appears to move faster at perihelion (currently around 3 January) and slower at aphelion a half year later. At these extreme points, this effect increases (respectively, decreases) the real solar day by 7.9 seconds from its mean. This daily difference accumulates over a period. As a result, the eccentricity of the Earth's orbit contributes a sine wave variation with an amplitude of 7.66 minutes and a period of one year to the equation of time. The zero points are reached at perihelion (at the beginning of January) and aphelion (beginning of July) while the maximum values are in early April (negative) and early October (positive).

Even if the Earth's orbit were circular, the motion of the Sun along the celestial equator would still not be uniform. This is a because of the tilt of the Earth's rotation with respect to its orbit, or equivalently, the tilt of the ecliptic (the path of the sun against the celestial sphere) with respect to the celestial equator. The projection of this motion onto the celestial equator, along which "clock time" is measured, is a maximum at the solstices, when the yearly movement of the Sun is parallel to the equator and appears as a change in right ascension, and is a minimum at the equinoxes, when the Sun moves in a sloping direction and appears mainly as a change in declination, leaving less for the component in right ascension, which is the only component that affects the duration of the solar day. As a consequence of that, the daily shift of the shadow cast by the Sun in a sundial, due to obliquity, is smaller close to the equinoxes and greater close to the solstices. At the equinoxes, the Sun is seen slowing down by up to 20.3 seconds every day and at the solstices speeding up by the same amount.

The equation of time was mean minus apparent solar time in the British Nautical Almanac and Astronomical Ephemeris. Earlier, all times in the almanac were in apparent solar time because time aboard ship was determined by observing the Sun. In the unusual case that the mean solar time of an observation was needed, the extra step of adding the equation of time to apparent solar time was needed. Since 1834, all times have been in mean solar time because by then the time aboard most ships was determined by marine chronometers. In the unusual case that the apparent solar time of an observation was needed, the extra step of adding the equation of time to mean solar time was needed, requiring all differences in the equation of time to have the opposite sign.

As the daily movement of the Sun is one revolution per day, that is 360° every 24 hours, and the Sun itself appears as a disc of about 0.5° in the sky, simple sundials can be read to a maximum accuracy of about one minute. Since the equation of time has a range of about 30 minutes, the difference between sundial time and clock time cannot be ignored. In addition to the equation of time, one also has to apply corrections due to one's distance from the local time zone meridian and summer time, if any.

Greenwich Mean Time (GMT)

Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is regularly used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone. It is also used to refer to Universal Time (UT), which is a standard astronomical concept used in many fields and is sometimes called Zulu time.

Noon Greenwich Mean Time is not necessarily the moment when the noon sun crosses the Greenwich meridian and reaches its highest point in the sky in Greenwich because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event can be up to 16 minutes away from noon GMT (this is called the equation of time). The fictitious mean sun is the annual average motion of the true Sun.

The term GMT has been used with two different conventions for numbering hours. The old astronomical convention before 1925 was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours.

As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees (this convention was internationally adopted in the International Meridian Conference of 1884). The chronometer on GMT did not affect shipboard time itself, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used worldwide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".

The daily rotation of the Earth is somewhat irregular and is slowing down slightly, atomic clocks have a more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world.

Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), the mean solar time on the meridian of Greenwich, and GMT is sometimes used loosely as a synonym for UTC. In fact the expression "Universal Time" is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1. All of these versions of UT are based on sidereal time, but with a scaling factor and other adjustments to make them closer to solar time.

Standard time, divided the world into twenty-four time zones, each one covering exactly 15 degrees of longitude. All clocks within each of these zones is set to the same time as the others, but different by one hour from those in the next zone. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the use of Greenwich Mean Time in order to set local clocks. This location was chosen because in 1884 two-thirds of all charts and maps already used it as their prime meridian.

In 1928, the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because the GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. The term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.

UTC, Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based.

UT1, is the principal form of Universal Time. UT1 is the same everywhere on Earth, and is proportional to the true rotation angle of the Earth with respect to a fixed frame of reference. Since the rotational speed of the earth is not uniform, UT1 has an uncertainty of plus or minus 3 milliseconds per day.

International Navigation Rules Exam

1. INTERNATIONAL ONLY A light used to signal passing intentions must be an __________.
a. all-round yellow light only
b. all-round white light only
c. all-round blue light only
d. alternating red and yellow light

2. INTERNATIONAL ONLY Which statement is TRUE, according to the Rules?

a. A vessel engaged in fishing while underway shall, so far as possible, keep out of the way of a vessel restricted in her ability to maneuver.
b. A vessel not under command shall keep out of the way of a vessel restricted in her ability to maneuver.
c. A fishing vessel while underway has the right-of-way over a vessel constrained by her draft.
d. A vessel not under command shall avoid impeding the safe passage of a vessel constrained by her draft.

3. INTERNATIONAL ONLY A light used to signal passing intentions must be an __________.
a. all-round white or yellow light
b. all-round yellow light only
c. all-round white light only
d. Any colored light is acceptable.

4. INTERNATIONAL ONLY Which statement is TRUE, according to the Rules?

a. A vessel constrained by her draft shall keep out of the way of a vessel engaged in fishing.
b. A vessel engaged in fishing while underway shall, so far as possible, keep out of the way of a vessel restricted in her ability to maneuver.
c. A vessel not under command shall avoid impeding the safe passage of a vessel constrained by her draft.
d. A vessel not under command shall keep out of the way of a vessel restricted in her ability to maneuver.

5. INTERNATIONAL ONLY A power-driven vessel pushing ahead or towing alongside will show sidelights, a stern light, and __________.

a. an all-round red light where it can best be seen
b. two yellow masthead lights in a vertical line
c. two masthead lights in a vertical line
d. a single white light forward

6. INTERNATIONAL ONLY Your vessel is crossing a narrow channel. A vessel to port is within the channel and crossing your course. She is showing a black cylinder. You should __________.
a. hold your course and speed
b. not impede the other vessel
c. exchange passing signals
d. sound the danger signal

7. INTERNATIONAL ONLY Your vessel is backing out of a slip in a harbor. Visibility is restricted. You should sound __________.

a. one prolonged blast only
b. one prolonged blast followed by three short blasts when the last line is taken aboard
c. one prolonged blast followed by three short blasts when leaving the slip
d. the danger signal

8. INTERNATIONAL ONLY You are approaching another vessel and will pass safely starboard to starboard without changing course. You should __________.

a. hold course and sound no whistle signal
b. hold course and sound a two blast whistle signal
c. change course to starboard and sound one blast
d. hold course and sound one blast

9. BOTH INTERNATIONAL & INLAND Which statement is TRUE concerning a situation involving a fishing vessel and a vessel not under command?

a. They are required to communicate by radiotelephone.
b. If the vessel not under command is a power-driven vessel, she must keep clear of the fishing vessel.
c. The fishing vessel must keep out of the way of the vessel not under command.
d. Both vessels are required to take action to stay clear of each other.

10. INTERNATIONAL ONLY Of the vessels listed, which must keep out of the way of all the others?

a. A vessel constrained by her draft
b. A vessel restricted in her ability to maneuver
c. A vessel pushing a barge
d. A vessel engaged in fishing

11. INTERNATIONAL ONLY Which vessel shall avoid impeding the safe passage of a vessel constrained by her draft?

a. A vessel not under command
b. A sailing vessel
c. A vessel restricted in her ability to maneuver
d. All of the above

12. INTERNATIONAL ONLY A signal of intent must be sounded in international waters by __________.

a. a vessel meeting another head-on
b. a vessel overtaking another in a narrow channel
c. a vessel crossing the course of another
d. the give-way vessel in a crossing situation

13. INTERNATIONAL ONLY To indicate that a vessel is constrained by her draft, a vessel may display, in a vertical line, __________.

a. three 360° red lights
b. two 225° red lights
c. three 360° blue lights
d. two 225° blue lights

14. INTERNATIONAL ONLY In addition to other required lights, a power-driven vessel pushing ahead or towing alongside displays __________.

a. two all-round red lights in a vertical line
b. two yellow towing lights in a vertical line
c. two white masthead lights in a vertical line
d. two lights on the stern, one yellow and one white

15. INTERNATIONAL ONLY Which statement is true concerning a vessel "constrained by her draft"?

a. She must be a power-driven vessel.
b. She is not under command.
c. She may be a vessel being towed.
d. She is hampered because of her work.

16. INTERNATIONAL ONLY When moving from a berth alongside a quay (wharf), a vessel must sound __________.

a. three short blasts
b. a long blast
c. a prolonged blast
d. No signal is required.

17. INTERNATIONAL ONLY You are in charge of a 250-meter freight vessel constrained by her draft proceeding down a narrow channel. There is a vessel engaged in fishing on your starboard bow half a mile away. According to Rule 9, which statement is TRUE?

a. You are not to impede the fishing vessel.
b. If you are in doubt as to the fishing vessel's intentions you may sound at least five short and rapid blasts on the whistle.
c. You are to slow to bare steerageway until clear of the fishing vessel.
d. You must sound one prolonged blast to alert the fishing vessel.

18. INTERNATIONAL ONLY The International Rules of the Road apply __________.

a. to all waters which are not inland waters
b. only to waters outside the territorial waters of the United States
c. only to waters where foreign vessels travel
d. upon the high seas and connecting waters navigable by seagoing vessels

19. INTERNATIONAL ONLY A towing light is __________.

a. shown at the bow
b. white in color
c. shown in addition to the stern light
d. an all-round light

20. INTERNATIONAL ONLY In a narrow channel, an overtaking vessel which intends to pass on the other vessel's port side would sound __________.

a. one prolonged followed by two short blasts
b. one short blast
c. two short blasts
d. two prolonged followed by two short blasts

21. INTERNATIONAL ONLY You are underway on the high seas in restricted visibility. You hear a fog signal of one prolonged and two short blasts. It could be any of the following EXCEPT a vessel __________.

a. minesweeping
b. engaged in fishing
c. constrained by her draft
d. being towed

22. INTERNATIONAL ONLY A vessel displaying three red lights in a vertical line is __________.

a. not under command
b. aground
c. dredging
d. constrained by her draft

23. INTERNATIONAL ONLY At night, a power-driven vessel underway of less than 7 meters in length where its maximum speed does not exceed 7 knots may show, as a minimum, __________.

a. sidelights and a stern light
b. the lights required for a vessel more than 7 meters in length
c. sidelights only
d. one all-round white light

24. INTERNATIONAL ONLY In a narrow channel, a signal of intent which must be answered by the other vessel, is sounded by a vessel __________.

a. meeting another head-on
b. crossing the course of another
c. overtaking another
d. Any of the above

25. INTERNATIONAL ONLY When two vessels are in sight of one another, all of the following signals may be given EXCEPT __________.

a. a light signal of at least five short and rapid flashes
b. four short whistle blasts
c. one prolonged, one short, one prolonged and one short whistle blasts
d. two short whistle blasts

26. INTERNATIONAL ONLY A power-driven vessel leaving a quay or wharf must sound what signal?

a. Three short blasts
b. A long blast
c. A prolonged blast
d. No signal is required.

27. INTERNATIONAL ONLY What whistle signal, if any, would be sounded when two vessels are meeting, but will pass clear starboard to starboard?

a. One short blast
b. Two short blasts
c. Five or more short blasts
d. No signal is required

28. INTERNATIONAL ONLY In a narrow channel, a vessel trying to overtake another on the other vessel's port side, would sound a whistle signal of __________.

a. one short blast
b. two short blasts
c. two prolonged blasts followed by one short blast
d. two prolonged blasts followed by two short blasts

29. INTERNATIONAL ONLY On open water, a power-driven vessel coming up dead astern of another vessel and altering her course to starboard so as to pass on the starboard side of the vessel ahead would sound __________.

a. two short blasts
b. one short blast
c. two prolonged blasts followed by one short blast
d. one long and one short blast

30. INTERNATIONAL ONLY If a vessel displays three all-round red lights in a vertical line at night, during the day she may show __________.

a. three balls in a vertical line
b. a cylinder
c. two diamonds in a vertical line
d. two cones, apexes together

Answers
1. B
2. A
3. C
4. B
5. C
6. B
7. A
8. A
9. C
10. C
11. B
12. B
13. A
14. C
15. A
16. D
17. B
18. D
19. C
20. D
21. D
22. D
23. D
24. C
25. B
26. D
27. D
28. D
29. B
30. B

Inland Navigation Rules Exam

1. INLAND ONLY Which term is NOT defined in the Inland Navigation Rules?
A. Towing light
B. Vessel constrained by her draft
C. In sight
D. Restricted visibility

2. INLAND ONLY For the purpose of the Inland Navigation Rules, the term "Inland Waters" includes
A. the Western Rivers
B. the Great Lakes on the United States side of the International Boundary
C. harbors and rivers shoreward of the COLREGS demarcation lines
D. All of the above

3. INLAND ONLY When two power-driven vessels are meeting on the Great Lakes, Western Riversl or waters specified by the Secretary, where there is a current, which vessel shall sound the first passing signal?
A. The vessel going upstream stemming the current
B. The vessel down bound with a following current
C. The vessel that is towing regardless ofthe current
D. Either vessel

4. INLAND ONLY When two power-driven vessels are meeting in a narrow channel on the Western Rivers, the vessel having the right of way is the one ?
A. moving upstream against the current
B. moving downstream with a following current
C. located more toward the channel centerline
D. sounding the first whistle signal

5. INLAND ONLY You are navigating in a narrow channel and must remain in the channel for safe operation. Another vessel is crossing the channel ahead of you from your starboard and you doubt whether your vessel will pass safely. Which statement is TRUE?
A. You must stop your vessel, since the other vessel is the stand-on.
B. You must sound one short blast of the whistle and turn to starboard.
C.You must sound the danger signal.
D.You must stop your engines and you may sound the danger signal.

6. INLAND ONLY Under the Inland Navigation Rules, what is the meaning of the two short blasts signal used when meeting another vessel?
A. I am turning to starboard.
B. I am turning to port.
C. I intend to leave you on my starboard side.
D. I intend to leave you on my port side.

7. INLAND ONLY Which lights are required for a barge, not part of a composite unit, being pushed ahead?
A. Sidelights and a stern light
B. Sidelights, a special flashing light, and a stern light
C. Sidelights and a special flashing light
D. Sidelights, a towing light, and a stern light

8. INLAND ONLY The stand-on vessel in a crossing situation sounds one short blast of the whistle. This means that the vessel __________.
A. intends to hold course and speed
B. is changing course to starboard
C. is changing course to port
D. intends to leave the other on her port side

9. INLAND ONLY Passing signals shall be sounded on inland waters by __________.
A. all vessels upon sighting another vessel rounding a bend in the channel
B. towing vessel when meeting another towing vessel on a clear day with a 0.6 mile CPA (Closest Point of Approach)
C. a power-driven vessel when crossing less than half a mile ahead of another power-driven vessel
D. All of the above

10. INLAND ONLY Your vessel is meeting another vessel head-on. To comply with the rules, you should exchange __________.
A. one short blast, alter course to port, and pass starboard to starboard
B. one short blast, alter course to starboard, and pass port to port
C. two short blasts, alter course to port, and pass starboard to starboard
D. two short blasts, alter course to starboard, and pass port to port

11. INLAND ONLY A vessel overtaking another in a narrow channel, and wishing to pass on the other vessel's port side, would sound a whistle signal of __________.
A. one short blast
B. two short blasts
C. two prolonged blasts followed by one short blast
D. two prolonged blasts followed by two short blasts

12. INLAND ONLY A fleet of moored barges extends into a navigable channel. What is the color of the lights on the barges?
A. Red
B. Amber
C. White
D. Yellow

13. INLAND ONLY At night, a light signal consisting of two flashes by a vessel indicates __________.
A. an intention to communicate over radiotelephone
B. that the vessel is in distress
C. an intention to leave another vessel to port
D. an intention to leave another vessel to starboard

14. INLAND ONLY A barge more than 50 meters long is required to show how many white anchor lights when anchored in a Secretary approved "special anchorage area"?
A. 2
B. 1
C. 3
D. None

15. INLAND ONLY Which type of vessel is NOT mentioned in the Inland Navigation Rules?
A. An inconspicuous, partly submerged vessel
B. A seaplane
C. An air-cushion vessel
D. A vessel constrained by her draft

16. INLAND ONLY Two vessels in a crossing situation have reached agreement by radiotelephone as to the intentions of the other. In this situation, whistle signals are __________.
A. required
B. not required, but may be sounded
C. required if crossing within half a mile
D. required when crossing within one mile

17. INLAND ONLY What lights are required for a barge being pushed ahead, not being part of a composite unit?
A. Sidelights and a stern light
B. Sidelights and a special flashing light
C. Sidelights, a towing light, and a stern light
D. Sidelights, a special flashing light, and a stern light

18. INLAND ONLY At night, a barge moored in a slip used primarily for mooring purposes shall __________.
A. not be required to be lighted
B. show a white light at each corner
C. show a red light at the bow and stern
D. show a flashing yellow light at each corner

19. INLAND ONLY Which statement is TRUE concerning the fog signal of a vessel 15 meters in length, anchored in a "special anchorage area" approved by the Secretary?
A. The vessel is not required to sound a fog signal.
B. The vessel shall ring a bell for 5 seconds every minute.
C. The vessel shall sound one blast of the foghorn every 2 minutes.

20. INLAND ONLY Which statement is TRUE concerning the Inland Navigation Rules?
A. They list requirements for Traffic Separation Schemes.
B. They define moderate speed.
C. They require communication by radiotelephone to reach a passing agreement.
D. All of the above

21. INLAND ONLY Two vessels are meeting on a clear day and will pass less than half a mile apart. In this situation whistle signals __________.
A. must be exchanged
B. may be exchanged
C. must be exchanged if passing agreements have not been made by radio
D. must be exchanged only if course changes are necessary by either vessel

22. INLAND ONLY Which is TRUE of a vessel downbound with a following current when meeting an upbound vessel on the Western Rivers?
A. She has the right-of-way only if she is a power-driven vessel.
B. She has the right-of-way only if she has a tow.
C. She does not have the right-of-way, since the other vessel is not crossing the river.
D. She must wait for a whistle signal from the upbound vessel.

23. INLAND ONLY A vessel of less than 20 meters in length at anchor at night in a "special anchorage area designated by the Secretary" __________.
A. must show one white light
B. need not show any lights
C. must show two white lights
D. need show a light only on the approach of another vessel

24. INLAND ONLY At night, which lights are required on barges moored in a group formation more than two barges wide?
A. Two unobstructed all-round white lights
B. All-round white lights placed at the corners of each barge in the group
C. Two unobstructed all-round yellow lights
D. Two red lights in a vertical line at the corner extremities of the group

25. INLAND ONLY A towing vessel pushing ahead on the Western Rivers above the Huey P. Long bridge must show __________.
A. sidelights only
B. sidelights and towing lights
C. sidelights, towing lights, and two masthead lights
D. sidelights, towing lights, and three masthead lights

26. INLAND ONLY A power-driven vessel, when leaving a dock or berth, is required to sound __________.
A. two short blasts
B. one long blast
C. one prolonged blast
D. the danger signal

27. INLAND ONLY Which is CORRECT regarding a "special flashing light"?
A. It must be yellow in color.
B. It must be placed as far forward as possible.
C. It must not show through an arc of more than 225°.
D. All of the above

28. INLAND ONLY For the purpose of the Inland Navigation Rules, the term "Inland Waters" includes __________.
A. the Western Rivers, extending to the COLREGS demarcation line
B. harbors and rivers to the outermost aids to navigation
C. waters along the coast of the United States to a distance of two miles offshore
D. None of the above

29. INLAND ONLY On the Western Rivers, a power-driven vessel crossing a river must __________.
A. maintain course and speed as you have the right of way over all vessels
B. keep out of the way of any vessel descending the river
C. keep out of the way of a power-driven vessel ascending or descending the river
D. None of the above

30. INLAND ONLY While underway at night, you see two yellow lights displayed in a vertical line. This should indicate to you a __________.
A. opening in a pipeline
B. vessel broken down
C. vessel pushing ahead
D. vessel fishing

Answers
1. B
2. D
3. B
4. B
5. C
6. C
7. C
8. D
9. C
10. B
11. B
12. C
13. D
14. A
15. D
16. B
17. B
18. A
19. A
20. A
21. C
22. A
23. B
24. A
25. B
26. C
27. D
28. A
29. C
30. C