Sunday, January 25, 2009

Celestial Navigation Information

Celestial navigation is a position fixing technique that was devised to help sailors cross the oceans without having to rely on dead reckoning to enable them to find land. Celestial navigation uses angular measurements (sights) between the horizon and a celestial object. You can use the Sun, Moon, Planets or one of the 57 navigational stars whose coordinates are tabulated in the nautical almanac.

How it Works
Celestial navigation is used to measure angles between celestial objects in the sky and the horizon to locate your position. At any given time, any celestial object the Sun, Moon, Planets, and Stars will be located directly over a particular geographic position on the Earth. Your location (latitude and longitude) can be found by referring to tables in the nautical almanac. The measured angle between the celestial object and the horizon is used to define a circle on the surface of the Earth called a celestial line of position (LOP). The size and location of this circular line of position can be found using mathematical or graphical methods. The LOP is significant because the celestial object would be observed to be at the same angle above the horizon from any point along its circumference at that instant.

Angular Measurement
Using a marine sextant to measure the altitude of the sun above the horizon has developed over the years. One method is to hold your hand above the horizon with your arm stretched out. The width of a finger is an angle just over 1.5 degrees. The need for more accurate measurements led to the development of a number of increasingly accurate instruments, including the kamal, astrolabe, octant and the sextant. The sextant and octant are more accurate because they measure angles from the horizon, eliminating errors caused by the placement of an instrument's pointers, and because their dual mirror system cancels relative motions of the instrument, showing a steady view of the object and horizon. Navigators measure distance on the globe in degrees, arc minutes and arc seconds. A nautical mile is defined as 1852 meters, it is also one minute of angle along a meridian on the earth. Sextants can be read accurately to within 0.2 arc minutes. So the observer's position can be determined within (theoretically) 0.2 miles, about 400 yards (370 m). Most ocean navigators, shooting from a moving platform, can achieve a practical accuracy of 1.5 miles (2.8 km), enough to navigate safely when out of sight of land.

Practical Navigation
Practical celestial navigation usually requires a marine chronometer to measure time, a sextant to measure the angles, an almanac giving the coordinates of celestial objects, a set of sight reduction tables to compute the height and azimuth, and a chart of your area. With sight reduction tables, the only math required is addition and subtraction. Most people can master simpler celestial navigation procedures after a day or two of instruction and practice, even using manual calculation methods. Modern practical navigators usually use celestial navigation in combination with satellite navigation to correct a dead reckoning track, that is, a course estimated from a vessel's position, angle and speed. Using multiple methods helps the navigator detect errors, and simplifies procedures. When used this way, a navigator will from time to time measure the sun's altitude with a sextant, then compare that with a precalculated altitude based on the exact time and estimated position of the observation. On the chart, you can use the straight edge of a plotter to mark each position line. If the position line shows you to be more than a few miles from the estimated position, you can take more observations to restart the dead-reckoning track. In the event of equipment or electrical failure, one can get to a port by simply taking sun lines a few times a day and advancing them by dead reckoning to get a crude running fix.

Latitude was measured in the past either at noon (the "noon sight") or from Polaris, the north star. Polaris always stays within about 1 degree of celestial north pole. If a navigator measures the angle to Polaris and finds it to be 10 degrees from the horizon, then you are on a circle at about North 10 degrees of the geographic latitude. Angles are measured from the horizon because locating the point directly overhead, the zenith hard to do. When haze obscures the horizon, navigators use artificial horizons, which are bubble levels reflected into a sextant. Latitude can also be determined by the direction in which the stars travel over time. If the stars rise out of the east and travel straight up you are at the equator, but if they drift south you are to the north of the equator. The same is true of the day-to-day drift of the stars due to the movement of the Earth in orbit around the Sun; each day a star will drift approximately one degree. In either case if the drift can be measured accurately, simple trigonometry will reveal the latitude.

Longitude can be measured in the same way. If you can accurately measure the angle to Polaris, a similar measurement to a star near the eastern or western horizons will provide the longitude. The problem is that the Earth turns about 15 degrees per hour, making the measurements dependent on time. A measure only a few minutes before or after the same measure the day before creates navigation errors. Before good chronometers were available, longitude measurements were based on the transit of the moon, or the positions of the moons of Jupiter. For the most part, these are too difficult to be used by anyone except professional astronomers.

Use of Time
The most popular method was, and still is to use an accurate timepiece to directly measure the time of a sextant sight. The need for accurate navigation led to the development of progressively more accurate chronometers in the 18th century. Today, time is measured with a chronometer, a quartz watch, a shortwave radio time signal broadcast from an atomic clock, or the time displayed on a GPS. A quartz wristwatch normally keeps time within a half-second per day. If it is worn constantly, keeping it near body heat, its rate of drift can be measured with the radio, and by compensating for this drift, a navigator can keep time to better than a second per month. Traditionally, three chronometers were kept in gimbals in a dry room near the center of the ship. They were used to set a watch for the actual sight, so that no chronometers were never risked to the wind and salt water on deck. Winding the chronometers was the duty of the navigator, logged as "chron. wound." for checking by line officers. Navigators also set the ship's clocks and calendar.

Modern Celestial Navigation
The celestial line of position was discovered in 1837 by Thomas Hubbard Sumner when, after one observation he computed and plotted his longitude at more than one trial latitude in his vicinity and noticed that the positions lay along a line. Using this method with two bodies, navigators were finally able cross two position lines and obtain their position in effect determining both latitude and longitude. Later in the 19th century came the development of the modern (Marcq St. Hilaire) intercept method, with this method the body height and azimuth are calculated for a convenient position, and compared with the observed height. The difference in arc minutes is the nautical mile "intercept" distance that the position line needs to be shifted toward or away. Two other methods of reducing sights are the longitude by chronometer and the ex-meridian method.

Celestial navigation is becoming increasingly redundant with the advent of inexpensive and highly accurate satellite navigation receivers (GPS), it was used extensively in aviation until 1960s, and marine navigation until recently. But since a prudent mariner never relies on any sole means of fixing his / her position, many national maritime authorities still require deck officers to show knowledge of celestial navigation in examinations, primarily as a back up for electronic navigation. One of the most common current usages of celestial navigation aboard large merchant vessels is for compass calibration and error checking at sea when no terrestrial references are available.