Monday, January 5, 2009

How to Use the Marine Sextant

The marine sextant is designed to measure angles, either horizontally or vertically. The most common use of the sextant is for celestial observations using vertical angles between celestial objects and the horizon. It can also be used to measure the horizontal angle between two terrestial objects to get a line of position.

Sun Sights
For a Sun sight, hold the sextant vertically and direct the sight line at the horizon directly below the Sun. Move the shade glasses into the line of sight, move the index arm outward along the arc until the reflected image appears in the horizon glass near the direct view of the horizon. Rock the sextant slightly to the right and left to make sure it is perpendicular. As you rock the sextant, the image of the Sun appears to move in an arc. The sextant is vertical when the Sun appears at the bottom of the arc. This is the correct position for making the observation. The Sun’s reflected image appears at the center of the horizon glass, one half appears on the silvered part, and the other half appears on the clear part. Move the index arm with the drum or vernier slowly until the Sun's exactly on the horizon. This takes some practice and some people will get a error by bringing the sun too far down.Some navigators will let the body contact the horizon by its own motion, bringing it slightly below the horizon if rising, and above if setting. At the instant the body is touching the horizon, note the time. This is the uncorrected reading of the sextant.
Moon Sights
When observing the Moon, use the same procedure as for the Sun. Sights of the Moon are best made during daylight hours or twilight when it is easier to see it and the horizon. At night you can get false horizons below the Moon because the Moon illuminates the water below it.
Star and Planet Sights
Most people find the Sun and Moon are easy to find in the sextant, but the stars and planets are harder to locate because your field of view is so small. There are several ways to help you locate a star or planet.

1. Set the index arm and micrometer drum on 0° and direct the line of sight at the body to be observed. Then keep the reflected image of the body in the mirrored half of the horizon glass, swing the index arm out and rotate the sextant down. Keep the sighted image in the mirror until the horizon appears in the clear part of the horizon glass. This way is hard to do, if the body is lost while it is being brought down, you might not find it, which means you will have to start over again.
2. Direct the line of sight at the body while holding the sextant upside down. Slowly move the index arm out until the horizon appears in the horizon glass. Then invert the sextant and take the sight in the usual manner. This is my preferred method, you will have less of a chance of losing the body.
3. Determine in advance the approximate altitude and azimuth of the body by the starfinder 2102-D. Set the sextant at the indicated altitude and face in the direction of the azimuth. The image of the body should appear in the horizon glass with a little searching. When measuring the altitude of a star or planet, bring its center down to the horizon. Stars and planets have no upper or lower limb that you can see, you must observe the center of the point of light. Because stars and planets don't have a limb and because their visibility may be limited, the method of letting a star or planet intersect with the horizon by its own motion is not recommended. As with the sun and moon, rock the sextant to get the perpendicularity.
Taking a Sight
Get the altitudes and azimuths for several stars or planets when preparing to take celestial sights. Choose the stars and planets that will give you the best bearing spread. Try to select bodies with a altitude between 20° and 65°. Take sights of the brightest stars first in the evening, take sights of the brightest stars last in the morning. Sometimes weather or other ships can obscure the horizon directly below a body that you want to observe. You could try a back sight using the opposite point of the horizon as the reference. For this you must face away from the body and observe the supplement of the altitude. If the Sun or Moon is observed, what appears in the horizon glass to be the lower limb is the upper limb, and vice versa. In the case of the Sun, it is usually best to observe what appears to be the upper limb. The arc that appears when rocking the sextant for a back sight is inverted, that is the highest point indicates the position of perpendicularity. If more than one telescope is furnished with the sextant, the erecting telescope is used to observe the Sun.
The collar that the sextant telescope fits may be adjusted in or out in relation to the frame. When moved in, more of the mirrored half of the horizon glass is visible to the navigator, and a star or planet is best observed when the sky is bright. Near the darker limit of twilight, the telescope can be moved out, giving a broader view of the clear half of the glass, and making the less distinct horizon more easily seen. When measuring an altitude, have an assistant note and record the time, with a "stand-by" warning when the measurement is almost ready, and a "mark" at the moment a sight is made. If a flashlight is needed to see the comparing watch, your assistant should be careful not to interfere with the navigator’s night vision. If an assistant is not available to time the observations, the observer holds the watch in the palm of his left hand, leaving his fingers free to turn the tangent screw of the sextant. After making the observation, note the time as quick as possible. The delay between completing the altitude observation and noting the time should not be more than one or two seconds.