The sextant is subject to a number of errors and adjustments. To find the true altitude of a celestial body from the observed these must be allowed and adjusted for.
These are:
1. Index Error
2. Dip
3. Refraction
4. Parallax
5. Semi-diameter
Index error is an instrumental error. When looking through a sextant at the horizon the exact level will seldom be seen to be at 0°.
horizon split.
Before you use the sextant the Index error should be determined.
If the error is less than 0° it should be added to whatever reading is obtained - if more subtracted.
Tip:
1. if its off, its on - add.
2. if its on, its off subtract.
Dip is an adjustment made for the height of the eye above sea level. In practice this is usually taken as 0.98 times the square root of the height of the eye in metres above sea level multiplied by 3.28.
Refraction is extracted from the Nautical Almanac. It allows for the bending of light rays as they travel through layers of varying density air.
Parallax corrections are needed if the observed body is a planet, the sun or the moon. From the Almanac.
Semi-diameter correction is needed if the observed body is the sun or the moon. In this case either the top or bottom of the celestial object (upper or lower limb) is made to touch the horizon. To obtain the center of the body this correction is applied, from the Almanac.
Once all the corrections are applied we have the true altitude. And this subtracted from 90 gives us the zenith distance. Which means we know exactly how far we are from that point on the earth which is at right angles to our observed celestial body.
