Measuring Altitude and Correcting the Measurement.
Measuring the Altitude. To measure the altitude of a celestial body, we use a sextant. As shown in the following diagram the horizon is viewed directly through the sextant telescope and the celestial body is viewed via two mirrors. The upper mirror is attached to the index bar. The index bar is moved until it reflects an image of the celestial body into the lower mirror which is fixed. The position of the index bar is finely adjusted until the image of the celestial body appears to sit on the horizon.
As the index bar is adjusted, it moves a pointer over a graduated scale and when the images are made to coincide, the angle indicated by the pointer is the altitude.
The altitude measured by a sextant is referred to as the Sextant Altitude.
Index Error. No matter how carefully a sextant is manufactured, there will usually be a very small error in its reading and this is known as index error and has to be corrected.
Other Corrections.
A number of other corrections have to be made to the sextant altitude before we arrive at the True Altitude. These include corrections for dip (height of eye), refraction, semi-diameter, parallax, horizontal parallax, temperature and atmospheric pressure. Details of these corrections can be found in my book Astro Navigation Demystified.
Links: Azimuth and Altitude Part 1 Azimuth and Altitude Part 2
Astro Navigation Demystified 
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