What these corrections do
A sextant measures the angle between a heavenly body and the visible horizon. That raw angle is not the true altitude. Index error is the instrument’s own zero error. Dip is the angle from the observer’s eye down to the visible horizon and depends on height of eye. Refraction bends the light so the body appears higher than it is. For the Sun, semi-diameter is also applied depending on whether the upper or lower limb was observed.
How this calculator works
Index error is added if off the arc and subtracted if on the arc. Dip in minutes is taken as 1.76 × square root of height of eye in metres. Refraction in minutes is approximated as 1 / tan(altitude). The result is a training estimate. For an official sight reduction use the Nautical Almanac correction tables, which also include parallax and semi-diameter.
Who should use it
Useful for cadets and officers practising celestial navigation. Do not use this approximate refraction for a legal or examination sight without checking the official tables.