The refraction for 0° is included in the above table in a situation where you have an accurate watch, tables and an almanac, but no sextant.
A horizon sight is when a celestial body just touches the horizon. If you use the sun you take the sight when either the upper or lower limb reaches the horizon. A
horizon sight has an apparent altitude of 0° and is worked out just like any other. Be warned. Horizon sights are famous for their inaccuracy, mostly because of the problems of allowing for refraction below altitudes of 5°.
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