Definition
Dip removes the depression of the visible horizon and is about 1.76 times the square root of height of eye in metres, in minutes of arc. Refraction, always subtractive, is around 1 minute at 45 degrees altitude but exceeds 30 minutes at the horizon. Semi-diameter, roughly 16 minutes, adjusts a sun or moon limb to its centre. Parallax in altitude matters chiefly for the moon, where it can reach nearly a degree, and slightly for Venus and Mars.
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