Augmentation (Moon)

Augmentation is the increase in the Moon’s apparent semidiameter as its altitude rises, reaching its maximum near the zenith. The cause is geometric: an observer on the Earth’s surface stands up to one Earth radius nearer the Moon when it is overhead than when it sits on the horizon, so the same disc subtends a larger angle. The effect is small, about 0.3 arcminutes at the zenith, and the Nautical Almanac semidiameter tables build it into the Moon altitude corrections. The navigator applies it when reducing a Moon sight to the limb, since semidiameter is added or subtracted to reach the center.