I thank Fred Espenak, Mark Littman, and Ken Willcox for the glossary they prepared for their book Totality: Eclipses of the Sun (Oxford University Press), for assistance in defining some of the terms related to solar eclipses.

Annular eclipse of the Sun. An eclipse of the Sun where the Moon's angular diameter is too small to cover the entire disk of the Sun. Thus, at the eclipse's central phase, a thin ring, or annulus, of sunlight surrounds the darkened Moon. As the Moon recedes from the Earth, more eclipses will be annular than total and, after about 600 million years, all central eclipses will be annular.

Annular-total eclipse of the Sun. If the Moon is very close to the diameter of the Sun, an eclipse that begins as annular at sunrise will turn to total when the shadow hits the full bulk of the Earth. See also hybrid eclipse.

Anomalistic month. The amount of time for the Moon to orbit the Earth, from one perigee (closest Moon–Earth distance) to the next. The amount of time is 27.55 days.

Anomalistic year. The amount time for Earth to travel from one perihelion (closest point to the Sun) to the next. That amount of time is 365.26 days.

Aphelion. When an object orbiting the Sun has reached its farthest point from the Sun, that point is called the aphelion.

Apogee. When an object orbiting the Earth has reached its farthest point from the Earth, that point is called the apogee.