Aberration (ab-er-ā´shun). A small apparent motion of the fixed stars, occasioned by the progressive motion of light and the earth’s annual motion in its orbit. By this they sometimes appear twenty seconds distant from their true situation.
Amplitude (am´pli-tud). An arc of the horizon intercepted between the true east and west points and the center of the sun, or a star at its rising or setting.
Anomaly (an-om´al-i). The angular distance of a planet from its perihelion, as seen from the sun; either true, mean, or eccentric.
Aphelion (af-ēl´yun). That point of a planet’s orbit which is most distant from the sun.
Apogee (ap´o-jē). That point in the orbit of the moon which is at the greatest distance from the earth.
Apparition (ap-par-ish´un). The first appearance of a star or other luminary after having been obscured.
Ap´pulse. The approach of a planet towards a conjunction with the sun or any of the fixed stars.
Apsis (ap´sis). The two points of a planet’s orbit in which it is at its greatest and least distance from the sun.
Aquarius (a-kwā´ri-us). The eleventh sign of the zodiac, which the sun enters about the 21st of January.