Apsis, plural apsides, the line joining the aphelion and perihelion points; or the major axis of elliptical orbits.
Arc, a part of a circle.
Ascension, right, the angular distance of a heavenly body from the first point of Aries, measured on the equator.
Asteroids (star-like), [162]; orbits of interlaced, [74].
Astronomical instruments, [43].
Astronomy, use of, [57].
Atom, size of, [255]; power of, [256].
Aurora Borealis, [143].
Axis, the line about which a body rotates.
Azimuth, the angular distance of any point or body in the horizon from the north or south points.