MAA Glossary


The apparent change in position of a star caused by the annual motion of the earth.

The point in the orbit of the moon or artificial satellite nearest the earth.

The point on an orbit nearest the sun.

Large spherical object shining by a star. Our planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto (+Charon ?)

Position angle:
For a binary star, the position angle isthe angle (measured counterclockwise) at the brighter star between a linedrawn due north to a linedrawn to the fainter star.

Precession of the equinoxes:
The axis of the earth's rotation slowly changes its direction, maintaining a constant tilt with respect to the ecliptic and making a complete rotation once every 26000 years. In consequence, the equinoxes on the intersections of the celestial equator and the ecliptic slowly drift westward.

Prime vertical:
A great circle on the celestial sphere from the east to the west and passing through the zenith.

Cloud of gas elevated above the solar surface.

Proper motion:
The apparent drift of a star across the sky, compounded from the motion of the star and the sun.