MAA Glossary

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

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

Perihelion:
The point on an orbit nearest the sun.

Planet:
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.

Prominence:
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.