| A | B |
|---|
| aphelion | the place in the orbit of a planet where the planet is farthest from the sun |
| astronomical unit | average distance from earth to the sun 1.5 x 10^8 km or 93 x 10^6 miles |
| celestial sphere | an imaginary hollow sphere upon which the ancients believed the stars were hung and carried around earth |
| constellations | an apparent group of stars originally named for mythical characters. the sky is presently divided into 88 constellations |
| declination | the angular distance north or south of the celestial equartor denoting the position of a celestial body |
| ecliptic | the cutting off of the light of one celestial body by another passing in front of it |
| equatorial system | a method of locating stellar objects much like the co-ordinate system used on earth's surface |
| geocentric | the concept of an earth-centered universe |
| heliocentric | the view that the sun is at the center of the solar system |
| lunar eclipse | an eclipse of the moon |
| mean solar day | the average time between two passages of the sun across the local celestial meridian |
| perihelion | the point in the orbit of a planet where it is closest to the sun |
| perturbation | the gravitational disturbance of the orbit of one celestial body by another |
| phases of the Moon | the progression of changes in the moon's appearance during the month |
| plane of the ecliptic | the imaginary plane that connects earth's orbit with the celestial sphere |
| precession | a slow motion of earth's axis that traces out a cone a period of 26,000 years |
| Ptolemaic system | an earth-centered system of the universe |
| retrograde motion | the apparent westward motion of the planets with respect to the stars |
| revolution | the motion of one body about another, as earth about the sun |
| right ascension | an angular ditance measured eastward along the celestial equator from the vernal equinox. Used with declination in a coordinate system to describe the position of celestial bodies |
| rotation | the spinning of a body, such as earth, about its axis |
| sidereal day | the period of earth's rotation with respect to the stars |
| sidereal month | a time period based on the revolution of the moon around earth with respect to the stars |
| solar eclipse | an eclipse of the sun |
| synodic month | the period of revolution of the moon with respect to the sun, or its cycle of phases |
