| A | B |
|---|
| retrograde motion | The apparent westward drift of the planets with respect to the stars. |
| sidereal day | The time interval of 23 hours, 56 minutes and 4 seconds, it takes for the earth to make one complete rotation (360o) with respect to a star other than the Sun |
| heliocentric | Sun-centered universe |
| sidereal month | The true period of the Moon’s revolution around Earth, 27 1/3 days. |
| geocentric | Earth-centered universe |
| rotation | The turning, or spinning, of a body on its axis. |
| astronomical unit (AU) | The distance unit that is the average distance from Earth to the Sun, about 150 million kilometers (93 million miles). |
| solar eclipse | A dark shadow cast on the Earth when the Moon moves in a line directly between Earth and the Sun. (an eclipse of the sun) |
| declination | The angular distance north or south of the celestial equator denoting the position of a stellar body. |
| 10. perturbation | An variance in the orbit of a body from its predicted path |
| perihelion | The point in the orbit of a planet where it is closest to the sun. |
| ecliptic | The apparent annual path of the sun against the backdrop of the celestial sphere. |
| celestial sphere | An imaginary, transparent, hollow sphere on which the ancient astronomers believed the stars traveled daily around Earth. |
| constellation | An apparent group of stars originally named in honor of a mythological character of great hero. |
| plane of the ecliptic | The imaginary plane that connects Earth’s orbit with the celestial sphere. |
| Ptolemaic system | The early Earth-centered model of the universe. |
| revolution | The motion of a body about another, as Earth about the Sun. |
| phases of the Moon | The progression of changes in the Moon’s appearance during the month. |
| right ascension | The angular distance of a stellar object measured eastward along the celestial equator from the position of the vernal equinox. |
| synodic month | The cycle of the Moon through its phases which requires 29 ½ days. |
| lunar eclipse | A dark shadow cast on the Moon when the Earth moves in a line directly between the Moon and the Sun. ( an eclipse of the moon) |
| equatorial system | A method used to locate stellar objects that is very similar to the latitude-longitude system used on Earth’s surface. |
| aphelion | The point in the orbit of a planet where it is farthest from the Sun. |
| precession | The very slight movement of Earth’s axis over a period of 26,000 years. |
| mean solar day | The time interval from one noon to the next averages about 24 hours. |
| eclipse | The cutting off of the light of one celestial body by another passing in front of it. |
