| A | B |
|---|
| astronomy | The study of the moon, stars, and the objects in space. |
| axis | An imaginary line that passes through Earth's center and the North and South poles, about which Earth rotates. |
| rotation | The spinning motion of a planet on its axis. |
| revolution | The movement of an object around another object. |
| orbit | The path of an object as it revolves around another object in space. |
| solstice | The two days of the year on which the noon sun is directly overhead at either 23.5° South or 23.5° North. |
| equinox | The two days of the year on which neither hemisphere is tilted toward or away from the sun. |
| vernal equinox | The day of the year that marks the beginning of spring in the Northern Hemisphere. |
| autumnal equinox | The day of the year that marks the beginning of fall in the Northern Hemisphere. |
| gravity | The force of attraction between earth and objects on it. |
| satellite | Any object that revolves around another object in space. |
| universal gravitation | The pull or force of attraction between universal bodies. |
| oblate spheroid | a slightly flattened sphere (shape of the earth) |
| inclination | Tilt of the earth on its axis |
| ellipse | The shape of the Earth's orbit around the Sun. An elongated, or stretched out, circle. |
| Circle of Illumination | The imaginary line on the surface of the earth that separates day and night |
| perihelion | The point in Earth's orbit when the earth is closest to the sun. |
| aphelion | The point in earth's orbit when the earth is farthest from the sun. |
| precession | the wobble of earth on its axis |
| winter solstice | December 21st, when the the northern hemisphere is tilted away from the sun. |
| summer solstice | June 21st, when the northern hemisphere is tilted toward the sun. |
| autumnal equinox | September 21st, when the northern hemisphere is neither tilted toward nor away from the sun |
| vernal equinox | March 21st, when the northern hemisphere is neither tilted toward nor away from the sun |
