| Term | Definition |
|---|
| Apogee | highest point; the point farthest from the earth; OP. perigee |
| Perigee | nearest point to the earth in the orbit of a satellite |
| Winter solstice | begins on Dec. 21st in northern hemisphere; northern hemisphere is tilted away from the sun, so it's the coldest time of the year |
| Summer solstice | : On June 21, it is the longest day in the Northern Hemisphere due to the fact that the sun is directly over the tropic of Cancer |
| Waning crescent | The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing. |
| Waning gibbous | phase of the moon between full and one-half when the lighted area of the moon appears to grow smaller |
| Waxing crescent | The Moon appears to be partely but lesss taht one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing. |
| Waxing gibbous | "The moon is moving toward the full moon phase, and presently more than a quarter of it is visible on Earth." |
| Vernal equinox | the day of the year that marks the beginning of spring in the northern hemisphere |
| Sidereal year | the time for the earth to make one complete revolution around the sun, relative to the fixed stars |
| Sidereal month | amount of time taken for the moon to return to the same position in the sky, relative to the stars |
| sidereal day | The time it takes Earth to rotate once relative to the stars or 23 hours 56 minutes and 4 seconds, which is 4 minutes less than a solar day, since during one sidereal day the Sun also moves 1° east along the ecliptic, and Earth has to rotate 4 additional minutes to complete one rotation relative to the Sun in one 24-hour solar day. |
| Satellite | A body that revolves around a larger body. For example, the Moon is a satellite of Earth. |
| North celestial pole | The sky's North Pole. The point in the sky directly above Earth's North Pole. |
| New moon | The Moon as it appears on Earth when it is positioned between Earth and the Sun at the beginning of a cycle of lunar phases. The new moon rises and sets with the Sun. |
| Midnight sun | The Sun when visible at midnight, which occurs only in the summer north of the Arctic Circle and south of the Antarctic Circle. |
| Lunar eclipse | An eclipse of the Moon caused when the Moon moves partially or wholly into the shadow of Earth and grows dark for up to a few hours. A lunar eclipse can be seen by everyone on the side of Earth facing the Moon. |
| Full moon | The Moon's appearance from Earth when it is positioned directly opposite the Sun. The Moon is full two weeks after new moon. The full moon rises at sunset and sets at sunrise when Earth is between the Moon and the Sun. |
| Astronomy | The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena. |
| Astronomical Unit | The average distance from the Earth to the Sun, or 149,597,870 kilometers (92,955,807 miles). |
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