ASTRONOMY TERMS

2. The time after sunset or before sunrise when the sky is not fully dark. Astronomical twilight ends after sunset (and begins before sunrise) when the Sun is 18° below the horizon.
6. Greek for “circle of animals.” It’s the set of constellations situated along the ecliptic in the sky, through which the Sun, Moon, and planets move.
8. The moment when a celestial object crosses the meridian and is thus at its highest above the horizon.
11. A group of people who get together to view the night sky. Astronomy clubs often hold star parties to introduce stargazing to the public.
13. When a planet or asteroid is opposite the Sun in the sky. At such times the object is visible all night — rising at sunset and setting at sunrise.
14. The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).
17. The edge of a celestial object’s visible disk.
18. A telescope’s main light-gathering lens or mirror.
19. A supermassive black hole gorging on gas at the center of a distant galaxy.
20. A measure of the atmosphere’s stability. Poor seeing makes objects waver or blur when viewed in a telescope at high magnification. The best seeing often occurs on hazy nights, when the sky’s transparency is poor.

1. A star whose brightness changes over the course of days, weeks, months, or years.
3. The angle between the plane of an orbit and a reference plane. For example, NASA satellites typically have orbits inclined 28° to Earth’s equator.
4. The point in the sky that’s directly overhead.
5. The measure of how much an orbit deviates from being circular.
7. Sunlight reflected by Earth that makes the otherwise dark part of the Moon glow faintly. It’s especially obvious during the Moon’s thin crescent phases.
9. Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.
10. The two times each year, around June 20th and December 21st, when the Sun is farthest north or south in the sky. At the summer solstice, the day is longest and the night is shortest, and vice versa at the winter solstice.
12. The fraction of the Moon or other body that we see illuminated by sunlight.
15. Latin for “cloud.” Bright nebulas are great clouds of glowing gas, lit up by stars inside or nearby. Dark nebulas are not lit up and are visible only because they block the light of stars behind them.
16. The imaginary north-south line that passes directly overhead (through the zenith).