Frequently Used Astronomical Terms
galaxy: a vast collection of stars, gas, and dust, typically 10,000 to 100,000 light-years in diameter and containing billions of stars.
light pollution: a glow in the night sky or around an observing site caused by artificial light.
light-year: the distance that light (moving at 186,000 miles per second) travels in one year. About 6 trillion miles.
limb: the edge of a celestial object's visible disk.
magnitude: the brightness of a star or other celestial object. The higher the magnitude, the fainter the object. A 1st-magnitude star is 100 times brighter than a 6th-magnitude star.
meridian: the imaginary north-south line that passes directly overhead (through the zenith).
Messier object: an entry in a catalog of star clusters, nebulas, and galaxies that was compiled by French comet hunter Charles Messier (pronounced mess-YAY) between 1758 and 1782.
Milky Way: a broad, faintly glowing band stretching across the night sky, composed of billions of stars in our galaxy too faint to be seen individually. It's invisible when the sky it lit up by artificial light or bright moonlight.
nebula: 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.
objective: a telescope's main light-gathering lens or mirror.
occultation: when the Moon or a planet passes directly in front of a more distant planet or star. A grazing occultation occurs if the background body is never completely hidden from the observer.
opposition: 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.