Frequently Used Astronomical Terms
The terminology used by astronomers can be somewhat confusing at first, but this excellent guide from our friends at Night Sky magazine will help clear things right up for you. Enjoy!
angular size and distance: the apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm's length spans about 1 degree, and your fist spans about 10 degrees.
aperture: the diameter of a telescope's main lens or mirror, and the scope's most important attribute.
asterism: any prominent star pattern that isn't a whole constellation (such as the Big Dipper).
astronomical unit: the average distance from Earth to the Sun, slightly less than 93 million miles.
averted vision: viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.
black hole: a concentration of mass so dense that nothing, not even light, can escape its gravitational pull once swallowed up. Many galaxies, including the Milky Way, have super-massive black holes at their centers.
celestial coordinates: a grid system for locating things in the sky. It's anchored to the celestial poles and the celestial equator (directly above Earth's equator). Declination and right ascension are the celestial equivalents of latitude and longitude.
circumpolar: denotes an object near a celestial pole that never dips below the horizon as Earth rotates.
compound telescope: a telescope with a mirror in the back and a lens in the front.