Frequently Used Astronomical Terms
terminator: the line on the Moon or a planet that divides the bright, sunlit part from the part in shadow. Its usually the most exciting and detailed region of the Moon to view through a telescope.
transparency: a measure of the atmosphere's clarity, how dark the sky is at night and how blue it is during the day.
twilight: the time after sunset or before sunrise when the sky is not fully dark.
Universal Time: Greenwich Mean Time, expressed in the 24-hour system. For example, 23:00 UT is 6 pm Eastern Standard Time or 7 pm Eastern Daylight Time. Astronomers use Universal Time to describe when celestial events happen in a way that is independent of an observer's time zone.
variable star: a star whose brightness changes over the course of days, weeks, months, or years.
zenith: the point in the sky that is directly overhead.
zodiac: Greek for "circle of animals." The set of constellations situated along the ecliptic in the sky, through which the Sun, Moon, and planets move.