Astronomy dictionary – Z Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science. Click on one of the letters below to search for a term. a b c d e f g h i j k l m n o p q r s t u v w x y z Zz Zenith The point on the celestial sphere that is directly above an observer. Zenithal hourly rate (ZHR) The number of meteors during a meteor shower that an observer at a dark site would see during the course of an hour with the radiant overhead. Zodiac Constellations that the Sun, Moon and major planets appear to pass through as the Earth moves along its orbit.