aphelion the point in the orbit of a heavenly body where it is farthest from the sun. See also perihelion. apocynthion apolune. apogee the farthest point in an orbit from the body being orbited. apolune the farthest point from the moon in a lunar orbit, as that of a spacecraft. Also called apocynthion. areography the study of the physical features of the planet Mars. celidography Archaic. a description of the surface markings of the sun or a planet. —celidographer, n. exobiology the branch of biology that studies life beyond the earth’s atmosphere, as on other planets. —exobiologist, n. meridian the highest point a planet or other orbiting heavenly body reaches in its orbit. —meridian, meridional, adj. occultation the process of one heavenly body disappearing behind another as viewed by an observer. pericynthion perilune. perigee the closest point in an orbit to the body being orbited. perihelion the point in the orbit of a heavenly body where it is nearest the sun. Also called perihelium. See also aphelion. perihelium perihelion. perilune the closest point to the moon in a lunar orbit, as that of a spacecraft. Also called pericynthion. planetarium 1. a representation of the planetary system, particularly one in which the movements of the planets are simulated by projectors. 2. a room or building housing such an apparatus. planetoid Astronomy. any of thousands of small celestial bodies that revolve about the sun in orbits chiefly between those of Mars and Jupiter ranging in diameter from one mile to 480 miles. Also called asteroids, minor planets. —planetoidal, adj. zenography the study and description of Jupiter. —zenographical, adj.
- plural form of planet