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retrograde motion
(Encyclopedia)retrograde motion, in astronomy, real or apparent movement of a planet, dwarf planet, moon, asteroid, or comet from east to west relative to the fixed stars. The most common direction of motion in the...light-year
(Encyclopedia)light-year, in astronomy, unit of length equal to the distance light travels in one sidereal year. It is 9.461 × 1012 km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars n...Oort, Jan Hendrik
(Encyclopedia)Oort, Jan Hendrik ōrt [key], 1900–1992, Dutch astronomer. He confirmed (1927) Bertil Lindblad's theory of the Milky Way galaxy's rotation. In the 1950s he and his colleagues used radio astronomical...Bode's law
(Encyclopedia)Bode's law [for J. E. Bode], also known as Titius's law or the Titius-Bode law, empirical relationship between the mean distances of the planets from the sun. If each number in the series 0, 3, 6, 12,...solar time
(Encyclopedia)solar time, time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial. When the cente...astrophysics
(Encyclopedia)astrophysics, application of the theories and methods of physics to the study of stellar structure, stellar evolution, the origin of the solar system, and related problems of cosmology. The distinctio...syzygy
(Encyclopedia)syzygy sĭzˈəjē [key], in astronomy, alignment of three bodies of the solar system along a straight or nearly straight line. A planet is in syzygy with the earth and sun when it is in opposition or...Scholz's star
(Encyclopedia)Scholz's star, dim binary star system, consisting of a red dwarf and brown dwarf, in the constellation Monoceros, apparent magnitude 18.3. The red dwarf is a tiny star, with less than ten percent the ...Mercury, in astronomy
(Encyclopedia)Mercury, in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon a...exobiology
(Encyclopedia)exobiology or astrobiology, search for extraterrestrial life within the solar system and throughout the universe. Philosophical speculation that there might be other worlds similar to ours dates back ...Browse by Subject
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