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altitude amount angle angular distance apparent astronomers average axis binary binary stars bodies brightness celestial equator celestial pole celestial sphere centimeter circle comet Consequently constellation craters density determined diameter direction diurnal diurnal motions earth earth's atmosphere eccentricity equal equatorial equinox eyepiece fact faint follows galaxy gases gravity greater horizon interval Jupiter known latitude length light light-years luminosity lunar eclipse magnitude Mars mass mean solar measured Mercury meteorites meteors miles molecules month moon Mount Wilson Observatory move nearly nebulae object oblate observed orbit parallax passes perihelion period of rotation phenomena photograph planetary planetoids planets polar position probably radiant energy radiation rays refraction relatively respect revolution revolve right ascension satellites Saturn sidereal solar eclipses solar system spectral classes spectral lines spectroscope spectrum sun spots sun's Suppose surface synodic synodic month telescope temperature theory tides tion Uranus velocity Venus visible wave-lengths Yerkes Observatory