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5th magnitude Al-Sufi Algol ancient apparent diameter Arabic Arcturus astronomers Astrophysical Journal binary star bright star brighter called centre century B.C. components constellations Cygni dark body density disc distance of Sirius earth eccentricity eclipses equal estimated faint stars fainter gaseous globular clusters Greek Harvard heavens Herschel thought Hipparchus Houzeau hypothesis Iapetus Jupiter known larger cloud Lick Observatory light luminosity magni magnitude by Ptolemy measures Messier Milky millions of miles modern moon moon's naked eye nebulosity nubecula number of stars objects observations Observatory Ophiuchi orbit Orion parallax period photographs Pickering planetary planets probably Procyon Professor proper motion Ptolemy reference regions remarkable ring rotation round satellite Saturn seems sidereal Sir John Herschel Sir William Herschel Sirian type Sirius small stars smaller solar system space spectroscopic binary spectrum spheres spots stellar stellar magnitude sun's mass supposed telescope theory tion tude Tycho Brahe Uranus variable stars velocity Yerkes Observatory