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absolute magnitudes actually angle apparent magnitudes appear astronomer atmo atmosphere atoms average density axis B-type binary star bright star brighter calculated called celestial Cepheids CHAPTER clouds colour comet comparatively component considerable constellation craters dark deduced diameter direction discovery distance double stars earth earth's surface eclipse galactic centre galactic system galaxy globe globular clusters gravitation heavens height hemisphere Herschel hundred Jupiter Kepler's known light light-changes light-curve light-years lines lunar Mars Martian measured Mercury meteors miles per second Milky million miles minor planets moon moon's moving Neptune observations Observatory open clusters orbital periods photographic plate planetary Pluto position proper motions radial velocities radiation revolve ring rotation satellites Saturn scale seen shown similar Sirius solar spectrum solar system space spectra spectroscopic speed spiral nebulae spots star's stellar stellar distances sun-spot sun's mass sunlight supposed telescope temperature terrestrial tions Uranus variable star Venus violet visible wave-length