Online Astronomy eText: Orbital Motions
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"Eppur si muove" ("And yet it does move") -- Galileo (apocryphal)

      Not that long ago our world was thought to be unique. Heavenly bodies -- the Sun, Moon, stars and planets -- were thought to be completely different from anything in our world, which sat at the center of the Universe, fixed and unmoving, in solitary splendor.
      We still believe that our world lies at the center of what we call “the observable Universe”, but we no longer believe that it is fixed or even necessarily unique. Instead, we think that there must be innumerable worlds, many of which support creatures who like us, look into the heavens and wonder whether they are alone.
      The difference between these two world-views is immense. On the one hand our world is all there is, immeasurably important yet utterly alone. Alternatively, it is merely one of a nearly infinite community of worlds, and of no more or less importance to that community than one person is to the population of our world. Either view can fill us with wonder and perhaps dismay; and for those who lived when the old world order was cast down and a new one raised on its ruins, wonder and dismay were frequent companions.
      What was it that cast down the old order? A simple question, really -- does the Earth move, or not? For if the Earth is, like other planets, in motion around the Sun, then they must be worlds of some sort as well; and if our world is no longer unique, then perhaps we are not the object of some special dispensation.
      It is for that reason that Galileo's scientific discoveries and those of his contemporaries changed our Universe, making the Earth a new planet, and the planets new worlds.