The Celestial Sphere
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  The Celestial Sphere

  • What is it? An imaginary sphere surrounding the Earth on which celestial objects appear to be located; thought to really exist in antiquity, today used as tool to identify locations in the sky.
  • Special places: Celestial Equator, North and South Celestial Poles, the Ecliptic; Solstices and equinoxes. At any location on Earth, the altitude of the Celestial Pole is the same as the latitude of that location.

  Locating Objects

  • Apparent daily motion: As the Earth rotates, the whole sky "moves backwards". As a consequence, most objects, except for Polaris (the North Star – What is the altitude of Polaris?) and other circumpolar ones, rise in the East and set in the West every day.
  • What can we do? Use coordinates on the celestial sphere, as opposed to altitude and azimuth; right ascension RA ("longitude" around the equator from the Spring equinox, in hours, minutes, seconds) and declination ("latitude" above the equator, in degrees).

Motions on the Celestial Sphere

  • Proper motion: Subtract the effect of Earth's daily rotation and yearly orbit, to find actual displacements on the celestial sphere.
  • The Sun and the Moon: They always move Eastward, the Sun along the ecliptic, the Moon on a faster orbit that produces different "phases".
  • Planet: They move in more complicated, slow paths near the ecliptic, usually West to East but sometimes with an apparent retrograde motion (East to West); Occasionally they are in conjunction, in opposition, or they transit the Sun.
  • Other objects: Small "nearby" objects (comets, asteroids, or even closer ones) move much faster, stars and more distant objects not at all, as seen with the naked eye.
  • Telescope view: Nearby stars show a slight back and forth motion every six months (parallax), or move in other ways over periods of years. (Forget completely about seeing galaxies move, although most of them are moving at millions of mph, at least!)
  • What have humans made of all this? They identified constellations and assigned meanings to their patterns; Tried to interpret or understand the motions of planets; Concluded that the Earth doesn't move, from evidence including the apparent lack of stellar parallax...

page by luca bombelli <bombelli at>, modified 29 sep 2012