First off, I apologize if I am breaking any site-etiquitte. Secondly, this is probably extremely elementary.

For a lab, I was given the question:

When we are sitting in a planetarium, where are we located on(or in) the "celestial sphere"?

To me, this seems like a silly question - because the answer is really dependent on where the operator places you. Is this assumption wrong? Is there a particular place in the celestial sphere that you would be?

I also thought that it might just be the very center of the celestial sphere.

  • $\begingroup$ many planetaria have seats surrounding an orrery, and if they don't they may place the projector's supposed location anywhere for which the stars and astral bodies are mapped. A planetarium may actually show you the film "powers of 10" in which case you would be travelling at increasing speeds away from earth, eventually faster than the speed of light. $\endgroup$ – DeltaEnfieldWaid Dec 22 '16 at 5:54

Well, at least you did some thinking and proposed a couple of personal thoughts instead of just asking for the answer to a homework question. Both observations you make are pertinent.

Remember, a celestial sphere doesn't actually exist. It is an imaginary concept, a simplification based upon our perspective of the universe.

A planetarium, whether it is an old-fashioned one that depicts an earth-bound perspective of the universe, or a modern digital projection system that can "fly" you through space to view the universe from a different place, is projecting a perspective of the view of the universe from a particular place, that is, you see the "celestial sphere" as it would appear from that place. In the case of a view from elsewhere in space, it would be a different celestial sphere from our celestial sphere.

So yes, you would be at "the centre" of the celestial sphere. Maybe not the "very" centre in the case of old fashioned projectors with fixed star fields, as they wouldn't account for the 150 million kilometre imprecision caused by the movement of the earth around the sun. But, as this is an insignificant distance in astronomical terms, that doesn't matter a great deal, so "the centre" is probably a sufficient description.

  • $\begingroup$ A well programmed planetarium might show the moon in two slightly different positions wrt the "fixed" stars as viewed from two widely spaced spots on earth... $\endgroup$ – DJohnM Feb 20 '14 at 0:49
  • $\begingroup$ @User58220 Yes - the moon isn't on the celestial sphere. You're still at the centre of the view of the celestial sphere. A fantastically well programmed planetarium might show you the different star field from two different positions on the opposite sides of the planet, but I doubt you'd see the difference. As I say, I'm not sure how much of a difference there would be to see from two viewing points at opposite ends of the long axis of our orbit around the sun. $\endgroup$ – Jeremy Feb 27 '14 at 1:37
  • $\begingroup$ The Earth, as seen from the moon, is close to 2 degrees across. This, in turn, means that for two Earth-bound observers located at the antipodes, the moon would be seen shifted the same 2 degrees against the fixed stars. That's four lunar diameters... $\endgroup$ – DJohnM Feb 27 '14 at 7:14
  • $\begingroup$ @User58220 Yes - the moon isn't on the celestial sphere. $\endgroup$ – Jeremy Mar 2 '14 at 23:00
  • $\begingroup$ Numerous Google search results for "moon on celestial sphere" would suggest a different opinion... $\endgroup$ – DJohnM Mar 2 '14 at 23:54

If we follow the Wikipedia definition of "celestial sphere", "the celestial sphere can be considered to be infinite in radius. This means any point within it, including that occupied by the observer, can be considered the center."

Hence the question could be answered, as you presume, by in the center of the celestial sphere.

Although, of course, if we aren't located in the center of the planetarium, the celestial sphere will look distorted to us.


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