I was reading about the retrograde motion of Mars. All explanations give reference to the stars in the background. But I do not understand why this is necessary. I say this since stars themselves also appear to move and can see and observe this without any references. This brings up the second point: if the stars also appear to move, why do explanations have these background stars frozen?

I am new to all this so this might be a naive question :)


Go outside.

Pick a star, any star and watch it for a while. It moves. As the Earth spins all the stars move across the sky, taking 23hr 56 min to move all around the sky to the same place. This is due to the rotation of the Earth (less than 24 hours due to the orbit of the Earth). We sometimes say the "celestial sphere" appears to rotate around the Earth, due to the Earth's rotation.

The planets also move across the sky every day, taking just less than 24 hours to all around the sky.

But the planets also move relative to the celestial sphere, and since th stars don't move relative to the celestial sphere we can say that the planets move relative to the stars. It is this motion that is of interest, since it is not due to the rotation of the Earth, but to the actual motion of the planet relative to the Earth. This motion is much slower than the apparent motion of the celestial sphere; Jupiter takes 12 years to make a full journey around the sky. But this motion is special, complex and interesting. It is this motion that earns planets the name "wandering stars" in ancient Greek.

The stars are at a very great distance, so to all intents and purposes they are "fixed". Relative to each other, the stars hardly move at all. You would need to wait centuries before you noticed any difference. So the background of fixed stars makes a very convenient way to refer to the motion of the planets

  • $\begingroup$ Yes. But what I don't understand is why the "relative to stars" is important. Even if one could not see the background stars, can one still not observe the planets move? (I view from the same lawn chair fixed in the ground every night. ) And about the last paragraph, so it is that the stars appear to move much slower than the planet ? $\endgroup$ – Sal_99 Jul 3 '20 at 22:27
  • $\begingroup$ Movenent needs a reference frame. The stars provide a very convenient frame of rest. $\endgroup$ – planetmaker Jul 3 '20 at 22:44
  • $\begingroup$ @sal_99 From your lawn chair, all the celestial bodies appear to rotate around the sky, due to the rotation of the Earth. The stars essentially rotate as a unit, as if they were stuck on a ball, the celestial sphere. So it's pretty easy to predict the location in the sky of a star, given your latitude & longitude, and the date & time. But the Sun, Moon, and planets move relative to the fixed stars on the celestial sphere. So to predict their positions we need to know how they move on the celestial sphere. $\endgroup$ – PM 2Ring Jul 4 '20 at 5:09
  • $\begingroup$ Maybe I'm missing something. What I see is : the planets and the stars all complete a cycle around the sky in a full day due to the rotation of the Earth. But over the year, the stars and the planets also move (if you view day by day at the same time from the same location ) . This is due to Earth's orbit. Now we can tell that the stars take this motion westwards ( like we can Orion go west over a year ). Now even if the stary background isnt visable, would I not see the planets move over the year? $\endgroup$ – Sal_99 Jul 4 '20 at 8:42
  • $\begingroup$ And since the stars also move westward over the year, why do we claim fixed stars ? Is their westward motion just much slower than that of the planets ? $\endgroup$ – Sal_99 Jul 4 '20 at 8:44

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