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Was it when heliocentric model was spreading in the whole world? I saw a model of the solar system that was made in the 18th century where the sun was in the middle and the planets and comets around it and the end it said: "firmament of fixed stars". Wasn't at that time accepted that stars were suns or atleast not attached to any firmament and fixed?

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    $\begingroup$ There are some similar questions to this that have already been answered. There isn't a single answer. Sometime between 500 BC and Anaxgoras' suggestion that stars are "firey stones" and Bessel's measurement of parallax in 1838. $\endgroup$
    – James K
    Sep 23, 2023 at 6:41
  • $\begingroup$ @JamesK, how does parallax work? Please keep it simple as possible. Not exactly gifted in the brains department. $\endgroup$ Sep 23, 2023 at 7:28
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    $\begingroup$ @AgentSmith: It’s the angle difference between two points of view. A common example is to look at your finger raised some distance from your eyes, while closing one eye. If you note the apparent position of your finger relatively to distant objects, then close the open eye and open the other one, you’ll notice your finger seems to have “moved” with respect to the background. The principle is the same for stellar parallax, except we use the Earth’s positions six months apart instead of the eyes, and the closer star replaces the finger while more distant stars are the background. $\endgroup$ Sep 23, 2023 at 8:06
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    $\begingroup$ @AgentSmith move your head from side to side. Notice how nearby objects appear to change position relative to distant objects. You can do the same with stars, only "move your head" is move the earth around the sun. Nearby stars will appear to move relative to distant ones, and by measuring how much they appear to move you can decide how far away they are. $\endgroup$
    – James K
    Sep 23, 2023 at 11:24
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    $\begingroup$ see astronomy.stackexchange.com/questions/40499/… or astronomy.stackexchange.com/questions/23516/… and astronomy.stackexchange.com/questions/36326/… Do any of these answer your question? $\endgroup$
    – James K
    Sep 23, 2023 at 11:26

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