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We take it for granted these days that the stars are unimaginably bigger than the planets and the moons. But when you look at the sky, it does not appear this way. The moon looks bigger and brighter than the stars. This leads me to wonder when astronomy first discovered the relative size of the stars. Did people believe that the moon was bigger than the stars for a long time? If so, where was this believed, and up until when? What discoveries or observations led people to conclude that the stars are larger?

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    $\begingroup$ There is no date before which people believed one thing and then the other, opinions differed since antiquity. Wikipedia has a detailed history article on plurality of worlds describing their evolution. $\endgroup$ – Conifold Nov 14 '17 at 0:32
  • $\begingroup$ @Conifold But there are other indications. Like a consensus among experts. And whether the masses have access to the opinions of experts. Certainly there is a date before and after which astronomers could say with consensus that the stars are bigger than the moon. We'll never know the exact date but we can probably ascertain the century. $\endgroup$ – ktm5124 Nov 14 '17 at 0:40
  • $\begingroup$ Certainly, there isn't. But it is a common mistake of approaching history with modern yardsticks. You are thinking in modern categories like "consensus", "experts", "masses" that did not even form until well into 19th century. How would we decide who the "experts" are, let alone have them reach "consensus" across continents in 14th century? Such questions become meaningless when projected not even too far into the past, history does not oblige. One only starts asking the right questions after learning more about what they are asking. $\endgroup$ – Conifold Nov 14 '17 at 1:14
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    $\begingroup$ You undoubtedly make good points, but I think there exists a way to make my question valid. It is possible to define some criteria which you can project into the past. For example, whether we have any manuscripts from antiquity supplying a conjecture about the stars. Or how many eminent astronomers held some position in the 17th century? And surely I could have been more specific but I preferred to ask a general question and receive a general answer. I'm very happy with the answer I received, so I think the question succeeded. $\endgroup$ – ktm5124 Nov 14 '17 at 1:24
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    $\begingroup$ There's no need to conduct a poll. It's easy to say "at least five eminent astronomers from Europe thought this as early as the 17th century". Besides, this comment exchange is unproductive. Clearly, you want me to ask better questions here. I'm not a frequent visitor, but I'll keep it in mind for the next time I ask. $\endgroup$ – ktm5124 Nov 14 '17 at 1:40
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It was known since antiquity that stars have no parallax visible by naked eye (parallax due to the displacement of the observer on the Earth). This indicates that they must be much further than the Moon (Moon has significant parallax which was known in antiquity).

The next step was heliocentric system of Copernicus. If one accepts the motion of the Earth around the Sun, the stars must have parallax due to this motion, unless they are VE-ERY far. Actually this was used as an important argument against the heliocentric system. The argument was that they cannot be THAT far. The universe cannot consist of mostly empty space. Why would God create such a strange universe?

As they are very far, it follows that they are very much BRIGHTER than the Moon and some of them have to be brighter then the Sun, because we see them.

Strictly speaking this does not imply that they are bigger (they can be brighter but smaller, in principle). But if we make a natural assumption that they are of similar nature to the Sun, then their brightness and distance indicate they they are much bigger than the Moon, and some of them are bigger than the Sun.

All this could be made precise when the parallax (due to the Earth motion around the Sun) was eventually discovered (in 18th century), and when it was proved that the nature of the stars is indeed similar to the nature of the Sun (by spectral analysis, in 19th century).

Direct measurement of the size is still impossible (except perhaps of the closest ones), they all look like dots, even with the most powerful telescope. But we have a theory about their inner structure, this theory is well confirmed and it predicts their actual dimensions.

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  • $\begingroup$ Ah, so it took a mathematical mind to realize that the stars have no parallax, and thus they must be far. And then we discovered that they are even farther than we thought when astronomy adopted a heliocentric view of the solar system, as it was a necessary consequence. $\endgroup$ – ktm5124 Nov 13 '17 at 21:23
  • $\begingroup$ I wonder if the average Athenian in the 5th century BC would have known that the stars are very far away and that they are bigger than the moon? Or would only the mathematicians, astronomers and highly educated have known? $\endgroup$ – ktm5124 Nov 13 '17 at 21:25
  • $\begingroup$ @ktm5124: what the average Athenian thought about the subject I cannot say, but even to astronomers before the heliocentric system it was not clear that they are bigger. The only thing they knew for sure was that they are further. $\endgroup$ – Alexandre Eremenko Nov 13 '17 at 21:33
  • $\begingroup$ @ktm5124: And when you say "average Athenians" you should specify when. Scientific astronomy began in Greece only in 2-nd century BC, that is long AFTER the "average Athenians" like Plato, Aristotle, Sophocles and Thucidides lived. $\endgroup$ – Alexandre Eremenko Nov 13 '17 at 21:36
  • $\begingroup$ Good point. Noted! $\endgroup$ – ktm5124 Nov 13 '17 at 21:37

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