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Since when do astronomers have conjectured that space is void, and not full of air like our immediate environment?

I am more interested about how long the notion has been around and how influential rather than when and how it has been scientifically demonstrated.

Background for the question: I heard a conference about philosophy of science explaining how Galileo revolutionized physics by postulating principles that where true, but defying immediate observation, like "all objects fall with the same acceleration" when in fact heavier ones tend to fall faster due to air resistance, or "undisturbed objects move in a straight line at constant speed" when such a trajectory can't be observed on Earth because of gravity and frictions.

I was wondering how he came to have these notions, then thought he might have been inspired by the motion of planets because he knew them very well and they move in a void so their movement is, as far as a XVI century astronomer is concerned, undisturbed.

But that would imply he knew space is actually void of air, which is not intuitive and requires observation technology he did not have access to (planes, weather balloon, sounding rockets...). So i was wondering since when this fact have been established or at least conjectured.

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    $\begingroup$ related: History of Science and Mathematics SE: Who was the first to postulate that space was a vacuum?, here in Astronomy SE: What was the first astronomical measurement which demonstrated that “the Earth is surrounded by vacuum”? $\endgroup$ – uhoh Jan 20 at 11:59
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    $\begingroup$ So apparently Galileo had the notion and his disciple demonstrated it after inventing the barometer. Thanks a lot. $\endgroup$ – armand Jan 20 at 12:06
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    $\begingroup$ If you feel you can write an answer, it's always okay to post a (reasonable) answer to your own question so that future readers will see it. $\endgroup$ – uhoh Jan 20 at 12:07
  • $\begingroup$ Rather than the actual proof, i am looking for how long has the notion been around and how influential it was. The answer from history of science SE is spot on, now is it a history of science question with an "Astronomy" tag, or an astronomy question with a "History" tag ? YMMV, so I posted a mash up answer along with links to the original response. $\endgroup$ – armand Jan 21 at 0:21
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(This is a mash up of the answers in Who was the first to postulate that space was a vacuum? and What was the first astronomical measurement which demonstrated that “the Earth is surrounded by vacuum”? )

The notion that earth is surrounded by vaccum has been around since at least the atomist philosophers like Democritus (c. 450 BC), albeit without demonstration outside of speculation it was open to debate.

Galileo himself had the notion that air is not weightless, which implies that the amount of air weighting on our bodies is limited, suggesting there is vacuum above it.

His disciple Torricelli invented the barometer, allowing him to estimate the amount of air above our head and demonstrate that space around the Earth is a vacuum.

Conclusion, Galileo most certainly had a clear notion that the planets are moving in a vacuum (which does not mean it is what inspired his laws of motion), and it was firmly established shortly after his death.

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