ii.The planets were notably bigger as well and this was important because this explained why the parallax of stars had yet to be detected. Remember that if the Earth is moving around the sun then we should be able to detect the parallax shifting of the stars. Tyco Brahi couldn’t detect that even with his high precision instruments but he worked out that in order for stellar parallax to be undetectable, that there must be an enormous amount of empty space between the stars and the planets. Nobody believed that that was possible so the lack of stellar parallax pointed towards the fact that the earth probably isn’t moving. iii.But Galileo showed through the telescope that the planets are hundreds of times bigger than the stars which showed that perhaps stars are hundreds of times more distant than the planets and that would explain why they appear hundreds of times smaller and that the earth CAN be moving around the sun without us being able to detect STELLAR parallax.
NATS1745 – Lecture 6iv.So Galileo had some observational evidence to contradict one of the main arguments against heliocentrism; his observation of the planets showed that just because stellar parallax couldn’t be seen, doesn’t mean that the earth can’t be moving4.Observation of JUPITERa.One of his most FAMOUS discoveriesb.With the telescope, Galileo saw that Jupiter is embedded within 4 faint star-like objects that are invisible to the naked eye and you can see them on the slide 10:35c.Over the course of several nights Galileo sketched Jupiter’s position relative to the four stars and he expected to find that Jupiter moves west to east relative to these 4 faint stars which would be from right to left on the sketch (that’s what all planets do except when they are retrograde) – what he found instead was that Jupiter shifted east-west relative to the 4 faint stars but among the other stars in the sky Jupiter was shifting west to east as it should and then a few nights later Jupiter shifted west to east relative to the 4 faint stars but yet during this entire time period, Jupiter was moving west to east relative to all the other stars in the sky. The back and fourth motion was only seen relative to the 4 faint stars around Jupiter. And sometimes 1 or 2 of these 4 faint stars would disappear as if Jupiter was eclipsing it and so you can see, in some observations he saw all 4 faint stars, some he only saw 3 or 2d.Galileo realized that the only way to reconcile this observation was to say that the 4 faint stars were shifting back and fourth around Jupiter meaning that the stars must be in orbit around Jupiter, and since stars don’t actually orbit planets, he thought that these 4 faint stars couldn’t be planets, they’re actually moons just like the earth’s moon; that’s the definition of a celestial body that orbits a planet.