But the biggest clue was that an inverse-square law would account for both surface gravity and the Moon’s orbit. Without that clue, it’d have been a lot harder, and might not have come until after Coulomb’s Law.
Oh, and
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(and I already checked; the QR code doesn’t work)
I’m just not seeing it. Sure, the Moon is literally big, but the motion of the planets had been studied for centuries, not to mention the Galilean moons. Inverse square was “in the air” at Newton’s time and multiple people had been considering it. Plus it’s a sort of obvious consequence of any point force in 3 dimensions. If light spreads out as inverse square, why not gravity?
There was, and would have been, massive interest in astronomy anyway; because the stars had been used since time immemorial for navigation, on land and on water.
What I wonder, however, is how long it would have taken us to have any concept of there being other solid worlds, as opposed to just lights in the sky, without a visible rock hanging there right over our heads within easy view.
Not longer than the invention of the telescope. Granted, that’s probably 250,000 years after the first person wondered if there’s anything interesting happening on the Moon.
Well, the invention of a telescope good enough to resolve persistent features on the surface of Mars. That takes something a lot better than what Galileo was using.
I’d think that just resolving them as disks would be enough, though it’s hard to say.
I swear I can actually see Jupiter as a disk with the naked eye. It’s not just the lack of twinkling; it just looks noticeably bigger than, say, Mars. Even if not, just a slight magnification would do the trick. Kinda surprising that no one figured this out before Galileo.
Sure, Galileo’s telescope could resolve disks. But it won’t show that they’re solid worlds. In particular, it won’t show that they have surface features. That was one of Galileo’s big contributions: He was able to discern that the Moon had mountains, which made it a thing like the Earth, rather than the perfect, featureless, unchanging ether that was assumed at the time.
I’m not familiar with that history, but you don’t need a telescope to determine there are mountains on the Moon. Just view a total solar eclipse through smoked glass. Could have been done 5000 years ago. Baily’s beads are a direct consequence of the Moon’s topography.
It is interesting. The Wiki page on Baily’s beads claims that Halley was the first to record observations of the beads:
About two Minutes before the Total Immersion, the remaining part of the Sun was reduced to a very fine Horn, whose Extremeties seemed to lose their Acuteness, and to become round like Stars … which Appearance could proceed from no other Cause but the Inequalities of the Moon’s Surface, there being some elevated parts thereof near the Moon’s Southern Pole, by whose Interposition part of that exceedingly fine Filament of Light was intercepted.[8]
Anyone could have done this for many thousands of years. I suppose it’s evidence that ~1600-1700 is the first time that modern observational/experimental science made an appearance in any kind of consistent form.
How much accuracy in astrometric measurements would you need to detect Earth wobbling in its orbit around the Sun?
Also, with a permanently moonless night, people would have a better view off the night sky.
I think it would be too hard to use parallax. Certainly with respect to the stars, and probably the planets as well. Maybe with some cleverness it could be done.
I think you could do it with radio or optical spectroscopy. If my math is right, the point on the surface of Earth opposite the barycenter is moving at about 29 m/s relative to the center. That’s on top of the 465 m/s that the surface already moves at due to rotation, but it should be possible to correct for that.
You’d then need some reference frequency. A modern atomic clock would make it trivial. You could observe how a pulsar changes frequency as the Earth-Moon system rotates around. Going back several decades, maybe you could use another pulsar in a different location as a reference, but you’d need some way to compare the frequencies. I’d have to think about this some more. I doubt you could do it more than 100 years ago, even with modern knowledge.
Thanks.
As a point of comparison, the speed of the Voyager probes can be determined to within 0.1 millimeters per second with Doppler. So 29 m/s is trivial! Even 50 years ago I think it would be fairly easy. But probably not 100 years.
Yes, but I don’t know whether anybody 5000 years ago would have leapt to that conclusion.
People who lived before electric lighting became common already had a massively better view of the night sky than almost anybody does today.
Converting that knowledge into a dependable calculation of just exactly how much each tide will ebb and flow and exactly when was complicated enough that Victorians devised an analog machine that could do differential calculus to do it.
Well, clearly either no one observed them or no one came to that conclusion. I just find that a little surprising! It’s one of those little bits of knowledge that you could bring back and prove to people with technology they had at the time.
Even if they didn’t conclude that they were mountains, they would at least know that the edge is rough in some fashion, and it isn’t a perfect celestial sphere.
That’s certainly true. It took until Fourier until they could really be solved properly. Still, the mathematical underpinnings were there.
First, it’s necessary to understand that what’s happening is that the moon is coming between the sun and the observer. While at least some people had figured that out by a couple of thousand years ago, I don’t think we know whether anybody understood it 5000 years ago. Certainly even a couple of thousand years ago some societies had different explanations.
The dark of the moon isn’t particularly visible in daylight.
I’m certainly not saying that everybody would have come to the correct conclusion. Just somebody! Though option 3 is that it was both observed and understood–just never written down.
There are a remarkable number of people who have somehow never seen the Moon at all in the daytime. Still, for those who have, I think it would be a good guess that if they see a crescent Moon cross the sky before and after an eclipse, there was probably a new Moon in between.
With such machines being used until the mid-20th century