The Roche phenomenon in the article is always an interesting one, for me. I know this will sound silly to most people here but my experience with the Bernoulli “explanation” for lift in aerofoils makes me feel a little uneasy about tidal forces in general.
I don’t mean that I find the Bernoulli explanation non-sensical — that’s a very common thing these days — more that the experience of listening to the falsehood presented as truth by so many people now means I am suspicious of other non-intuitive explanations.
In this specific case, I can’t get a good intuition about how tidal forces explain (1) Earth’s moon causing ocean bulges on both sides of Earth; and (2) tidal friction making Earth’s moon stop spinning and move further away. I feel like it’s one of those phenomena, like aerofoil lift, whose explanation is glossed over far too quickly given how odd the explanation is.
> I can’t get a good intuition about how tidal forces explain (1) Earth’s moon causing ocean bulges on both sides of Earth
Try to imagine that all the Earth is liquid, so no landmass that resist deformation. And now ask yourself a question: which form this Earth should take in presence of Moon? You see the reason for a bulge facing the Moon, but what kind of shape should be the other side? Anti-bulge, depression? Or maybe an approximate spherical surface? Or another bulge facing from the Moon?
> tidal friction making Earth’s moon stop spinning and move further away.
Deformed Earth is rotating and the bulge under the Moon tends to move faster than Moon (in terms of angular velocity). So the bulge is sort of "in front" of the Moon and it pulls it forward speeding it up. The other bulge works in opposite direction, but it is more distant so the force is lower.
> So the bulge is sort of "in front" of the Moon and it pulls it forward speeding it up. The other bulge works in opposite direction, but it is more distant so the force is lower.
Interesting! I had never caught that key idea and need to read more about that. thanks
> Earth’s moon causing ocean bulges on both sides of Earth
That's not really a thing, and tides are way more complicated than that. For example, at any given moment, part of New Zealand's coast is at high tide, and part is at low tide. Same with the coasts around the North Sea, and the Atlantic coast of Patagonia, and Hudson Bay.
I agree, I find those deeply counterintuitive too. For other readers, the tidal explanation I have read is yes, the moon’s gravity pulls on the sea to raise a tide on the side facing the moon. The reason there is a tide on the opposite side is that the moon also pulls on the earth’s solid mass (also water, but the solid part it’s important here), creating displacement that causes the water on the other side to rise. There is a difference in the gravitational acceleration across the diameter of the earth and it’s less on the far side, thus the unequal pull, thus displacement. Since they are orbiting, this gravitational pull doesn’t cause the two (earth and moon) to meet, though.
This is an amateur’s explanation and I’d welcome input from someone with more understanding. For tidal friction, I can’t answer at all, I need to research.
Some simple mental models:
Think of the gravity connecting the moon and the earth as a string, and both bodies are rotated around its gravitational midpoint (which is afaik actually slightly inside the earth, but this still works), and you'll see that this will make the ocean bulge on both sides.
But then we have the earth's own rotation to think about. The earth turns faster than the moon orbits the earth, and the bulges will drag behind, and the moon is affected by that, like the lure at the end of a string when you swing a fishing rod.
As for the Bernoulli explanation for lift.. try holding a sheet of paper in front of you, it's not solid so it'll bow down, with a nice wing-shaped form. Then blow gently over it, see what happens.
The tidal bulges are easy to understand if you think through a force diagram with the different forces on the various parts of the Earth. The gravitational force is weaker on the more distant parts of the Earth from the moon.[1]
So everything is pulled towards the moon, but the more distant parts are pulled less, and the sides are pulled inwards, so the resultant shape is the familiar ellipsoid
Brilliant, thank you. The killer part of this video is quantifying the effects of the standard tidal force lift/stretch explanation. They are tiny: around 1 micro-g. Compare this with the total hydraulic pressure of tidal forces on a perfect global ocean predicting a 75cm tide (which, on Earth, is amplified by the coastlines of the continents.)
What's the probability that a moon was occluded? The star, the moon, and the Earth would have to be perfectly aligned on a straight line. It seems a vanishingly small probability. A dense asteroid belt seems more likely except that the star was only occluded once.
If it is an asteroid belt, maybe it is on a different (high inclination) plane, which is why the star only hit one part of it.
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The Roche phenomenon in the article is always an interesting one, for me. I know this will sound silly to most people here but my experience with the Bernoulli “explanation” for lift in aerofoils makes me feel a little uneasy about tidal forces in general.
I don’t mean that I find the Bernoulli explanation non-sensical — that’s a very common thing these days — more that the experience of listening to the falsehood presented as truth by so many people now means I am suspicious of other non-intuitive explanations.
In this specific case, I can’t get a good intuition about how tidal forces explain (1) Earth’s moon causing ocean bulges on both sides of Earth; and (2) tidal friction making Earth’s moon stop spinning and move further away. I feel like it’s one of those phenomena, like aerofoil lift, whose explanation is glossed over far too quickly given how odd the explanation is.
> I can’t get a good intuition about how tidal forces explain (1) Earth’s moon causing ocean bulges on both sides of Earth
Try to imagine that all the Earth is liquid, so no landmass that resist deformation. And now ask yourself a question: which form this Earth should take in presence of Moon? You see the reason for a bulge facing the Moon, but what kind of shape should be the other side? Anti-bulge, depression? Or maybe an approximate spherical surface? Or another bulge facing from the Moon?
> tidal friction making Earth’s moon stop spinning and move further away.
Deformed Earth is rotating and the bulge under the Moon tends to move faster than Moon (in terms of angular velocity). So the bulge is sort of "in front" of the Moon and it pulls it forward speeding it up. The other bulge works in opposite direction, but it is more distant so the force is lower.
> So the bulge is sort of "in front" of the Moon and it pulls it forward speeding it up. The other bulge works in opposite direction, but it is more distant so the force is lower.
Interesting! I had never caught that key idea and need to read more about that. thanks
> Earth’s moon causing ocean bulges on both sides of Earth
That's not really a thing, and tides are way more complicated than that. For example, at any given moment, part of New Zealand's coast is at high tide, and part is at low tide. Same with the coasts around the North Sea, and the Atlantic coast of Patagonia, and Hudson Bay.
Animation of today's tides: https://www.tpxo.net/
Some good tide explanation: http://physics.stackexchange.com/questions/121830/does-earth..., discussed three months ago https://news.ycombinator.com/item?id=44065458
I agree, I find those deeply counterintuitive too. For other readers, the tidal explanation I have read is yes, the moon’s gravity pulls on the sea to raise a tide on the side facing the moon. The reason there is a tide on the opposite side is that the moon also pulls on the earth’s solid mass (also water, but the solid part it’s important here), creating displacement that causes the water on the other side to rise. There is a difference in the gravitational acceleration across the diameter of the earth and it’s less on the far side, thus the unequal pull, thus displacement. Since they are orbiting, this gravitational pull doesn’t cause the two (earth and moon) to meet, though.
This is an amateur’s explanation and I’d welcome input from someone with more understanding. For tidal friction, I can’t answer at all, I need to research.
Some simple mental models: Think of the gravity connecting the moon and the earth as a string, and both bodies are rotated around its gravitational midpoint (which is afaik actually slightly inside the earth, but this still works), and you'll see that this will make the ocean bulge on both sides. But then we have the earth's own rotation to think about. The earth turns faster than the moon orbits the earth, and the bulges will drag behind, and the moon is affected by that, like the lure at the end of a string when you swing a fishing rod.
As for the Bernoulli explanation for lift.. try holding a sheet of paper in front of you, it's not solid so it'll bow down, with a nice wing-shaped form. Then blow gently over it, see what happens.
The tidal bulges are easy to understand if you think through a force diagram with the different forces on the various parts of the Earth. The gravitational force is weaker on the more distant parts of the Earth from the moon.[1]
So everything is pulled towards the moon, but the more distant parts are pulled less, and the sides are pulled inwards, so the resultant shape is the familiar ellipsoid
[1] http://nicholasbsullivan.com/page_oceanography/Meeting6/01_t...
Here's the ultimate explanation of tides from PBS SpaceTime: https://www.youtube.com/watch?v=pwChk4S99i4
Brilliant, thank you. The killer part of this video is quantifying the effects of the standard tidal force lift/stretch explanation. They are tiny: around 1 micro-g. Compare this with the total hydraulic pressure of tidal forces on a perfect global ocean predicting a 75cm tide (which, on Earth, is amplified by the coastlines of the continents.)
What's the probability that a moon was occluded? The star, the moon, and the Earth would have to be perfectly aligned on a straight line. It seems a vanishingly small probability. A dense asteroid belt seems more likely except that the star was only occluded once.
If it is an asteroid belt, maybe it is on a different (high inclination) plane, which is why the star only hit one part of it.
Article in RNASS research notes https://iopscience.iop.org/article/10.3847/2515-5172/adfeda