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Discovery of a new satellite or ring arc around Quaoar

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49 pointswglb16 comments

Paper: https://iopscience.iop.org/article/10.3847/2515-5172/adfeda

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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 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.

See the PBS SpaceTime explanation hyperlinked-to elsethread and then the Physics StackExchange one. Gravity "pulling on" rock and water differently is a misleading explanation, and really an attempt to tweak an explanation that is bad to start with.

Tidal forces are fictitious forces that are what orbital mechanics look like in the accelerated frame of reference of the thing doing the orbiting. They are not a uniform "pull" in a single direction. That will get you to the idealized Waterworld explanation of PBS SpaceTime. What will get you further is the realization that PBS SpaceTime is almost on-point with the "assume a perfectly spherical cow" joke, which is where the Physics StackExchange explanation comes in, with the reality that the water is moving around and over the rock and that it's a lot more complex.

Which then gets you to Tom Scott and part of Cornwall rising and falling twice per day. (-:

* https://youtube.com/watch?v=lCA0II1sVZA

Thankyou. I will read up on this!

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...

> 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

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.)

You might also enjoy this Coding Adventure episode:

https://www.youtube.com/watch?v=8nIB7e_eds4

> 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

tides are spaghettification , like what is theorized as you fall into a black hole. You’re stretched into an oblong shape by the fact that the stronger gravity at your feet accelerates faster than your head.

With water and surface tension the net result is an oblong oval ish shape

And the tides are basically a teeny tiny version, where the spherical earth water is squeezed to an oblong shape

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

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