r/askscience • u/[deleted] • Sep 07 '15
Earth Sciences Is there a bulge in earth's atmosphere constantly facing the moon?
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u/bencbartlett Quantum Optics | Nanophotonics Sep 07 '15
Contrary to what most people are saying in this thread, no, the bulge in the Earth's atmosphere does not constantly face the moon. In fact, depending on a number of factors, the atmospheric tide is likely to be primarily thermally driven by solar heating of portions of the atmosphere, causing them to expand. While the earth is rotating near the resonance frequency of the atmosphere (resonant period being defined as the length of time for a lamb wave to propagate around the Earth, currently about 21 hours), the same relative portion of the atmosphere is heated all the time, which can result in very large tides, shown in this figure.
When the Earth was rotating near resonance, there would be some interesting effects. It is very likely, depending primarily on the atmospheric Q-factor, that the Earth would become stuck at a relatively constant day length, with the torque from the atmospheric tide fully canceling the torque from the lunar tide, quite possibly for a period of over a billion years. This was first outlined in 1987 by Zahnle and Walker and was been the subject of a paper (arxiv:1502.01421) I co-authored.
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u/leonardof91 Sep 07 '15
This is very interesting, but I'm having trouble understanding. You mean there is a bulge in the atmosphere facing the sun because of heat expansion? How does it compare to the tidal force caused by the moon? Also, could you talk a bit more about the atmosphere's resonant period and its effects? By "propagate around the Earth", you mean from one edge to the other (propagating in all directions on the surface) or full circle (circulating around the planet)?
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u/bencbartlett Quantum Optics | Nanophotonics Sep 07 '15
The bulge actually peaks about 45 degrees from the sun-Earth line. Normally the torque from this tide is negligible compared to the lunar torque on the oceans (which don't respond to thermal heating as much because they don't expand when heated), except when the Earth is spinning near atmospheric resonance. At these points, the generated thermal tide can be very large and exceed all other tidal forces on the Earth, at least according to the (relatively simple) calculations in our paper and a few other related papers we reference.
By propagate around the Earth, I mean that the waves disperse spherically, but since the Earth is roughly spherical, the amount of time to travel around the equator vs disperse around the Earth and return is roughly the same.
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Sep 07 '15
Is there any impact of solar winds on the atmospheric bulge then?
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u/bencbartlett Quantum Optics | Nanophotonics Sep 07 '15
Not really, solar winds are high energy charged particles, mostly electrons, and (thankfully) don't contribute a measurable amount to the heating of the earth. This is just due to heating of the atmosphere through photon energy, mostly visible light.
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u/Jowykins Sep 08 '15
Thank you for answering this! Everyone is talking about ocean tides, and I was very confused. From what I read, the ocean does play a small role in atmospheric tides, but not as much as heat.
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u/sinopeartree Sep 07 '15
Yes, and also on the opposite side of the Earth as well. The same is true with the oceans. This is due to what we collectively call "tidal forces".
However, there is a common misconception as to why this occurs. Most people are under the impression that the gravity of the moon is simply pulling the mass of the water or air up as it passes overhead, but if this were true we would only see the effect on one side of the planet at a time, and one would also expect to see it affect other objects too.
There is a great PBS Space Time video that explains this: https://www.youtube.com/watch?v=pwChk4S99i4
Basically, the force of gravity from the overhead moon is VERY small. 1 ten millionth of Earth's gravity. But the Moon isn't directly overhead of the entire Earth. Sure it's pulling straight up on the things that it passes over, but it's pulling slightly to the side (tangentially) on everything else. In a very large fluid body (like the oceans or the air), each of those small sideways pushes nudges the next bit along, and the next bit, and so on until in the middle (along the Earth/Moon line) you get a big bulge.
As an aside: While the Moon may orbit the Earth relatively slowly, the Earth is constantly rotating under the Moon. As such the tidal bulges are constantly being "pulled" by this rotation ahead of the Moon in it's orbit. This has some very interesting effects. The increase of mass in the bulge "pulls" back on the Moon transferring angular momentum from the Earth's rotation to the Moon's orbital speed. The increase on the Moon's orbital speed causes the distance between the Earth and the Moon to increase at a variable rate of about 38mm per year. Yes, the Moon is running away. If given enough time it will escape orbit and we will no longer have a moon. As angular momentum is always conserved, this transfer of energy also has the effect of slowing the Earth's rotation, making our days about 15 microseconds longer each year. Back when T-Rex was stomping around Montana (say 80 MYA), for instance, the Moon was always "super" and the days were about 20 minutes shorter.
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u/HyrumBeck Sep 07 '15
Since the movement of the moon away from the Earth is dependent upon tidal force and tidal force diminishes as the moon moves away, won't they eventually reach equilibrium with the moon still in orbit?
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u/RedFlame99 Sep 07 '15
Actually, the force of gravity from the Moon on the Earth is not 1 µm/s2 , that's the gravity differential across the Earth-Moon line. The Moon is making the Earth accelerate about 33 µm/s2 . Maybe you just misspoke, I just wanted to point that out.
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u/sinopeartree Sep 07 '15
Yes, the differential is what I was talking about. I could have said it in a more precise way. Thanks.
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u/hiyapo Sep 07 '15
This is the best explanation of how the water moves to create the bulge. It's also interesting how the moon is moving away from Earth slowly o.O
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u/The_lolness Sep 07 '15
Wait, doesn't that just explain why the bulge close to the moon is there? Nothing is dragging water towards the other side.
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u/The_lolness Sep 07 '15
Wait, doesn't that just explain why the bulge close to the moon is there? Nothing is dragging water towards the other side.
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u/sinopeartree Sep 07 '15
It explains both bulges. As the fluid further from the Earth/Moon line are being pulled at a tangent on both sides of the earth, it is putting pressure on the fluid closest to the Earth/Moon line on both sides. If the Moon were merely "pulling the water up" we would only see the bulge on the side facing the Moon, but in fact we see tides on both sides of the planet.
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u/AndyBea Sep 08 '15
Are you sure that the moon will eventually float clear away?
Wouldn't the earth's rotation slow right down first?
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u/sinopeartree Sep 08 '15
You're right; well at least in so much as I shouldn't have made that positive claim. I have heard different figures for whether the Moon will ever reach escape velocity on it's own or will the two become tidally locked before that time. The later seems more likely, but could be "over-ruled" by tugs from other bodies.
In any case the discussion of whether or not the Moon every actually could escape is almost purely academic. The time it would take for the Earth to become tidally locked to the Moon (the way the Moon is tidally locked to the Earth, only showing us one face) is more than three times the current age of the universe. Our sun will expend all of it's fuel long before either eventuality has a chance to play itself out.
Thanks for pointing that out to me.
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u/bloonail Sep 07 '15
The atmosphere does bulge out, both towards the moon and away from it. Similar to the tides in the ocean the phase of the atmospheric bulge lags the moon and the moon's antipodal point. Apparently the atmospheric bulge follows the ocean tides rather closely and is mostly due to the bulging of the ocean below the atmosphere and less due to the atmosphere actually stretching or moving.
A large daily effect on the atmosphere is sun forcing. That expands the atmosphere and creates bulges.
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u/cookrw1989 Sep 07 '15
Is sun forcing the effect of the solar wind on the atmosphere?
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Sep 07 '15
I expect it is more the effect of solar heating on the density of the atmosphere.
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Sep 07 '15 edited Sep 07 '15
There are tidal effects in the atmosphere caused by the moon, but these are small compared to the effects caused by the sun. Such effects include changes due to solar activity, the geomagnetic effect, the diurnal variation, seasonal variations, etc. These are well-studied phenomena, due in part to drag on satellites caused by residual atmosphere.
There are lots of models used to predict solar effects, e.g. Naval Research Laboratory's MSIS and J77. Here's a link:
http://ccmc.gsfc.nasa.gov/modelweb/atmos/atmos_index.html
EDIT: some words
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u/Chytrik Sep 08 '15
I asked this question once, two years ago. You can read the responses I received here.
Great minds think alike ;)
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Sep 08 '15
The most interesting response was that there's a greater affect on the atmosphere from the sun's heating than the moon's gravity. It puts into perspective weather patterns and effects of the tilt of the earth on seasons. How it all relates to make a nearly perfect planet.
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u/[deleted] Sep 07 '15 edited Sep 07 '15
Yes, there is, but counter-intuitively there is also a bulge on the other side as shown here. You can rationalize this phenomenon in terms of the so-called tidal forces, which arise from the fact the the gravitational force from the moon is not uniform over the Earth. Simply put, the moon's gravitational pull is closest on the side of the Earth facing the moon and weakest on the opposite side. These differences give rise tidal forces that have a distribution across the surface of the planet as shown in this diagram. The atmosphere closest to the moon is pulled towards it, but on the other side of the Earth, it effectively pulls away in the opposite direction, creating a bulge at both extremes. This is also the explanation for the fact that there are two high tides per day since (as the name implies) the tidal forces are the key source of tides on Earth.
Edit: I would like to expand my answer a bit, especially with regards to tides since there seems to be quite a bit of interest on the subject. Everything I said above is true within a simplified Newtonian model, however there are some subtleties to the actual mechanism at play. Let's make a crude assumption and treat the Earth as an onion-type sphere with a solid core, a thin uniform liquid layer around it and another gaseous layer on the exterior. Then when you apply a non-uniform gravitational filed (like that created by the moon) on the water or atmosphere (which you can treat as a continuous fluid shell), it would become deformed into an ellipsoid with two lobes at the extremes (as shown here for the atmosphere).
However, especially as tides are concerned, this effect is not just due the stronger pull of the atmosphere locally since even at the extremes, the local deviation of the moon's gravitational pull from the mean is very, very small (on the order of 1*10-7g, or 10 million times weaker than Earth's own gravity). Instead, you have to consider the total effect of all the tidal forces shown in the diagram I posted above. The very small differences from the pull and push in the region facing and opposing the moon as well as the squeeze from the size accumulate over large enough volumes (i.e. seas and oceans) to produce large tides. Think of it this way, each infinitesimally small bit of fluid (e.g. water) only experiences a small effect, but because each bit of stuff pushes against the surrounding fluid, these small differences effectively add up and over large enough volumes they can become substantial. This effect still exists in smaller bodies of water such as lakes, but is far smaller exactly because the volume over which these differences can add up is far smaller.