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Q & A: lunar tides

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Most recent answer: 01/03/2014
Q:
When the moon gets closer to earth, it effects the ocean with high tides. Why?
- Joseph McNeary (age 33)
Maryland Heights, MO USA
A:
The moon is in a nearly circular orbit around the earth, so the changing distance between them is not the key to the tides. The key is that the moon is at different distances from different parts of the earth.  Newton figured out that the gravitational pull from one object on another goes inversely with the square of the distance between them, at least for distances big compared with the sizes of the objects. That means that the parts of the ocean near the moon are getting pulled extra-hard toward the moon. The parts on the opposite side of the earth are getting pulled relatively weakly, compared to the average for the whole earth. So you'd expect two bulges in the oceans- one on the side toward the moon, because it's pulled up a lot. The other is on the side away from the moon, which is not pulled down enough to keep up with the earth. Of course in real life, the actual bulges are complicated by the odd shapes of the oceans, friction, weaker tides due to the same sort of effect from the sun, etc. Nevertheless Newton's key idea, explaining why there are two high tides per day, is correct.

Mike W.

It's not hard to work out the heights of the tides assuming that the earth is solid and that water can flow freely.   The solid earth assumption is needed for the pull of the moon averaged over the whole earth to make some sense -- the earth has to be pulled away from the water on the far side, and this is assuming it's rigid.  But the earth is made up of mostly molten components, and only the top crust is solid.  The ground also swells up and down with the tides, but less so than the oceans, because it is in fact a bit more rigid.  If the earth were as fluid as the water, you standing on the seashore wouldn't notice any tide at all because the seashore would move up and down in precisely the same way as the water.

Tom

(published on 10/22/2007)

Follow-Up #1: moon tides

Q:
Could It be the earth and the moon are both very large magnets and when the two get closer their magnetic fields react to each other? I know the earth is a magnet, what about the moon? Also, I live in missouri where there is no ocean, how can I predict when the moon is cloest to me as in a high tide without using a computer to do the work for me?
- Joseph McNeary (age 33)
Maryland Heights,MO USA
A:
This question means a lot to me personally. That was the first scientific idea I had (almost 50 years ago, also in Missouri), to try to connect the way one side of the Moon always faces the Earth with the way magnets and compasses behave. As it turns out, however, (as my parents told me) the Moon is not magnetic. It lacks the sort of core that gives the Earth its magnetism.

As for whether the Moon is directly overhead or directly opposite, yes Iím sure that you can track that (easily on Moon-lit nights) but Iím not sure where to find some program to help fill in between the times you can see.


Mike W.

Both the moon and the sun affect the tides, the moon having the larger effect.  VERY ROUGHLY speaking, when the moon is full high tides occur at noon and midnight. Low tides would then occur at 6 am and 6 pm.  To get accurate results consult a table, there are many you can find with Google.  A nice pictorial explanation can be found at
      
Lee H

(published on 10/22/2007)

Follow-Up #2: Tidal air currents

Q:
I am really pleased to see that you say The ground also swells up and down with the tides. What about the air? Does that move around like the sea? Its less viscous than the sea and the partly molten/crusty land. And its less dense and more easily pushed around. I imagine being at the bottom of the ocean away from the coast and I expect that the sea-tide would not be very evident. So similarly being at the bottom of the atmosphere (on the ground) the air-tides would not be very evident. But near a NorthĖSouth mountain range, high up, would one expect to get air-tides flowing past one just like the sea-tides one notices on the surface between large bodies of water Ė like the straights of Gibraltar. Surely there must be massive air movements caused by the moon in the upper atmosphere? How come there is no mention of them in the books etc.?
- John Grassby (age 62)
Guildford UK
A:
This is a very interesting question!   I confess itís something I have never considered.  I happen to have worked in a field where the solid earth tides are an important effect; the Virgo Gravitational Wave Interferometer in Pisa, Italy, where the small but finite earth tides affect the distance between two suspended mirrors spaced 3 kilometers apart by about 0.2 millimeters.  This is due to the tidal bulge of the earth.  See   
I couldnít find anything relevant to your question after a quick search of the web. No doubt the effect you suggest exists.   I suspect,  but could be wrong, that it is small in comparison to that from other sources of energy contributing to the movement of air in our atmosphere, for example, the large diurnal temperature fluctuations which drive atmospheric pressure gradients.    As an example of external forces affecting tides:  extra strong wave motion in the sea caused by a hurricane (erroneously called 'tidal surge') can overwhelm the usual tides by a large factor.

LeeH




(published on 11/23/2007)

Follow-Up #3: tidal vocabulary

Q:
Hello. Will you please tell me if it is correct in English to speak of the "ups and downs" of the tides. I need it for a technical work of translation. If it is correct, will you please give me the reference of a text/texts where I could find that expression. Thank you very much for your attention. Rocío Cerrudo
- Rocío (age 41)
Madrid-Spain
A:
Those would be unconventional words, although they would be understood. We usually refer to "high and low" tides.
Wikipedia has an article using these words:  http://en.wikipedia.org/wiki/Tide

Mike W.

(published on 06/28/2011)

Follow-Up #4: land, sea, and tides

Q:
Why in tides only sea water is attracted and not solid rocks etc
- Sona (age 22)
India
A:

Everything in any region is pulled the same way by the gravity that causes the tides. The water is free to flow but the solid rocks are held too rigidly by electrostatic (chemical) forces to flow.

Mike W.


(published on 01/03/2014)

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