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The Moon and Tides

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611232.  Fri Sep 11, 2009 6:41 am Reply with quote

In the Beavers, Bulges, Bacteria, etc. episode (featuring Anneka Rice) Stephen Fry mentioned how strong the tides would be on the moon if it had a sea (because of the effect of the gravitational pull of the Earth).

I do not believe this is correct. Tides on the Earth are caused by the Moon circulating around the planet 'dragging' the sea along with it. The Moon, however, keeps the same face towards the Earth at all times so tidal bulge would be static.

Excepting normal circulation expected within a large body of water (i.e. by heating from the Sun), surely there would no tides on the Moon, and certainly not massive surges as implied by Mr. Fry?

Please deduct him a suitable number of points for this oversight.

611243.  Fri Sep 11, 2009 7:01 am Reply with quote

That sounds like a good point to me, and suggests a whimsical sort of a notion that it would always be high tide on one side and low on the other. (I'm aware of the flawed logic in this idea - it's just a whimsy.)

Curious Danny
611369.  Fri Sep 11, 2009 10:25 am Reply with quote

The moon rotation and orbit are the same length of time - which is why we only ever see one side of the moon - so the sea would probably remain static and therefore there would probably be no tides at all.

611374.  Fri Sep 11, 2009 10:50 am Reply with quote

Hi Jimski, welcome to the boards. That is an interesting point and you are correct to say that the Moon is tidally locked to the Earth and thus (assuming it had oceans) it would not experience the same regular rising and falling tides we see on Earth.

As the Moon always shows the same face to the Earth it would have a permanent high tide on this ‘starside’. The other face (the ‘darkside’) would also have a permanent high tide, although this would be slightly less due to the ‘darkside’ being slightly further away from Earth.

Of course, the Earth’s tides are not solely the product of the Moon’s gravitational pull; the effect of the Sun causes spring and neap tides. The Sun would also have the same effect on any lunar ocean.

Then there is the point that the Earth’s gravitational effect changes throughout the Moon’s elliptical orbit as does the speed of orbit itself. These variations would cause further minor differences in the height of any lunar tides.

And now it has all become horribly complicated …


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