Gravity and it's effects on tides.

Am I the only guy in the world who thinks the moons gravity could not be strong enough to influence tides on earth? (if there is so much gravity there why do astronauts have to wear giant heavy boots to stay on the surface?) I have another idea. The earth spins around with its own gravity holding the water on the surface.(or rather the atmospheric pressure holds it onto the surface blah blah blah). Anyway I think the oceans are just “sloshing” back and forth as the earth turns. Also, I have spent some time in the north and in the south. Alaska has a huge tidal swing whereas Jamaica has a very small tidal swing. Coincidence? or are the tides smaller the closer you get to the equator? I have read some on the subject but I dont buy it. According to someone (who’s name i forgot) the highest tides are along the equator, the way it was measured was dumb though, they used an altimeter, duh spinning earth water being thrown outward creates the bulge thus the altitude. So Cecil, could modern thinking be way off base? could there be some merit to the “slosh” theory? I am a first time user (doper?) my kid actually told me about this site. your books are the bomb-diggity and wayyyyy funny keep up the good work!!

Yes, the moon’s gravity is in fact strong enough. The astronauts didn’t need to wear big boots to stay on the surface; their boots were big and bulky for the same reason that the rest of the spacesuit is big and bulky. You’ve got to hold in the air, and protect the guy inside from extremes in temperature, and it’s got to keep on doing this even if you trip and fall or snag it against a sharp rock.

You would only get sloshing from a turning Earth if the Earth had only recently started turning (or if it had recently changed its rotation somehow). Such sloshing would quickly dampen and settle down to a more-or-less equilibrium state.

As for altimeters, you’re not measuring the distance from the center of the Earth, but the difference from the average sea level in that area. And more extreme equatorial tides means that low tides are actually lower than they are in polar regions, so it’s not just because of the equatorial bulge.

And finally, tides are affected by the local terrain. If the Earth were completely smooth and completely covered by water, then the tidal height would depend only on latitude, and the tides would be more extreme the closer you got to the equator. But the Earth isn’t completely smooth, and the water can’t move completely freely, since there are a bunch of continents in the way. This can result in water piling up against the land in some areas, giving some places higher tides than others.

Are there any effects on Earth or the Moon associated with solid tides?

Nope, it is actually the gravity that keeps the water there; gravity also keeps the atmosphere in place, compressing it to create the atmospheric pressure…

Read some more - LOTS more!


The Earth exerts enough of a tidal force that the Moon’s rotation is synchronized to its orbit, and always keeps the same hemisphere pointed towards us. The Moon is slowing down the Earth’s rotation too, so the days are very gradually getting longer.

The tidal bulge caused by the Moon’s gravity is pulled forward by the Earth’s rotation, which in turn tugs on the Moon and adds momentum to its orbit, causing it to recede.

We cannot forget the Sun’s tidal action either.
Tidal raising power, decreases with the cube of the distance. The Sun is incredibly massive, but is about 388 times the distance from the Earth as the Moon is. Therefore the Moon’s tidal force is greater than that of the Sun (2.2 times greater to be precise).

This does cause some interesting tidal variations during a month. When the Moon is in a New or Full Phase, we have the highest high tides and the lowest low tides. This is due to the Moon and Sun working with each other to their greatest degree. These are called “spring tides”.
When the Moon is in first quarter or third quarter phase, the Sun and Moon are working against each other to their greatest degree and we get the lowest high tides and the highest low tides. These are called “neap tides”.

I think other SDMB folks are going to join in this discussion. Tides are very interesting things to study.

Just to be pedantic, but it’s not tidal raising going on. The moon’s tidal effects stretch the sphere of water on Earth to an ellipsoid and pull the solid Earth a bit as well, which is why you get a tide on the far side as well. So-called “tidal forces” in GR measure the convergence or divergence of nearby geodesics.

Out of 6,429,169,546 people* on earth, I doubt that you are the only one.
:rolleyes: [sup]Does that make you feel better?[/sup]

***** And counting.