I know why the oceans bulge up when the moon is (more or less) overhead: gravity. But why does the water bulge up on the side of the earth opposite the moon?
And don’t tell me “conservation of angular momentum.” That’s like saying “magic.” Tell me about the mechanics of it; what force causes the oceans to bulge? Somehow this was not covered in my college progress to a bachelors in Mechanical Engineering…
Basically, the pull of the Moon’s gravity pulls the water up on the near side of the Earth, while the centrifugal force (yes, physicists, I know) of the Earth’s motion around the common center of the Earth-Moon system tends to “throw” the water outwards on the opposite side. Tides are caused by the difference in gravitational pull between the near and far sides.
If you like, you can also think of the Moon pulling the Earth away from the water on the far side (which amounts to about the same thing as pulling the water away from the Earth).
Well, the lunar pull on water (actually lunar-gravity partial counteraction of Earth’s gravity) is of course strongest at the point where the Moon is directly overhead, or as close in the Southern/Northern sky as it ever gets to directly overhead outside the tropics. The pull is reduced from there as one observes the circumference of the Earth. The point where it draws water away the strongest is of course 90 degrees off from directly overhead. And the point where the Moon’s pull is the weakest is 180 degrees off from directly overhead. Hence high tide is when the Moon is directly overhead – and when it’s directly underfoot, and low tide is when it’s on the horizon (or would be if the horizon were perfectly flat).
Still another way to look at it. An elemental unit of the earth on the side nearest the moon is pulled toward the moon with a stronger force than is the same elemental piece on the far side. This means that there is a net force trying to pull the earth apart. The result is that the earth is pulled into a slightly oval shape with the long axis pointing in the direction of the moon. The earth’s rotation pulls the long axis slightly away from pointing directly at the moon, but that’s another story.
…and he says pooh-pooh on centrifugal force (sorry Q.E.D.!)
Ok, then this leads me to wonder: are the two bulges necessarily the same shape? Seems like they wouldn’t be, and the two different tides would have slightly different characteristics. Is this the case? Is it so small as to not be noticeable?
The two bulges are not the same shape. The “moon-side” bulge is larger than the “opposite-side” bulge. This is why there are two high tides every day, with one of them being slightly higher than the other.
For small tides, they’re approximately the same shape. But for large ones, they differ. In the extreme case, you’d get a sort of teardrop shape, rounded on the far side, and coming to a point directly below the thing causing the tides. This is never seen for planets, but can be seen with large stars orbiting close to other stars.
I’m surprised the GR/astrophysics go-to guy didn’t mention the whole reason they’re called “tidal forces”.
Basically, think of each little bit of water separately. The moon pulls on each, and each is pulled towards the center of the moon with a force that falls off as the square of the distance. So if we start with a sphere of water, the side closer to the moon is pulled harder than the side farther away. Also the whole sphere will tend to squish in a bit since the parts off the direct line between the center of the sphere and the center of the moon will get pulled towards that line. The effect is to deform the sphere into an ellipsoid. Put a big hunk of rock in the center of that sphere of water and BAM, tides.