One-Slide Explanation of Tides

I realize that this Bill O'Reilly quote is two weeks old, which in Internet time is a substantial fraction of the age of the Universe. And, the Internet being what it is, a top conservative commentator can't say something this butt-ignorant without having bloggers jump all over him within seconds. So, yes, I realize that I'm way, way behind the times, sort of like somebody getting all snarky to the dinosaurs because they didn't invest in programs tracking near-Earth asteroids. But, still, I think it bears repeating, to remind ourselves collectively the kind of people who are shaping the agenda of an entire political party in the USA right now.

Here's my one-slide explanation of how the tides work:


Click image for larger version

This slide does go along with some speaking, normally. Indeed, it is one (of 28) slides that I'll be using in the talk I'm giving in Second Life in about half an hour, all about interacting galaxies and whether or not they're connected to the phenomenon of active galactic nuclei. (Really, tides are relevant to this story!)

17 responses so far

  • Kevin Fairchild says:

    Rob, I've always found it useful with my high school students to explain tides in terms of squeezing. I start with the "what if you had two apples in free fall, and you were falling with them, they'd seem to move together" scenario. Then I draw gravitational field lines from the moon, and show how they tend to "squeeze" the earth. Then I draw the field lines from the Sun, and show how there's not as much squeeze (but there is a little) since the lines are more parallel. This seems to be a good visual, especially for why there are tides on *both* sides of the earth. I'd be interested to hear your thoughts on this strategy.

  • rknop says:

    Yes, that's a good one. The apples are a neat idea. Photos of Shoemaker Levy 9 may als0 be good as part of that -- round comet stretched out into a line of smaller bits.

    What I usually say when I show this slide is that the gravitational force on the "solid" earth is (effectively) acting at its center, so it in a sense gets "pulled out from under" the water that piles up on the far side. This doesn't address the squeezing in the middle though -- it just says three things line up: the water closest gets pulled strongest, then the Earth in the middle, then the water on the far side being pulled weakest. And, it *is* true that there's squeezing in the middle as well.

  • Thony C. says:

    Don't you know that the tides are caused by the movements of the Earth, diurnal and annual rotations, like water slopping around in a moving bowl. Galileo said it so it must be true ;)

  • The red gravity at the surface of the earth is meant to be the gravity exerted by the moon? I feel like one of my genetics students - "I thought I understood it until I saw your explanation".

  • rknop says:

    Rosie -- yes, the red arrows are the gravity of the Moon acting on the close and far sides of the Earth.

  • milan_va says:

    It's funny that just yesterday I've started to wonder why there's a spring tide when the Earth is *between* the Sun and the Moon. IMHO, if your explanation is correct, the tide should be smallest at that time?

  • rknop says:

    milan_va -- I'm not familiar with the term 'spring tide'. However, notice above that there are blobs of water both on the side close to the moon, and the side opposite the moon. The effect of tidal forces is to try to stretch things out-- the gravity closer to the moon is stronger than average, so things are pulled closer, and the gravity farther from the moon is weaker, so things aren't pulled in quite as much, and thus tend to stay farther away than the bulk of the planet. The Earth is solid, and doesn't stretch very much, but the liquid parts are able to respond more easily to that. So, you'll get two high tides a day-- one when the moon is directly overhead, and one when the moon is directly opposite overhead.

    Of course, the Sun affects the Earth's tides as well, so in reality the tides show up as a result of the combination of the Sun and the Moon. The tides will be biggest when the Sun and Moon are contributing in exactly the same way. This happens at full moon -- when the Earth is between the Sun and the Moon -- and at new moon -- when the Moon is between the Earth and the Sun. When all three are in a line, the stretching of the Earth due to the differential gravity of the Moon and the Sun are both pulling in the same direction, so the total effect is amplified compared to when they're pulling in different directions.

    • milan_va says:

      I've understood the situation with Moon, but the Sun pulling in the opposite direction has confused me.
      You are right, of course. I should have drawn the four arrows myself before posting... It's a kind of "X=not not X" situation, hard to do in head (my head at least).

  • Gingerbaker says:

    "the gravity closer to the moon is stronger than average, so things are pulled closer, and the gravity farther from the moon is weaker, so things aren’t pulled in quite as much, and thus tend to stay farther away than the bulk of the planet."

    It seems to me you have this exactly backwards. The (Earth's) gravity closer to the moon should be less, because the moon's gravity would be pulling in the opposite direction of the gravity of the Earth, which pulls material toward its core.

    On the far side of the Earth, both the Earth and the moon exert gravitational force on the water there, but now both gravitational forces pull in the same direction - toward the Earth's core.

    So, the effective gravitational force on the far side of of Earth is stronger, hence your slide is mislabeled.

  • rknop says:

    Gingerbaker -- the red arrows indicate the force *on the Earth* due to the gravitational attraction of the Moon.

    The fact that the near side of the Earth is closer to the moon than the far side means that the gravitational attraction *due to the Moon* is different accross the Earth. The effect of that is to try to stretch the Earth out along that direction. The Earth itself doesn't stretch much, but the water can flow a bit.

    What matters here is not the absolute gravitational attraction on the water. Rather, it's the differential attraction. The solid earth itself is held together as a solid, whereas the water can move about. The attraction of the Moon's gravity on the solid earth is thus effectively at the center of mass of the Earth, which is *stronger* than the attraction of the Moon's gravity on the water -- so the Earth is "pulled out from underneath" the water there.

    The Earth's gravity *does* matter of course; if it weren't for the Earth's gravity, the water wouldn't just blob up a bit and give us high tide, it would end up stretched out into long thin streamers extending away from the Earth.

  • Scott Free says:

    The gravitational attraction of the sun and moon are the driving forces but the water responds in a very complex fashion based on the natural resonance frequencies and damping forces of the specific body water. Consider the east coast of North American. From north to south the tidal range goes from 40' in the Bay of Fundy to just about 1/2' in New Jersey to 10' again in Georgia before declining to 2-3' in Florida. More dramatically, consider the opposite ends of the Cape Cod Canal, a mere 6 miles apart. At the north end the tidal range is 10' and the other end its 4'. The times are also 2 hours apart. Wicked currents up and down the canal!

    Given observations of the tides at any point scientists can precisely predict future tides but forecasting tides without measurement just on the basic physics is beyond the current state of the art.

  • jane hay (evodevo) says:

    Yes, there are "spring" tides and "neap" tides. See - http://oceanservice.noaa.gov/education/kits/tides/media/supp_tide06a.html

  • Bau Ur says:

    I just realized why I missed 6 points on an oceanography test in 1983. I am embarrassed that I did not look it up for myself in the intervening time. Thanks Rob.

  • Frank Lovell says:

    "How'd the moon get there? How'd the sun get there? You can't explain those to me." -- Bill O'Reilly

    I believe Bill -- we can explain those to others just fine, but we cannot explain those to him -- and we know why we can't explain it to him, don't we?!