The Tides

[Jamie123]

Hi everyone - it's been a long while since I bugged you with my meanderings, so I thought I'd have a drone about something that puzzled me as a kid – and a young adult – and still does plague my imagination. Namely “why do we get two tides a day”? Two high tides and two low tides. The Earth rotates, and the moon pulls the water on one side of the earth – to the water under the moon should be at high tide and the water opposite the moon should be low tide. And yet the water opposite the moon is also high tide! Why?

Well let me tell you what isn’t the answer: Waves.

Here's an example of a geography teacher invoking the “waves” answer.

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It’s not altogether obvious why you get this second bulge on the side of the earth opposite the moon. Well…umm…just think about it as a big wave – like water sloshing around a bowl. It’s bouncing around and just a little bit of tug from the moon is keeping it going.

Two things wrong with this. Firstly we all know how fast water waves move. We see them in the bathtub and in swimming pools. We see surfers riding them. A quick Google search shows its 1.97m/s. The tidal bulges circulate around the world every 24 hours, meaning that they travel  40,000 km every 24 hours – that’s 463m/s – or about half a kilometre per second. Ever seen a surfer surf that fast?

But even if it were true, what does it explain? Why do there need to be two wave crests instead of just one? (After all, there’s only one moon!) Ask a geography teacher this, and he’ll say “Enough about tides, let’s talk about how oxbow lakes form!”

(I’m a teacher myself, and I’ve often fallen into this trap: the temptation to talk garbage and/or to change the subject when put on the spot with a question you can’t answer is intense! But honesty is usually the best policy.)

OK, next solution:

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The moon pulls the water from the earth (making the bulge on the moon-side) and also pulls the earth from the water (making the anti-moon bulge).

This looks very convincing when you draw it out. I first heard this explanation from Patrick Moore on “The Sky at Night” and later saw an illustration in Richard Feynmann’s “The Character of Physical Law” which seemed to confirm it. But shortly afterwards another idea struck me: the earth is not fixed in space by an invisible God-like hand holding it like a ball, with the moon free to move around it. The earth and moon rotate together around a common centre of gravity, so the anti-moon side receives a net centrifugal force, lifting the water to produce the second tidal bulge.

(If you’re one of those physics teachers hate the phrase “centrifugal force”, then nyaaa nyaaa nyana nyaaa to you, I’m going to use it!)

However, when I actually READ Feynmann’s explanation in “The Character of Physical Law” I found out that wasn’t what he was saying at all: his argument was that the force of the moon’s gravity was different on both sides of the earth, and the water moon-side got a greater tug than the water anti-moon side, so the entire earth is “stretched” by the moon. The earth of course is rigid, but the ocean does indeed stretch, and this is what makes the tides. You can see him make the same argument in this video around 5:29

(Notice he also talks about the “centrifugal force”.) This would also explain why the comet Schumacher-Levy 9 split into fragments due to "tidal forces" from Jupiter. Around that time I read another book called “Bad Astronomy” (I can’t remember the author) which as far as I can remember made the same argument. And now you can hear Neil deGrasse Tyson say the same thing:

Sounds great till you do the maths. Given that the moon’s orbit is about 400,000km in radius and the diameter of the earth is about 10,000km, the difference in the moon’s gravity on either side is (look up the formula and do the sum yourself!) about 5 millionths of a Newton per kilogram of water - about 1% the strength of an ant! And I know ants are incredibly strong for their size, but could an ant lift 100 kilograms of water through 4 metres in the 6 hours between low and high tide? 

So what is going on? 

At last I think I’ve found someone who has an explanation that (I think) works! Check out...

 

You've got to love the analogy to squeezing a zit!  You don't hear that one much from geography teachers! 

 

Edited May 8, 2018 by Jamie123