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SYFY WIRE Bad Astronomy

Time and Tide: Crash Course Astronomy Episode 8

By Phil Plait

I’m fascinated by tides. Not so much the movement of the ocean, as you might think. That’s a product of tides; what I mean is the change of gravity over distance stretching things.

Say what?

Let me explain. In fact, let me explain in the latest episode of Crash Course Astronomy: Tides!

This episode was one I was looking forward to writing ever since Hank Green asked me to do the series. Tides affect everything! The length of the day, the Moon always showing one face to us, the Moon’s recession from the Earth, and yes, even the oceans’ rise and fall. These are all due to a series of interlocked steps in physical logic that starts with the simple fact that gravity gets weaker with distance. Start with that, and the rest is inevitable.

Tides affect stars orbiting each other, galaxies when they collide, and even black holes as they gobble down matter. That’s so cool!

I know the idea that the Earth has two tidal bulges confuses a lot of people, so hopefully my explanation in the video makes it clear why they both exist, and not just one bulge pointing toward the Moon. There are a lot of ways to explain this; the actual vector analysis in a nonrotating frame is the best way, but I opted not to get into that in this short, basic video. Duh.

Also? I love the graphics Thought Café did for this episode, especially the one at the very, very end. This article I wrote may help you get that joke. There is, after all, a tide in the affairs of men.

P.S. Yes, of course I’ve read Shakespeare. His work has a lot of astronomical overlap.

Correction, March 6, 2015, at 14:45 UTC: I originally misstated that this was Episode 7. Sorry about that; I forgot to correct for time dilation.

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