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How Do The Tides Work?

Anyone who lives close to ocean is familiar with the tides. And you probably know they have something to do with the Moon. But how do the tides work? Do other planets experience tides?

Just what the heck are tides? Some kind of orbit jiggle jello effect from the magic Etruscan space-whale song? Is it an unending slap-back of gravitometric Malthusian resonance originating from the core of the Sun’s crystalline liver-light organelles? Is it all the plankton agreeing to paddle in the same direction at their monthly oceanic conferences?

As certain as I am that you enjoy my word terminology salads, with apologies to Papa Bear, we both know tides are caused by the gravitational interaction with the Moon. You would think we’d have only one high tide and one low tide, with the Moon pulling the Earth’s water towards it. Moon goes one side, water rushes over to that side, moon goes to other side, water chases around to follow it. But the tides make the water levels appear to rise twice a day, and lower twice a day in 6 hour increments. So, it’s clearly more complicated than that.

Tides

Tides. Image Credit: NOAA

The gravity from the Moon does pull the water towards it. That’s what gives you the highest tide of the day. It’s a bulge of water that follows the Moon around and around as the Earth rotates. This makes sense to us. But then Earth itself is pulled with a little less gravity than the water towards the Moon and, the water on the opposite side of the Earth is pulled with even less gravity, and so you wind up with another bulge on the opposite side of the Earth.

So from our perspective, you end up with a bulge of water towards the Moon, and a bulge away from it. The part of the Earth with the water getting pulled towards the Moon experiences a high tide, and same with the part on the opposite side of the Earth with the other bulge. Correspondingly, the parts of the Earth at right angles are experiencing low tides.

It would be hard enough to predict with a simple spherical Earth covered entirely by water, but we’ve got continents and coastlines, and that makes things even more complicated. The levels that the tides rise and fall depend quite a bit on how easily the water can move around in a region. That’s why you can get such big tides in places like the Bay of Fundy in Canada.

The Moon over Gulf Islands National Seashore near Navarre Beach, Florida. Credit: Mindi Meeks.

The Moon over Gulf Islands National Seashore near Navarre Beach, Florida. Credit: Mindi Meeks.

Our Sun also contributes to the tides. Surprisingly, it accounts for about 30% of the them. So when the Sun and the Moon are lined up in the sky, you get the highest high tides and the lowest low tides – these are Spring Tides. And then when the Sun and Moon are at right angles, you get the lowest high tides and the highest low tides. These are Neap Tides.

Tidal forces can be very powerful. They can tear galaxies apart and cause moons to get shredded into pieces. Perhaps the most dramatic example is how Jupiter’s enormous gravity pulls on Io so strongly that its surface rises and falls by 100 meters. This is 5 times greater than the Earth’s biggest water tides. This constant rise and fall heats up the moon, giving it non-stop volcanism.

What do you think? Share your favorite tidal science fact in the comments below. And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Comments on this entry are closed.

  • Homonymous Anonymous August 18, 2014, 11:15 AM

    Tide goes in, tide goes out, even Bill O’Reily knows it has nothing to do with gravity!
    ;)

  • psalazar94121 August 19, 2014, 11:12 AM

    Tides are a great subject for illustrating how bodies in space have a direct and measurable impact here on Earth. I wrote this post recently to illustrate why kayakers get better ebb tides after midnight in the summer and after noon in the winter. http://urbanastronomer.blogspot.com/2014/07/sun-and-moon-and-ebb-tides.html

    Love the site, Fraser. Great articles.

    Paul

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