How Does the Moon Cause Tides?

Question posed by William.

You're probably well aware of the fact that you'd have a different weight on different planets. On Mars, for example, you'd weigh less than you do here on Earth*, and if you were to visit Jupiter (and could find a solid surface to stand on) you'd weigh considerably more than you do here. This is because bodies (i.e. planets) with more mass have a stronger gravitational field.

Gravity has another feature, though: it gets weaker as you move away from a body that's exerting it**. That means that if you weigh yourself with a (very, incredibly accurate) set of scales in an aeroplane you'd find you weigh slightly less than you do standing in your living room. The effect is similar to that of magnetism: if you play with a magnet and something magnetic (a nail, perhaps***), you find that the magnet has less of an effect on the nail the further apart they are.

A diagram showing the direction of tidal forces on a body
(longer arrows indicate stronger force)
If the blue circle is Earth, the 'Satellite' indicated is the Moon.
From Wikimedia Commons
Now, you know that the Earth keeps the Moon in orbit with its gravitational field, but sometimes it's easy to forget that the Moon is also attracting the Earth with its gravity as well. And this is where I start answering your question...

Because of gravity losing its strength as you get further away from a massive body, the water on the surface of the Earth that's facing the Moon gets attracted towards the Moon a bit more than any of the rest of the water on Earth. This makes it bulge outwards a bit, so the water's a bit deeper there than it is elsewhere. As the Earth rotates, the Moon tugs on slightly different areas, and so the bulge appears to move across the surface of the Earth, and it's this moving bulge that makes the tides rise and fall each day.

Because of the decreasing effect of gravity as you move further away from the source, there's also a bulge on the opposite side of the Earth from the Moon (this water is being tugged on less, so it swells up). This is why we get two tides every day: one is caused by the bulge of the Moon tugging on it, the other is caused by the twin bulge on the opposite side of the planet.

Tidal Forces
What you think of as 'tides'- water going up and down the beach- are only one aspect of this gravitational phenomenon. 'Tidal forces' act not just upon the water of Earth, but on the entire planet: the rocks that make up the crust, the molten rock under the surface and even the iron-rich core are subject to tidal forces.

The effect is not limited to Earth, either. Everywhere you find two bodies orbiting each other, you find tidal forces. It is thought by some astronomers that Jupiter's moon, Europa, may be kept warm under the surface by tidal forces kneading its innards, much like blu-tak gets warm when you play with it for a while. An even more forceful example of the power of tides could be found in the history of Saturn's beautiful ring system: One possibility for their formation is that they are the result of a large icy moon orbiting too close to its parent Saturn, and being ripped apart by the stronger tidal forces experienced there.

* I'm assuming you're on Earth. If I'm wrong, then welcome to my blog. Come and visit some time.
** Everything that has mass has a gravitational field. Even you.
*** Excuse my lack of imagination: I have man flu.


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