Tides! How Do They Work?

In honor of the recent super moon, I’ve decided to write about how the tides work. I had never really considered the process before, and found it to be surprisingly complex. Or, at least, I was surprised it was more complex than just “the moon.”

I have to preface this post with these words: I am a molecular biologist by training.  I am not an astrophysicist or nor am I an amateur anything-to-do-with-planets. This is my humble understanding along with some cartoons. Feel free to correct me!

There are two major players here ““ the Earth and the moon. The sun is important a little later on, but we’ll get to that.

First of all, the Earth and the moon are involved in an intricate dance.  The moon’s gravity is constantly pulling on the Earth, which doesn’t do all that much to the Earth other than pull the Earth’s oceans toward the moon. This causes a bulge in the oceans on the side of the Earth facing the moon.

The moon's gravity pulls the ocean toward the moon. This gravitational pull is indicated by the yellow arrow.

At the same time, there is another force working on the Earth’s oceans.  This is an outward force called centrifugal force.  Even though we like to think of the moon as orbiting our planet, both planets actually rotate around a common center of gravity. Centrifugal force pushes the water outward, away from the moon, resulting in a bulge of ocean water on the exact opposite side of the ocean water being pulled toward the moon.

The centrifugal force of the Earth and the moon spinning around their centers of gravity also causes the ocean to bulge. This is indicated with the blue arrow.

We have two bulges in ocean water, where tides are high.  The thinning of ocean water, at points opposite, are where the tide is low.  This results in two high tides a day for most places on Earth, although there are exceptions.

The sun also exerts gravitational powers upon the oceans of Earth.  This is why when the Earth, moon and sun line up and we see a new or full moon, the tides are bigger! More gravity is being exerted; larger tides occur!

This is relevant because the super moon of this weekend was at the perigee of the elliptical orbit; the point closest to Earth in 18 years.  And because the moon was full, the sun was also adding some gravitational powers to the tides.  According to NASA, however, the difference in the tides was only some 15 or so centimeters, so while the moon was gorgeous, the tides weren’t dangerous.

Unless you believe that the super moon was actually Gallifrey coming back.

Super moon or Gallifrey?

Special thanks to Calisee, who asked me how to explain how tides worked, and when I said, “The moon,” pointed out that that couldn’t be all that was going on.

As ever, if you have questions or issues, feel free to email me at AskDorilysAboutScience@PersephoneMagazine.com!

Interesting link:

http://www.youtube.com/watch?v=CTQ6ciHENgI – A guy explains tides with a cookie, a pickled onion and an orange.

8 thoughts on “Tides! How Do They Work?”

  1. After a year of teaching environmental ed on the beach, I am excellent at drawing this in the sand with my foot. And after spending a weekend on physical oceanography seminars, I’m way more confused about it than I thought possible! There’s places where the tides start? They work their ways down coastlines, moving out from that central point in the middle of the ocean? So weird.

  2. So am I the only dumbass who was surprised the moon has gravity? I thought there wasn’t gravity on the moon…but clearly I’m confused.

    Thanks for the tutorial, and now my explanation will be “The gravity of the moon and centrifugal force”.

    So here’s my question — how dangerous is it to swimming during a thunderstorm? I was discussing this with my 5 yr old and I told her there was a charge in the lightning, and that water conducts electricity. And electricity hurts. Was I close?

    1. Gravity and the moon! The moon does have gravity. However, because the moon has less mass than the Earth (it only has about 17% of the mass of Earth), that’s why the astronauts could jump higher on the moon.

      Regarding the swimming and the thunderstorm. Yes. She could be electrocuted. Water is an excellent conductor of electricity.

    1. It took me a while to work out, in my mind, why water wouldn’t be flying around everywhere if centrifugal force was playing a role. You know, how if you get on one of those mini-merry-go-rounds at the park as a kid, you go flying in all directions?

      But, as far as I can tell, the placement of the moon and center of gravity is the reason for that!

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