November 14 – Supermoon!

Today’s factismal: Not all full moons happen when the Moon is closest to the Earth.

There’s a good reason that they call it “rocket science” (OK, actually they call it “astrophysics”); figuring out what is happening in the sky is hard. That’s because things are moving all around and interacting with each other in all sorts of weird ways. Let’s take something simple for example – the Earth-Moon-Sun system. You’d think that with just three bodies orbiting each other there would be an exact mathematical description of how things will move. And you’d be wrong! Though we can solve certain special cases, in general we cannot tell how the motion of the three bodies will change more than a short time into the future (a million years or so).

The Sun, Earth, and Moon, drawn to scale (almost - the Moon is three times as large as it should be)

The Sun, Earth, and Moon, drawn to scale (almost – the Moon is three times as large as it should be)

But we can tell some things for sure. For example, we know that a full moon happens when the Moon and Sun are on the opposite sides of the Earth. And we know that the Moon’s orbit around the Earth is not a perfect circle; instead it is an ellipse that slowly moves around the Earth. Since an ellipse has a part that comes closer to the Earth and a part that is farther away, sometimes the full moon happens farther away from the Earth and sometimes it happens closer. Things being what they are, most of the time the full moon happens farther away. But when it happens close to the Earth (what astronomy wonks call perigee {“close to Earth” in Greek}), we get a Supermoon.

The Earth-Moon system, draw to scale. Notice how the Moon's orbit is slightly elliptic.

The Earth-Moon system, draw to scale. Notice how the Moon’s orbit is slightly elliptic.

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How super is a Supermoon? Not very. Because the Moon’s orbit is an almost perfect ellipse, the Supermoon is only about 14% larger than a normal full moon. Unless you are very, very observant you’ll never notice the difference. You can see the exact difference in the images below. This first image shows a full Moon over China, taken by Expedition 48 Commander Jeff Williams on the ISS. The second image shows what a Supermoon would look like.

A full Moon over China (Image courtesy Expedition 48 Commander Jeff Williams, NASA)

A full Moon over China
(Image courtesy Expedition 48 Commander Jeff Williams, NASA)

What a Supermoon would look like

What a Supermoon would look like (Modified image)

Now even though the Supermoon isn’t spectacular, that doesn’t mean that the Moon isn’t special. It is our nearest neighbor and can tell us a lot about how the Earth and the rest of the Solar System formed. If you’d like to get in on the fun of discovering more about the Moon, then head on over to Moon Mappers where folks just like you are telling scientists what they see on the super-duper Moon!
https://cosmoquest.org/x/science/moon/

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