It's a Bird, It's a Plane, It's Supermoon!

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Connections - Physics and the Everyday World

It's a Bird, It's a Plane, It's Supermoon!

The Astronomy of Supersized Moons


Brooke Adams

Florida Institute of Technology

June's supermoon, which appeared as much as 13.5 percent larger and 30 percent brighter than a typical full Moon, is shown here behind the Washington monument. Image courtesy of NASA/Bill Ingalls.We’ve all seen a full Moon. But did you ever notice that sometimes the full Moon is larger?

Popularly referred to as a supermoon, this larger apparent size happens when the Moon is closest to the Earth. The effect on June 22 and 23 was special because the Moon was approximately 356,991 kilometers away, the closest it will be to the Earth until August 2014. These different sizes remind us that our moon’s orbit is not a perfect circle, but rather an ellipse, following Kepler’s laws.

The giant impact hypothesis best explains the Earth-Moon system’s current orbital patterns, as well as why the Moon’s chemical composition is so similar to Earth’s. This popular explanation of the Moon’s origin says that when our planet was young, a gigantic collision with another body created debris, which then started orbiting Earth. Eventually the debris collected into the single body we now call the Moon.

Because the Moon orbits Earth more slowly than the rate at which the planet spins, it pulls on Earth and slowly reduces Earth’s rate of rotation. This tug-of-war between the Earth and Moon also means the Moon’s orbit is getting larger, at a rate of about 4 centimeters per year.

If enough time were to pass, the Moon would eventually drift away altogether, as the Earth’s gravitation will not have enough pull to keep it. The Sun will probably turn into a red giant (in its late phase of stellar evolution) before that happens. Both the Moon and Earth will be destroyed by the expanding Sun.

This won't happen for billions of years, though, so we should always have our Moon to light up the night sky. But as the Moon drifts outward, our supermoons will become just a tiny bit less super every year. //

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