Tonight, the full moon will appear larger than it has since the 1940s. The moon will appear around 14 percent bigger and 30 percent brighter compared with the smallest full moons. It will be worth taking a step outside to see this super supermoon.
What is a supermoon, and why does it happen?
The moon’s orbit around Earth is not a perfect circle. It’s an ellipse, a saucer shape that’s longer than it is wide. That means as the moon follows this orbit, it’s sometimes closer to the Earth and sometimes farther away. At perigee, the closest spot in its orbit to the Earth, it’s around 31,068 miles closer to Earth than at apogee, when it’s farthest away.
Meanwhile, we see different phases of the moon -- full, crescent, waxing, and waning gibbous — depending on if the sun-facing side of the moon is facing the Earth.
These two phenomena don’t always match up, but when they do, astronomers call it a perigee full moon (a.k.a. super moon). This occurs about one in every 14 full moons, Jim Lattis, an astronomer at the University of Wisconsin Madison, notes.
To be sure, this isn’t an enormous difference compared with a regular full moon. Neil deGrasse Tyson has called the frenzy around supermoons overblown. “If you have a 16-inch pizza, would you call that a super pizza compared with a 15-inch pizza?" he said on the StarTalk radio show.
Whether or not you’re impressed, it’s a good enough excuse to go outside and marvel at the beauty of the cosmos. Sky and Telescope reports that the night of the 13th or 14th should make for good viewing (perigee will occur at 6:23 am EST on the 14th). But “[t]he tiny difference between the two evenings will be undetectable,” the magazine writes. You may want to try to catch the moon earlier, when it rises (for those of us on the East Coast, around 5:30 pm), because near the horizon, an optical illusion will make the moon appear absolutely huge. Like this:
But why is this supermoon extra special?
The moon will appear slightly larger than it has in decades because of mere chance. The moon will reach fullness just three hours after perigee on November 14. Because perigee and the full moon are so closely timed, this full moon will be the largest (relative to our perspective on Earth) since 1948. The next-closest supermoon will be in 2034, as NASA reports.
Again, though, the differences in size between a really close supermoon and a typical one would be are pretty negligible. The full moon Monday will be just 30 miles closer to Earth than the last record in 1948, National Geographic reports. In astronomical terms, that’s tiny.
The moon will also be a bit brighter than usual — also due to the fact that it will be a bit closer to the Earth.
And if you miss it, fear not. We’re currently in a run of supermoons, and there will be another on December 14 (it won’t be quite as big). After that, we’ll have to wait until December 2017 for the next one.
Correction: This article originally misstated the how the moon’s phases occur. They do not occur due to the moon’s passing in and out of the Earth’s shadow. Rather, they’re due to the changing angles between the sun, Earth, and moon.