This Monday, the early morning night sky will be treated to an extra special supermoon. You may be asking yourself "what’s the point?"; It feels like there’s a new supermoon every other month, and there are other bigger things to worry about right now. Well, here’s the deal: it’s going to be big — the closest supermoon the Earth has seen in the past 69 years.
The Moon’s distance to Earth isn’t a constant because our satellite is in elliptical orbit — meaning its proximity to our planet is always changing. Meanwhile, the Moon goes through phases, depending on where it is in relation to the Sun and how much sunlight it receives. Whenever the full moon phase coincides with the Moon’s closest approach to Earth — known as perigee — that’s when we call it super.
These two events happen at different rates, so it takes a while for them to sync up. "Just like if you’re in your car, and you’ve got your turn signal on and you’re behind a car with its turn signal on, every once in awhile the turn signals sort of appear to sync up and fall out of sync," Noah Petro, deputy project scientist for NASA’s Lunar Reconnaissance Orbiter mission, tells The Verge.
There are differing definitions for a supermoon, though. One interpretation is that a supermoon occurs when the full Moon is within 90 percent of its closest approach to Earth. That’s why it may seem like we get a supermoon every other weekend. But for NASA and others, the definition is a little more exclusive: it’s the closest full moon that occurs during the 14-month cycle of full moons. Under that umbrella, we really only get a supermoon about once a year.
Tomorrow’s supermoon satisfies the latter definition. The Moon’s average distance from Earth is more than 238,800 miles, but tomorrow, it’ll be 7.26 percent closer than that. That means it'll look about 14 percent larger than your average full moon, and 30 percent brighter. And this year, the supermoon is extra close to Earth. Not only is it the closest supermoon we’ve had since January 1948, but it’s going to be closer than 99.7 percent of all full moons in the past 200 years.
But here’s the sobering reality: It’s not going to be that much closer than your regular supermoon. This one is only going to be about half a percentage closer than most, and the Moon is going to be about 30 miles farther away than the supermoon in 1948. But that small percentage translates to a thousands of miles in distance, and Petro says it should serve as a great excuse for people to go out and stare up our natural satellite. "People don’t normally look at the Moon. You wouldn’t necessarily notice it’s really different," says Petro. "It’s basically an excuse for people to go out and start looking."
So go. Stare at the Moon. Let it remind you of our beautiful, cosmic neighborhood that is so much bigger than ourselves. The best time to go out and look is Monday morning just before dawn. Perigee is set to occur at 6:22AM ET. But the Moon will be extra luminous tonight and Monday night for those in the US who don’t want to get up that early.