Look out for a supermoon, king tides, and plenty of darkness

The moon will be at its closest over the next few days, reaching perigee on Monday, a mere 222,135 miles away.

People watch a supermoon rising over Plymouth Harbor in 2016. –John Tlumacki/Globe Staff/File

There’s a full moon coming this weekend, and while I think all full moons are interesting, the combination of generally clear skies and the fact that this is a “supermoon’’ should make this one particularly beautiful.

First, what is a supermoon? This phenomenon occurs when the moon is full and makes a close pass to Earth. Usually, the moon is about 238,000 miles away from Earth. However, because the moon’s orbit is elliptical rather than a perfect circle, the distance between the moon and the Earth changes throughout the month and over the course of the year.

The moon will be at its closest over the next few days, reaching perigee (closest pass) on Monday (Dec. 4), a mere 222,135 miles away. This proximity is what creates the phenomenon known as a supermoon. The moon appears bigger and closer to us than usual — because it actually is.

The moon is officially full Sunday, but it rises at more than 99 percent from Saturday to Monday, so you have three days to enjoy the experience.

Big tides, too


When the moon gets this close to Earth, it brings big tides with it — “king tides,’’ as they’re sometimes known. It’s a good thing we don’t have a nor’easter because if we did, there would be a potential for flooding.

We could see tides as high as 12 feet in Boston Harbor from Sunday to Tuesday.

Plenty of darkness

The chart below shows moonrise times. The sun sets just after 4 p.m. this weekend — we are now into the earliest sunsets of the year. Saturday’s moonrise takes place prior to sunset, but Sunday’s and Monday’s will occur after the sun has sunk below the horizon. It gets dark early now, but there is some good news if you don’t like the early sunsets: After Dec. 9, the sunsets start getting later, ever so slowly. You probably won’t notice it until around Christmas, but the days will start getting a little longer, bit by bit.

The sun isn’t just setting earlier; the angle of the sun is also very low right now. This is why there is solar glare almost the entire day. The sun only reaches about 25 degrees above the horizon by noon, the same height it would be at around 8 a.m. in the summer. The low sun angle also means less energy reaching the Earth, and, of course, it’s dark for many more hours than it is light.


There will be about 14 hours of darkness per night this weekend, but much of it will be illuminated by a supermoon. Take an opportunity to look up and enjoy the view!


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