It’s coming up Sunday night: the closest and largest Moon of 2014. What’s cool is that the Moon reaches an extreme perigee the very same hour in which it is full. As a result, Sunday’s Full Moon will be about as large as possible.
Now, the actual size-boost is about six percent above an average Full Moon, and 11 percent over a small one, like this past January’s Full Moon. Even 11 percent is not at all dramatic, visually. The human eye can barely see the difference. The size enhancement is dwarfed by a separate factor: the Moon illusion.
When you see any Moon rising, it looks enormous. So between 8 and 8:30 p.m. Sunday evening, you will see a gigantic-looking Full Moon low in the east. But this is mostly a psycho-optical effect imposed on all celestial objects near the skyline. Even the Big Dipper looks supersized when it’s low, as you can see these nights at around midnight when it scrapes the northern horizon.
The science behind all this may actually be cooler than the visual observation. You see, the Moon’s orbit changes shape. Sometimes it’s rounder; at other times it’s more squashed and oval. Well, it so happens the Moon’s orbit will reach its most stretched-out elliptical shape this weekend. The result will be a Moon that is able to approach our planet more closely than at any other time this year. That it happens the very same hour in which it is full is no coincidence: The configuration is what helps stretch out the Moon’s orbit in the first place.
Bottom line: Saturday night and again Sunday night, find a place with an unobstructed eastern horizon. Right around sunset, watch the Moon rise. It will look huge. The media, fed by sources that include our community observatory SLOOH, is calling this the MegaMoon.
As usual, there are caveats: On the one hand, yes, this is the year’s largest Full Moon. Indeed, the Moon hasn’t come this close to us since March 2011. And yes, it will indeed look huge on Saturday night from 7:10 to 7:40 p.m., and this Sunday evening about an hour later. It’s worth a look. But remember, the big size is mostly due to the Moon illusion. This weekend’s Full Moons will look less imposing in the middle of the night, when they’re high up.
A more dramatic consequence involves tides. The Moon’s tidal effect varies with the cube of its distance. This lunar close approach will thus significantly raise the tides everywhere. In addition to the fact that it coincides with the Full Moon, the syzygy or lineup of the Earth, Moon and Sun creates its own especially high spring tides. The combo of spring tides (meaning that it’s the New or Full Moon) plus this close perigee produces what are sometimes called proxigean tides. These are maximal tides. Now, all we’d need along the coast are onshore winds, as in a storm, and there’d be major coastal flooding. A storm would add further, since a one-inch drop in air pressure raises the seas one foot.
The exact moment of Full Moon is 2:09 p.m. Sunday afternoon. So the Moon will look almost equally full late Saturday night and early Sunday night. But tides usually max out one day after the peak lunar influence. The seas need a little time to catch up to what’s happening. So expect the highest tides on Monday.
If you’re at the beach, the highs will reach almost to the boardwalk in some places, while the lows will go way out and make clamdiggers very happy. In our region, at places like the Saugerties Lighthouse, the Hudson River tides will be dramatic. The path to the lighthouse will be underwater. It’s a dramatic connection between Earth and the heavens.
For all these reasons, we shouldn’t miss this weekend’s MegaMoon.
This week’s column was adapted from Bob Berman’s newest book, Zoom.