On Tuesday (April 7) at 2:08 p.m. EDT (1808 GMT), the moon will arrive at its closest point to Earth in 2020: a distance of 221,772 miles (356,907 kilometers) away. And 8 hours and 35 minutes later, the moon will officially turn full.
Although a full moon theoretically lasts just a moment, that moment is imperceptible to ordinary observation, and for a day or so before and after most will speak of seeing the nearly full moon as "full," although if you look carefully enough, you'll be able to tell that on Monday night and Wednesday night, the moon will appear ever-so-slightly out of roundness compared to Tuesday night. The narrow strip of darkness will appear on the left side of the moon on Monday and the right side of the moon on Wednesday.
What was once called a "perigean full moon" is now referred to in popular parlance, as a "supermoon."
In addition, the near coincidence of Tuesday's full moon with perigee will result in a dramatically large range of high and low ocean tides; high tides will run higher than normal and low tides will be lower than normal.
Any coastal storm at sea around this time will almost certainly aggravate coastal flooding problems. Such an extreme tide is known as a perigean spring tide, the word spring being derived from the German "springen" — meaning to "spring up" — and is not, as is often mistaken, a reference to the spring season.
Tides lag behind the moon
The highest tides will not, however, coincide with the perigee moon, but will actually lag by up to a few days depending on the specific coastal location. For example, at Wilmington, North Carolina, the highest tide (5.6 feet, or 1.7 meters) will be attained at 11:21 p.m. EDT on Thursday, April 9. For New York City, high water (6.1 feet 1.9 m) at The Battery comes at 9:34 p.m. EDT on Wednesday, April 8, while at Boston Harbor a peak tide height of 12.1 feet (3.7 m) comes at 1:20 p.m. EDT on Friday, April 10, nearly three days after perigee.
But then, if you live in the Canadian province of Nova Scotia, likely you're very familiar with the tidal antics of the Bay of Fundy, where the vertical tidal range can increase 10 to 20 feet (3 to 6 m) when the moon is at perigee. Quite a range indeed!
In contrast, later this year, on Halloween (Oct. 31), the full moon will closely coincide with apogee, its farthest point from the earth. In fact, on that night the moon will appear 13.8 percent smaller than it will appear this weekend. Some refer to that as a "micromoon" or a "minimoon."
Oops! Wrong month...
In the 2020 edition of the Observer's Handbook of the Royal Astronomical Society of Canada is a notation that the full moon of March 9 was the "largest in 2020."
But that was a mistake. The moon was actually 71 miles (115 km) farther away last month, making the moon appear ever-so-slightly smaller compared to April's full moon. The Handbook is a highly reputable publication; the "Bible" for assiduous skywatchers. That innocent notation that the March full moon was the largest (in apparent size) in 2020 was unfortunately promulgated in many publications.
What happened? Space.com asked Patrick Kelly, who compiles "The Sky Month by Month" section for the Handbook, who conceded the error and explained: "I am not sure why I picked March."
Kelly added, "I find calling anything a 'supermoon' just leads to disappointment as there is no visible difference. To quote [American philosopher] William James: 'A difference which makes no difference is no difference at all.'"
So, while Tuesday's full moon will be the largest one of the year, the variation of the moon's distance from Earth is not readily apparent to observers viewing the moon directly.
Or is it?
A "bloated" moon on the horizon
When a so-called "supermoon" lies close to the horizon, it can look absolutely enormous. That is when the famous "moon illusion" combines with reality to produce a truly stunning view.
For reasons not fully understood by astronomers or psychologists, a low-hanging moon looks incredibly large when hovering near to trees, buildings and other foreground objects. The fact that the moon will be much closer than usual this week might only serve to amplify this strange effect.
So, a "supermoon" either rising in the east at sunset or dropping down in the west at sunrise might seem to make the moon appear so close that you could almost touch it. Try it and see for yourself.
Lastly, Tuesday's full moon is the first full moon of the spring season . . . the so-called "Paschal Moon," that helps set the date of Easter. The general rule is, the first Sunday after the occurrence of the Paschal Moon is designated as Easter. And indeed, after this full moon on Tuesday, April 7, the following Sunday (April 12) will be Easter.
Editor's note: If you have an amazing supermoon photo you'd like to share for a possible story or image gallery, you can send images and comments in to email@example.com.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook