The first total lunar eclipse of 2021 occurs early next week and features the largest Full Moon of the year.
Ready for the lunar eclipse drought to come to an end? It’s been a while since we’ve watched the Moon pass through the Earth’s dark inner shadow, to be sure. 2020 featured four lunar eclipses… all of which were faint penumbrals. In fact, you have to go all the waaaaay back to January 21st, 2019 (remember 2019?) to remember the last total lunar eclipse. But that wait ends next Wednesday morning on May 26th, with a very short total lunar eclipse, centered on the Pacific region.
The eclipse transpires in the early morning hours for western North and South America, and at dusk for Far East Asia and Australia. Europe, Africa and the Middle East miss the event entirely, and most of eastern North America only sees the very early partial phases of the eclipse underway at sunrise and moonset.
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Eclipses occur when the node crossing (where the Moon’s orbit intersects the ecliptic) occurs near New or Full Moon. By definition, a lunar eclipse must occur near a Full Moon. This also marks the start of eclipse season, with an annular solar eclipse occurring two weeks later on June 10th.
A ‘Super, Flower, Blood-Moon’ Eclipse
No doubt, you’ve heard next week’s event termed as a ‘Super Blood Moon’ eclipse. That’s because Wednesday’s Full Moon and eclipse occurs only nine hours after the second closest lunar perigee of the year, with the Moon 357,309 kilometers distant. The perigee Moon will appear 33’ 28” across during the eclipse, slightly larger than its normal 30’ across. Can you spy the difference?
The Circumstances For Wednesday’s Eclipse
The penumbral stage for the eclipse begins at 8:47 Universal Time (you can convert UT to your local time here), though it will take another half hour or so before you’ll notice a slight tea-colored shading on the SE lunar limb. The real action begins around 9:45 UT, when the Moon starts to enter the inner dark umbra of the Earth’s shadow, and the partial stage of the eclipse begins. Totality starts at 11:11 UT, and mid-eclipse occurs at 11:20 UT. Be ready for a very short eclipse, with just 14 minutes and 30 seconds of totality. The process then reverses itself, with the Moon quitting the umbra at 11:26 UT and the eclipse ending at 13:50 UT.
This is indeed a very short eclipse, as total lunar eclipses go. In fact, next week’s total lunar eclipse is the second shortest for the 21st century: only the April 4th, 2015 total lunar eclipse was shorter, at just 4 minutes and 42 seconds in duration. The third runner up for the century on (mark your calendars) October 21st, 2097 comes close, with a totality of 15 minutes and 12 seconds in duration.
…and you may not see the Moon disappear entirely. The 2015 eclipse was extremely bright, with many observers remarking that, to their eyes, the eclipse did not appear to ever entirely hit totality. This sparked a lively discussion on the estimated versus apparent diameter of the Earth’s inner shadow umbra at the Moon’s distance. Certainly, the actual edge of the Earth’s shadow appears as ragged and diffuse, versus sharp and distinct. Here’s a fun project: without looking at the precise times of contact, try and judge with the partial phases of the Moon get underway, versus the start and end of totality. Were you close?
Things to Watch For
During totality, watch for the southwestern limb of the Moon to take on a bright orange to dark red tinge, the sanguine ‘blood red’ of the ‘blood moon’. With this eclipse being barely total, expect the Moon to take on a bright multi-hued tone, from saffron-yellow near the edge, to darker red near the core. This is the result of a thousand sunsets around the rim of the Earth, filtered into the planet’s shadow onto the Moon. Standing on the Earthward surface of the Moon and looking back, you’d see a total solar eclipse. Not all total lunar eclipses are the same, and the color of the Moon’s appearance during totality is a function of how deep it passes through the core of the umbra, and how much suspended dust and aerosols are present in the Earth’s atmosphere at the time.
The Danjon scale describes the apparent color of an eclipse, from 0 (very dark) to 4 (very bright). Though binoculars or a telescope can enhance the view, you can enjoy a total lunar eclipse using nothing but a working pair of 1x Mark-1 eyeballs. You’re seeing visual confirmation that the world is indeed round during a total lunar eclipse, as the curve of our planet crosses the face of the Moon.
Lunar Eclipses as a Medival ‘GPS’
Before the advent of accurate chronometers, lunar eclipses were useful to attain a one-time fix on your longitude at sea. Columbus claimed to have attempted to find his position in the Atlantic in 1494 and 1504 using eclipses, though he favored a smaller model for the true size of the Earth, and was less than successful. Captain James Cook fared better, effectively using eclipses and the lunar distance method for celestial navigation during his voyages across the Pacific to gauge his position.
Tales of the Saros
Wednesday’s total lunar eclipse is part of lunar saros series 121 member 56 of 84 eclipses. This is the final total lunar eclipse of this saros, which started waaaay back on October 6th, 1047 and produced its first total lunar eclipse on July 13th, 1516. If you caught the total lunar eclipse on May 16th, 2003, you saw the last member of this same lunar saros family.
Take the ‘Selenelion’ Challenge
Well-positioned observers Wednesday may have a shot at a unique visual observation on Wednesday morning/evening: a chance to see—very briefly—the totally eclipsed Moon and the Sun above the horizon shortly after sunrise… at the same time. This is known as a selenelion.
What’s going on here? Doesn’t the Moon have to be opposite to the Sun for a lunar eclipse to work?
This happens because the Earth’s shadow isn’t precisely the size of the Moon, but actually about three times larger. This means that the Moon can linger a bit in that upper shadow, after sunrise but before moonset. And of course, the curve of the planet and atmospheric refraction can deflect the apparent versus actual observed position of the Sun and Moon even further. A fascinating 2018 study demonstrated that the observed versus predicted sunrise and sunset times on your favorite weather app may be off by as much as five minutes.
The line to see a selenelion Wednesday (between the U3 to U2 line on the map) runs from eastern Texas to Montana and western Canada and Alaska at dawn, and China and southeast Asia at dusk. Be sure to find as flat a horizon as possible to carry out this unique feat of visual athletics, and the more elevation you can get, the better.
The next lunar eclipse(s) aren’t until November 19th 2021 (a partial lunar), and May 16th 2022 (the next total lunar).
Clouded out, or simply reside on the wrong side of the planet to see Wednesday’s total lunar eclipse? The good folks at the Virtual Telescope Project have got you covered, with a webcast covering the eclipse hosted by astronomer Gianluca Masi starting at (10:00 UT/6:00 AM EDT) on May 18th.
Don’t miss next Wednesday’s total lunar eclipse: its worth getting up early (or staying up late) for.
Lead image: Totality! The January 2019 total lunar eclipse. Image credit and copyright: Szabolcs Nagy.