Big Moon, Short Eclipse — Don’t Miss it! – Sky & Telescope

April 15, 2014 total lunar eclipse
The totally eclipsed Moon glows red from sunlight refracted into Earth’s umbral shadow during the April 15, 2014, lunar eclipse.
Bob King

If it feels like a while since the Moon took a deep dive into Earth’s shadow, you’re right. The last total lunar eclipse occurred on January 21, 2019, followed by four penumbral eclipses in 2020. Wait no more! On May 26th, observers in the western half of North America, western South America, East Asia, and Australia will once again see the Moon fully eclipsed.

Lunar Eclipse May 26, 2021
This year’s first lunar eclipse is just barely a total affair. The timing favors western North America.
Leah Tiscione / S&T; Source: USNO

This eclipse will be short and sweet much like the total solar variety, with the Moon spending just 15.9 minutes inside the umbra, Earth’s central shadow. The brief visit is due to two factors. First, the Moon passes well north of the umbra’s center with its northern limb nearly tangent to the umbra’s edge. Minutes after it enters the shadow, it pushes out the other side.

Perigee (supermoon) vs. apogee
The diameter of the full Moon at perigee (closest approach to Earth) compared to apogee is obvious when the two Moons are placed side by side, but notoriously difficult to see when there’s no second Moon with which to compare.

Secondly, the eclipse happens to coincide with a perigean full Moon (known popularly as a supermoon). In fact, May’s Flower Moon is the closest full Moon of the year. Perigee, the point in its orbit of closest approach to the Earth, occurs on May 25th at 9:53 p.m. EDT, only about 9.3 hours before mid-eclipse. Distance from the Earth affects both the Moon’s apparent size and velocity. The full Moon’s average apparent diameter is 31′, but during May’s totality it will be 33.6′ across, a difference of about 8%. This extra-large Moon makes for a slightly tighter fit within the umbra, leading to a quicker egress.

The Moon will also be traveling faster along its orbit. On May 11th when it was at apogee, its most distant point from the Earth, it clocked in at 3,528 kilometers per hour (2,192 mph). During totality on the 26th, our satellite will be sailing along almost 250 kilometers an hour (155 mph) faster. The extra speed hastens its exit from the shadow.

Eclipse phases across the U.S.
Eclipse simulations are shown for Philadelphia, Minneapolis, and Denver. In each, the Moon will hover very low in the southwestern sky at maximum eclipse. Because the eclipse occurs during morning twilight over the eastern half of the Americas, the shadow will appear pale and less contrasty. Be sure to bring a pair of binoculars to see the shading and colors more distinctly.
Stellarium with additions by the author

While totality is restricted to the regions described earlier, half the planet will witness a partial lunar eclipse, from eastern South America to India. Across the U.S. and Canada, the farther west you live, the deeper the Moon appears in eclipse.

May 26, 2021 Lunar Eclipse Coverage Map
How much of the eclipse you’ll see depends upon your location.
Leah Tiscione / S&T; Source: USNO

From the Eastern seaboard only the early penumbral phase will be visible before the Moon sets around sunrise. From the Midwest about half the Moon will be covered, while from Denver westward, observers will enjoy a total eclipse. Only in the far western states will totality be visible in a dark sky, otherwise this is a morning twilight event. Because the Moon lies in Scorpius well south of the ecliptic, it will shine low in the southwestern sky across much of the U.S. and Canada, so be sure to scout out a location with an unobstructed view in that direction.

Lunar Eclipse: Umbra and penumbra
A total lunar eclipse occurs during a full Moon when the Sun (to the left, outside the diagram), Earth, and Moon line up in the order shown. The Moon first crosses into the pale penumbral shadow before dipping into the umbra. Light from the Sun (yellow lines) skirts the Earth’s atmosphere, which reddens and refracts it into the umbra.
S&T illustration

At the start of the eclipse, the Moon enters the Earth’s outer shadow, called the penumbra. Here the planet’s globe only partially blocks the light of the Sun, diluting the shadow and making it a challenge to see. But if you pay close attention, you’ll notice a gray shading across the eastern third of the Moon about 20 minutes before the Moon enters the dark, inner umbra, marking the start of the partial eclipse.

Penumbral eclipse begins 4:47 a.m. 3:47 a.m. 2:47 a.m. 1:47 a.m. 12:47 a.m. 10:47 p.m. (May 25)
Partial eclipse begins 5:45 a.m. 4:45 a.m. 3:45 a.m. 2:45 a.m. 1:45 a.m. 11:45 p.m.
(May 25)
Totality begins _____ _____ 5:11 a.m 4:11 a.m. 3:11 a.m. 1:11 a.m.
Maximum eclipse _____ _____ 5:19 a.m. 4:19 a.m. 3:19 a.m. 1:19 a.m.
Totality ends _____ _____ 5:27 a.m. 4:27 a.m. 3:27 a.m. 1:27 a.m.
Partial eclipse ends _____ _____ _____ 5:53 a.m. 4:53 a.m. 2:53 a.m.
Penumbral eclipse ends _____ _____ _____ _____ _____ 3:50 a.m
Eclipse phase times are shown for the contiguous U.S. time zones as well as Alaska (Alaska Daylight Time / AKDT) and Hawai‘i (Hawai‘i-Aleutian Standard Time / HAST). A blank indicates that aspect isn’t visible. These times use the new Herald-Sinnott method that has been adopted in lunar eclipse predictions in the recently published 21st Century Canon of Lunar Eclipses and the Eclipse Almanac: 2021 To 2030.

Partial eclipse begins when the umbra takes that first nibble from the lunar limb. Not long after, when roughly a quarter of the Moon has entered the inner shadow, its reddish hue becomes noticeable without optical aid. The color comes by way of our atmosphere, which refracts reddened sunlight grazing the planet’s circumference into the depths of the umbra. Sans atmosphere, the Moon would be completely invisible during totality.

Danjon scale
The Danjon Scale is used to estimate the color of the totally eclipsed moon. Astronomers and climatologists use that information to determine how clean or “dirty” the stratosphere is.
Alexandre Amorim

The Moon’s color in the shadow ranges from a bright copper to a deep maroon-brown, influenced by how centrally it passes through the umbra as well as the amount of suspended aerosols in the Earth’s atmosphere at the time of eclipse. The more material, the more light absorbed and the darker the Moon.

I suspect May’s totality will be a bright one if only because the Moon barely scrapes through the umbra. During its brief foray, watch for ozone in the upper stratosphere to work its magic. Ozone absorbs red light and colors the outermost fuzzy fringe of the umbra blue, a sight best seen in binoculars or a telescope.

View from the Moon
When we watch the May 26th total lunar eclipse, a hypothetical lunar astronaut would look back to see the Earth totally eclipse the Sun. The red ring is sunlight refracted by Earth’s atmosphere. Venus and Mercury would also join the scene.

During any phase of the eclipse — but especially during totality — watch for potential meteoroid impacts. They’ll appear as momentary flashes of light on the darkened moonscape. Amateurs captured one of the best-known impact flashes on video during the 2019 eclipse. If you have a second telescope and video capability, consider dedicating that instrument to impact recording. For still photography tips I recommend Fred Espenak’s How to Photograph a Lunar Eclipse.

If you’re lucky enough to see totality from the countryside, you’ll experience the full eclipse aesthetic as the sky transitions from bright moonlight to black velvet. With the Moon in shadow, thousands of stars return to view along with the grand arch of the summertime Milky Way. Maybe it’s just synesthesia, but when moonlight is quenched at totality, a hush seems to settle over the land.

The eclipse will take place in the head of Scorpius not far from the double star Beta (β) Scorpii, also known as Acrab. This view shows the scene from San Francisco near the start of dawn.

During the eclipse, the Moon will appear very close to a pair of 4th-magnitude stars, Omega1 and Omega21, ω2) Scorpii, and about 1° southeast of the beautiful double star Beta (β) Scorpii. Antares lies about 6.5° southeast. One or both of the Omegas will be occulted around eclipse time from Mexico, Central America, and parts of South America. Beta will be occulted prior to the eclipse from southern South America.

Whatever percentage of the Moon is covered, I wish you clear skies. Don’t forget. You’re part of this celestial alignment, too!


If you’re interested in reading more about this and other celestial events this month click here to purchase a copy of the May issue of Sky & Telescope.

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