The moon can only avoid Earth's shadow for so long. The last total lunar eclipse occurred on Jan. 20, 2019. This Wednesday, May 26th, observers in the western half of North America, western South America, East Asia, and Australia will see it fully eclipsed once again. Those of us in the eastern half of the U.S. will only see a partial eclipse because the moon will set before totality.

The distance between the Earth and moon varies during its orbit from closest (perigee) to farthest (apogee). The difference amounts to nearly 30,000 miles (48,000 km). (NASA)
The distance between the Earth and moon varies during its orbit from closest (perigee) to farthest (apogee). The difference amounts to nearly 30,000 miles (48,000 km). (NASA)

Coincidentally, the eclipse takes place within hours of the moon reaching perigee, the point in its orbit where it's closest to the Earth. These days, a perigean full moon is better known as a supermoon. The moon travels around the Earth in an ellipse, a shape that resembles an oval. When closest to the Earth, it appears larger than at apogee, its most distant point. May's supermoon will be the closest full moon of 2021 and appear about 8 percent larger and 15 percent brighter than a typical full moon. Will you be able to tell the difference?

Total lunar eclipses can only occur at full moon when the sun, Earth and moon are lined up in that order. The moon first glides through Earth's outer shadow, the penumbra, and then into the dark, inner shadow, the umbra. Sunlight refracted by the atmosphere "leaks" around the edges of the Earth to color the moon inside the umbra. (NASA)
Total lunar eclipses can only occur at full moon when the sun, Earth and moon are lined up in that order. The moon first glides through Earth's outer shadow, the penumbra, and then into the dark, inner shadow, the umbra. Sunlight refracted by the atmosphere "leaks" around the edges of the Earth to color the moon inside the umbra. (NASA)

Our planet casts dual shadows — a dark, inner umbra and a paler, outer shadow called the penumbra. From within the umbra, if you look back at the Earth, the solid body of the planet completely blocks the sun. That's why it's so dark in there. Looking back from inside the outer shadow, the planet only partially blocks the sun. Partial sunlight mixes with shadow to dilute the penumbra's gloom, turning it from black to middle gray. It can be difficult to see penumbral shading until just before the moon enters the umbra. Then it becomes visible as a "graying" of the moon's edge.

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Penumbral eclipse begins4:47 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 begins5:45 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.

The table above gives the times of each stage of the eclipse for each time zone. From left: EDT (Eastern Daylight); CDT (Central Daylight); MDT (Mountain Daylight); PDT (Pacific Daylight); AKDT (Alaska Daylight Time) and HAST (Hawai‘i-Aleutian Standard Time). The blanks indicate parts of the eclipse that won't be visible. Penumbral shading should be obvious about 20-30 minutes before the start and after the end of the partial eclipse.

Eclipse simulations are shown for Philadelphia, Minneapolis, and Denver. In each, the Moon will hover very low in the southwestern sky at maximum eclipse. From Duluth, Minn., the partial eclipse will start at 4:45 a.m. and end at moonset at 5:28 a.m. with just less than half the moon in shadow. (Stellarium with additions by the author)
Eclipse simulations are shown for Philadelphia, Minneapolis, and Denver. In each, the Moon will hover very low in the southwestern sky at maximum eclipse. From Duluth, Minn., the partial eclipse will start at 4:45 a.m. and end at moonset at 5:28 a.m. with just less than half the moon in shadow. (Stellarium with additions by the author)

From the eastern seaboard states, the moon sets before the start of the partial eclipse. If you live in Philly or Cleveland, only the penumbral shading will be visible before moonset. Skywatchers in the Midwest will get to see a partial eclipse, with the umbra moon taking an ever-growing "bite" from the lunar disk until moonset. The farther west you are, the deeper into shadow it goes.

This map shows the visibility of the eclipse from the Americas. (NASA)
This map shows the visibility of the eclipse from the Americas. (NASA)

From Detroit, about 10 percent of the moon is shadowed before sunrise. In Chicago, that increases to 37 percent; Duluth, MN., 38 percent; Minneapolis 55 percent; Grand Forks, ND 62 percent and Denver 100 percent. These numbers are approximate depending on close to the horizon you can still see the moon before it sets. Across most of the Americas, the eclipse takes place in the hour or two before sunrise (at dawn) with the moon low in the southwestern sky.

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

Make sure you find a place with an unobstructed view in that direction before eclipse day for the best views. Also, since it happens at dawn, the shadowy bite will appear pale in the growing light compared to seeing it in a dark sky. To better appreciate the changing colors, use a pair of binoculars.

The eclipsed moon's color varies depending on the state of Earth's atmosphere at the time. If it's relatively transparent and free of particulates, the shadowed moon glows yellow or bright orange. But if a recent volcanic eruption has saturated the atmosphere with dust and chemicals its color deepens to red or even a rusty brown. During the 1982 total lunar eclipse, ash and acids released by the massive eruption of the El Chichón volcano in Mexico darkened the moon so much, it was barely visible during totality.

Use the Danjon scale to estimate the moon's brightness during totality. L=0 is a very dark eclipse, while L=4 is a bright one. (Images: Bob King)
Use the Danjon scale to estimate the moon's brightness during totality. L=0 is a very dark eclipse, while L=4 is a bright one. (Images: Bob King)

I suspect this eclipse will be a bright one because the moon just squeaks inside the umbra, spending only about 16 minutes there before gliding back out into the penumbra. Earlier, I wrote that from within the umbra, the Earth completely blocks the sun from view. So why isn't the moon black and invisible during a total eclipse? Thank the air you breathe.

The atmosphere bends (refracts) sunlight that grazes the circumference of the Earth and directs it into the umbral shadow. Only red and orange "leak" inside because the other colors are scattered away just like they are when you see a red sun grazing the horizon at sunset and sunrise.

Whenever we see a total eclipse, a hypothetical lunar astronaut would look back to see the Earth totally eclipse the Sun. This is a simulation of what that astronaut would see on May 26th standing on the moon. The red ring is sunlight refracted by Earth's atmosphere. Venus and Mercury would also join the scene. (Stellarium with additions by the author)
Whenever we see a total eclipse, a hypothetical lunar astronaut would look back to see the Earth totally eclipse the Sun. This is a simulation of what that astronaut would see on May 26th standing on the moon. The red ring is sunlight refracted by Earth's atmosphere. Venus and Mercury would also join the scene. (Stellarium with additions by the author)

If you're able to view totality, watch for potential meteoroid impacts. They'll appear as momentary flashes of light on the darkened moonscape that are otherwise completely lost in the glare of a normal full moon. Amateurs captured one of the best-known, eclipse impact flashes on video during the 2019 totality.

For still photography, the partial phases are easy to capture with a camera on a tripod. Since most of us will be taking pictures in a brightening dawn sky, you may find that your auto settings will work just fine. Otherwise you can set your camera and lens to manual (M) and experiment with different exposures. When in manual mode, make sure that you sharply focus the moon before taking photos.

Start at ISO 400 with you lens set to f/8 and a shutter speed around 1/500th of a second. That should do for the early partial phases. As the moon moves deeper into the shadow, gradually open up the lens to f/5.6 and f/4 as you lengthen the exposure time to about 1/15th of a second. When the moon is fully eclipsed, increase your ISO to 800, open your lens to f/4 or wider and expose from 1/2 to 2 seconds. Check out Fred Espenak's How to Photograph a Lunar Eclipse for more tips.

If bad weather interferes or if you want to continue watching the eclipse after moonset, the entire event will be livestreamed from three different sites on the island of Hawai'i. One of those streams will take a unique approach and focus on the how the dimming moonlight affects the view on the scene on the ground and in the sky. Griffith Observatory in Los Angeles will also host a live online broadcast on its YouTube channel on May 26 from 4:45 a.m. to 9 a.m. EDT.

Clear skies!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.