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Astro Bob: Watch a colorful total lunar eclipse Sunday night, May 15-16

We have a complete viewing guide for you. Observers should be alert for an unusually dark eclipse because of the recent Tonga volcanic eruption.

November 2021 lunar eclipse
I made this sequence of the November 19, 2021, near-total lunar eclipse with a small refracting telescope. It begins with the moon in penumbral eclipse, upper right, and ends shortly after the moon exited the Earth's umbral shadow, bottom left. Sunday night's eclipse will be total.
Contributed / Bob King
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Good news! If you live in the Americas, you won't have to get up at 2 in the morning to see this colorful celestial event. For instance, from the Midwest, the partial phase begins at 9:28 p.m., with the Moon entering total eclipse at 10:29 p.m. Farther west, it starts even earlier.

Eclipse visibility
May's total lunar eclipse will be visible across the Americas, Africa and Europe. Skywatchers in the eastern half of the U.S. will witness the entire eclipse, From the mountain states, the moon will rise already in partial eclipse, while from the West Coast, it rises in total eclipse.
Contributed / NASA, Fred Espenak

If you're a parent with kids, let 'em stay up late for one of the easiest-to-see and most beautiful events the cosmos has to offer. Neither special equipment or a dark sky is required — just walk outside and open your eyes! Lunar eclipses are infrequent enough that I try not to miss a single one. For the eastern half of the U.S., the last total lunar eclipse occurred in January 2019, more than two years ago. I'm pumped.

Lunar eclipses occur somewhere on Earth twice a year on average but can be as few as zero and as as many as three. This year, we have two totals — Sunday night and a second in the wee hours of November 8.

Total lunar eclipse free use Starry Night anno full.jpg
Lunar eclipses occur only at full moon, when the moon passes directly behind the Earth into its shadow. If the Earth were airless, the Moon would look completely black during a total lunar eclipse. But the atmosphere refracts reddened sunlight grazing the edge of the planet into the Earth's umbra (inner shadow), coloring the lunar disk during totality. (Not shown to scale.)
Contributed / Starry Night with additions by Bob King

A total eclipse occurs when the full moon passes through the two-part shadow cast by the Earth. The outer shadow, called the penumbra, appears gray because the globe of the Earth — from the perspective of someone standing on the moon — only partially covers the sun. Some sunlight filters into the shadow, diluting its darkness.

Penumbral shadows
We don't often notice, but shadows cast on Earth also have two parts — an outer fringe or penumbra, where a bit of sunlight leaks in, and a dark core, where direct sunlight is hidden, as shown in the photo of the tree. When the moon enters Earth's penumbral shadow, it appears somewhat shaded (right) but not black.
Contributed / Bob King

If you pay close attention just before the start of the partial eclipse, you'll notice a grayish shading covering the left (eastern) half of the moon. Some observers can spot it more than a half-hour ahead of partial eclipse. But it should be obvious five minutes before Earth's umbra — the dark, inner shadow — takes its first "bite" out of the moon's edge.

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Moon moves through shadow
This diagram shows Central Daylight times of the moon's progress through Earth's outer (penumbral) and inner shadow (umbra) during Sunday's eclipse. Note that the partial phase times are for when 50 percent of the moon is covered. For detailed times of all eclipse phases, see below.
Contributed / NASA's Scientific Visualization Studio

That bite marks the beginning of the partial phase of the eclipse. It's dark because the moon is entering the umbra or inner shadow, where the Earth's globe completely blocks the sun from view for someone standing on the moon. It takes about an hour for the moon, traveling at an average speed of 2,200 miles an hour (1 km/second), to fully enter the shadow. At that point, the total eclipse begins.

Totality lasts almost an hour and a half. At least two things will strike your eye during this phase: the color of the moon and how dark the sky becomes. You'd think that the moon would go completely black once inside the umbra, since it's fully engulfed in shadow. This doesn't happen because light from the sun filters through the bottom of the atmosphere around the entire 360° circumference of the globe.

Jan 20, 2019 total eclipse
During totality, color can vary across the moon's face as it shares the sky with stars that would otherwise be lost in full-moon glare. This photo is from the Jan. 20, 2019 total lunar eclipse.
Contributed / Bob King

It's highly reddened for the same reason the sun glows red at rising or setting — blue and green light in the sun's spectrum are scattered away, leaving the warmer hues of yellow, orange and red to pass. The air acts like a lens or prism and bends (refracts) the reddened light into the umbra to color the moon.

How cool is it that the air we breathe is responsible for the moon's color during totality?

Like so many things in nature, each eclipse is different depending on the moon's path across the umbra. Darker and longer eclipses occur when the moon passes directly across the umbra's center. This time, the moon passes south of the center, which should make for a moderately dark totality.

How "clean" the atmosphere is at the time of eclipse also effects the moon's visibility. Large volcanic eruptions can dirty up the air by launching massive amounts of ash, dust and aerosols (mostly sulfuric acid) to great altitudes. The material absorbs sunlight and can noticeably darken the moon during totality.

That very thing happened back in January when a submarine volcano in Tonga erupted , ejecting a plume of debris that reached the unheard of altitude of 36 miles (58 km) — higher than the stratosphere! Sunday's eclipse will be the first opportunity to see to what degree, if any, the Tongan dust and ash will affect the moon.

Danjon scale
The Danjon scale, rated from "0" to "4," can help observers determine the brightness of the moon during totality. Make your observation at maximum eclipse.
Contributed / Bob King

Use the Danjon scale diagram to help you estimate the moon's brightness and color. Please share your results on my Facebook page . During one especially dark eclipse in the early 1980s, you couldn't even find the moon during totality unless you knew exactly where to look.

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I mentioned that the sky also turns dark at total eclipse. With the moon effectively cut off from sunlight, we'll have almost 90 minutes of true darkness. Watch for your shadow to disappear and the stars and Milky Way to return to view. Eclipse darkness feels very special, even a little sacred.

Eclipse timetable
Use this table to determine when the different phases of the eclipse begin for your time zone.
Contributed / Bob King

Be sure to look at the moon in binoculars during this time. Floating among the stars of Libra it will look almost three-dimensional. You'll also better appreciate the subtle colors that play across the disk — deep yellow, orange, rust, vermilion and brown. Prior to totality, telescope users may notice a blue border to the encroaching umbral shadow caused by the ozone layer, which absorbs red light.

The first sliver of light along the moon's left (east) edge alerts us to totality's end. The moon soon returns to partial eclipse but with phases in reverse as it slides out of Earth's shadow and back into sunlight. If you're interested in eclipse photography I recommend Fred Espenak's guide How to Photograph a Lunar Eclipse . Cell phones capable of time exposures up to several seconds should work well to capture the totally eclipsed moon along with the local skyline.

If you have bad weather, Italian astronomer Gianluca Masi will live-stream the eclipse on his Virtual Telescope site on May 16th starting at 2:15 Universal Time (9:15 p.m. CDT on May 15th). You'll find additional streaming options at Griffith Observatory and timeanddate .

Or do like I do when clouds threaten. Consult the latest satellite weather photos and maps, find the nearest clear sky, then get in the car and go! Here are a few helpful links:

Weather Network interactive satellite cloud viewer
U.S. 7-Day Cloud Cover Forecast
GOES-East and GOES-West live satellite images

Good luck and clear skies!

Read more from Astro Bob
Two fresh craters pock the moon after a rocket booster impact. We're still trying to figure out whose it is.

"Astro" Bob King is a freelance writer for the Duluth News Tribune.

Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune.
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