North American observers can watch the Moon turn a reddish hue as it flirts with Earth’s shadow on the night of November 18–19 in the longest partial eclipse of the century.

Contacts:
Diana Hannikainen, Observing Editor, Sky & Telescope
+1 617-500-6793 x22100, diana@skyandtelescope.org

Gary Seronik, Consulting Editor, Sky & Telescope
gseronik@skyandtelescope.org

Susanna Kohler, Communications Manager and Press Officer, American Astronomical Society
+1 202-328-2010 x127, susanna.kohler@aas.org


Note to Editors/Producers: This release is accompanied by high-quality graphics; see the end of this release for the images and links to download.

Very late Thursday night and early Friday morning, you have a chance to watch the Moon darken and turn a reddish hue as it slips into Earth’s shadow. However, unlike the total lunar eclipse on May 26th this year, the November 18–19 event is a partial eclipse. But it’s a very deep one: About 99% of the lunar disk’s area (97% of its diameter) will pass through the umbra — the dark inner part of Earth’s shadow — leaving a tiny, silvery sliver of the Moon’s southern edge peeking out.

Weather permitting, viewers across almost all of North America will witness the entirety of the event. November’s eclipse favors night owls and early risers, because the full Moon passes through the umbra after midnight on November 19th for the four main U.S. time zones, and mid-eclipse occurs in the wee hours of the morning (at 4:03 a.m. EST and 1:03 a.m. PST). For Hawai’i and Alaska, the event starts to unfold before midnight. 

Northeasternmost Asia will also see the entire event. In South America the eclipse happens as dawn is brightening, while for most of Australia the Moon will already be eclipsed as it rises. Much of western Europe should see the early penumbral stages before the Moon dips below the horizon.

What to Look For

The partial eclipse is predicted to last longer than usual, nearly 3½ hours. In fact, not only will it be the longest partial lunar eclipse this century, it’s also the longest partial lunar eclipse between February 18, 1440, and February 8, 2669. It happens 1.7 days before the Moon reaches its apogee, or the farthest point from Earth, in its orbit. This means that the Moon’s orbital motion will be slower, thereby increasing the duration of the event.

“The Moon will be in Taurus and pleasingly placed some 6° — approximately the width of three fingers held together at arm’s length — lower left of the pretty Pleiades open star cluster at the time of maximum eclipse,” says Diana Hannikainen (pronounced HUHN-ih-KY-nen), observing editor at Sky & Telescope. “This should provide great opportunities for some fun photos.”

The stages of the eclipse occur simultaneously for everyone who can see the Moon, but the actual clock times depend on your time zone. See the table below and diagrams at the end for times of key events for the lunar eclipse:

Eclipse EventUTASTESTCSTMSTPSTAKSTHST
Moon enters penumbra
6:002:00 a.m.1:00 a.m.12:00 a.m.*11:00 p.m.*10:00 p.m.*9:00 p.m.*8:00 p.m.
Partial eclipse begins7:183:18 a.m.2:18 a.m.1:18 a.m.12:18 a.m.*11:18 p.m.*10:18 p.m.*9:18 p.m.
Maximum eclipse9:035:03 a.m.4:03 a.m.3:03 a.m.2:03 a.m.1:03 a.m.12:03 a.m.*11:03 p.m.
Partial eclipse ends10:476:47 a.m.5:47 a.m.4:47 a.m.3:47 a.m.2:47 a.m.1:47 a.m.12:47 a.m.
Moon exits penumbra12:067:06 a.m.6:06 a.m.5:06 a.m.4:06 a.m.3:06 a.m.2:06 a.m.
*November 18th. Eclipse phase times are shown for the contiguous U.S. time zones as well as Alaska (Alaska Standard Time / AKST) and Hawai‘i (Hawai‘i-Aleutian Standard Time / HST).

Mechanics of a Lunar Eclipse

A lunar eclipse happens when the Sun, Earth, and a full Moon form a near-perfect lineup in space, in what is known as syzygy. The Moon gradually glides into Earth's shadow, until most of the lunar disk turns from silvery grey to an eerie dim orange or red. Then events unfold in reverse order, until the Moon returns to full brilliance.

The events that happen during a total lunar eclipse are more complex and interesting than many people realize. The eclipse occurs in five stages, each with different things to watch.

(1) The Moon's leading edge enters the pale outer fringe of Earth's shadow: the penumbra. You probably won’t notice anything until the Moon is about halfway across the penumbra. Watch for a slight darkening on the Moon's leading edge. The penumbral shading becomes stronger as the Moon moves deeper in.

The penumbra is the region where an astronaut standing on the Moon would see Earth covering only part of the Sun's disk.

(2) The Moon's leading edge enters the umbra, the cone of Earth's shadow within which the Sun’s completely hidden. You should notice a dramatic darkening on the upper left edge of the lunar disk as seen from North America. With a telescope, you can watch the edge of the umbra slowly engulfing one lunar feature after another, as the entire sky begins to grow darker.

(3)  The trailing edge of the Moon slips into the umbra for the beginning of maximum eclipse. The Moon will glow some shade of coppery red or burnt orange.

Why is this? The Earth’s atmosphere scatters and bends (refracts) sunlight that skims its edges, diverting some of it onto the eclipsed Moon. It’s the same effect that happens at sunset. If you were on the Moon during a lunar eclipse, you’d see the Sun hidden by a dark Earth rimmed with the reddish light of all the sunrises and sunsets ringing the world at that moment.

The red umbral glow can be quite different from one eclipse to the next. Two main factors affect its brightness and hue. The first is simply how deeply the Moon goes into the umbra as it passes through; the center of the umbra is darker than its edges. The other factor is the state of Earth's atmosphere. If a major volcanic eruption has recently polluted the stratosphere with thin global haze, a lunar eclipse can be dark red, ashen brown, or occasionally almost black.

In addition, blue light is refracted through Earth's clear, ozone-rich upper atmosphere, above the thicker layers that produce the red sunrise-sunset colors. This ozone-blue light tints the Moon also, especially near the umbra's edge, but you’ll need binoculars or a telescope to see this effect.

During this deep partial, a sliver of the southern edge of the Moon’s disk will shine brightly even at mid-eclipse.

(4) As the Moon continues moving along its orbit, events replay in reverse order. The sliver of light will grow and expand upwards along the Moon’s leading edge.

(5) When all of the Moon escapes the umbra, only the last, penumbral shading is left. Some time later, nothing unusual will remain visible.

"You only need your eyes to see the drama unfold, but if you have binoculars or a backyard telescope, they'll give a much-enhanced view," says Hannikainen.


Read more on this eclipse in the November 2021 issue of Sky & Telescope and on S&T's website: A Near-Miss Total Lunar Eclipse.

If it's cloudy where you are, you can follow a live stream of the eclipse on the Virtual Telescope website starting at 7:00 UT on November 19th.


Sky & Telescope is making the illustrations below available to editors and producers. Permission is granted for nonexclusive use in print and broadcast media, as long as appropriate credits (as noted) are included. Web publication must include a link to skyandtelescope.org.


Lunar Eclipse November 2021 UT
Events for the deep partial lunar eclipse on the morning of November 19, 2021. This version is labeled for Universal Time (UT). Other versions are available for EST, CST, MST, PST, and unlabeled. Please refer to the table above for corresponding times in various time zones. Click here or on the image for a higher-resolution version.
Gregg Dinderman / Sky & Telescope; Source: USNO
Lunar Eclipse November 2021  coverage map
The map shows locations worldwide from which the November 18–19 lunar eclipse is visible, weather permitting. Because an eclipsed Moon is always full, the Moon sets (or rises) at almost the same time as the Sun rises (or sets) on the opposite horizon. Click here or on the image for a higher-resolution version.
Gregg Dinderman / Sky & Telescope; Source: USNO
Why does the Moon turn red? When the Moon falls in Earth’s shadow, not all sunlight is blocked. A sliver of light will travel through Earth’s atmosphere, refracting — or bending — toward the Moon to then reflect off its surface. For the same reason that the sky is blue, the blue wavelengths of sunlight are scattered in Earth’s atmosphere. Longer, redder wavelengths can pass through without scattering — this is the same phenomenon that turns the Sun red when it’s near the horizon. These photons are the ones that make it through to shine on the Moon. In a sense, the Moon is seeing all of Earth’s sunrises and sunsets all at once. Rollover the icons to see mid-eclipse views from Earth and from the Moon. Click here for a high-resolution, annotated diagram.
Sky & Telescope
Before Image After Image
Why isn't every full Moon an eclipse? The Moon’s orbit around Earth is tilted a small amount, 5.14°, compared to the plane of Earth’s orbit around the Sun (exaggerated in the figure for clarity). That means that most times, as the Moon orbits Earth, it evades Earth’s shadow — and when the Moon is opposite the Sun, we see it as a full Moon on Earth, its whole disk shining. However, roughly twice a year, the full phase of the Moon coincides with a node, where the Moon’s orbital plane crosses Earth’s orbital plane around the Sun. Depending on how close the node is to the full Moon, we may see a partial or total lunar eclipse at those times. Use the slider to see how the setup changes.
Sky & Telescope
Earth's umbra at Moon
Earth's umbra at the Moon's distance spans about 9,000 kilometers or 1.3°, which is 2.6 times the diameter of the Moon. Given the Moon's orbital tilt of 5.1° it's easy to see why eclipses don't happen every full Moon. (Earth's umbra is shown here as a dark circle to demonstrate the concept, but it wouldn't be visible in this way during the eclipse itself.)
Bob King

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