Longest lunar eclipse in centuries happens Friday morning

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A blood red moon lights up the sky during a total lunar eclipse on April 4, 2015 in Auckland, New Zealand. (Photo by Phil Walter/Getty Images)

(NASA) – On November 19, 2021, in the very early morning hours, the Moon passes into the shadow of the Earth, creating a partial lunar eclipse so deep that it can reasonably be called almost total.

What is an “almost total” lunar eclipse?

A lunar eclipse occurs when the Sun, Earth, and Moon align so that the Moon passes into Earth’s shadow. In a total lunar eclipse, the entire Moon falls within the darkest part of Earth’s shadow, called the umbra. In this eclipse, up to 99.1% of the Moon’s disk will be within Earth’s umbra.

How can I observe the eclipse?

The best viewing will be right around the peak of the eclipse, on November 19th at 9:03 UTC/4:03 AM EDT/1:03 AM PDT. This part of the eclipse is visible in all of North America, as well as large parts of South America, Polynesia, eastern Australia, and northeastern Asia.

A world map showing where the eclipse is visible at the time of greatest eclipse. Earlier parts of the eclipse are visible farther east, while later times are visible farther west.
A world map showing where the eclipse is visible at the time of greatest eclipse. Earlier parts of the eclipse are visible farther east, while later times are visible farther west. Credit: NASA’s Scientific Visualization Studio

What can I expect to observe?

Listed times in bold are in UTC on November 19, 2021.

6:021:02 am EST/ 10:02 pm Nov. 18 PST on Nov. 18Penumbral eclipse beginsThe Moon enters the Earth’s penumbra, the outer part of the shadow. The Moon begins to dim, but the effect is quite subtle.
7:192:19 am EST/ 11:19 pm Nov. 18 PSTPartial eclipse beginsThe Moon begins to enter Earth’s umbra and the partial eclipse begins. To the naked eye, as the Moon moves into the umbra, it looks like a bite is being taken out of the lunar disk. The part of the Moon inside the umbra will appear very dark.
8:453:45 am EST/ 12:45 am PSTRed color becomes visibleMore than 95% of the Moon’s disk is in the umbra and the Moon will appear red. The color might be easier to see in binoculars or a telescope. Using a camera on a tripod with exposures of several seconds will bring out the color, at the expense of overexposing the lit part of the Moon.
9:034:03 am EST/ 1:03 am PSTEclipse peakThe peak of the eclipse occurs at 9:03 UTC. This is the best time to see the red color.
9:204:20 am EST/ 1:20 am PSRed color no longer visibleThe redness fades as less than 95% of the Moon is in the Earth’s umbra. It appears that a bite is taken out of the opposite side of the Moon from earlier.
10:475:47 am EST/ 2:47 am PSTPartial eclipse endsThe whole Moon is in Earth’s penumbra, but again, the dimming is subtle.
12:047:04 am EST/ 4:04 am PSTPenumbral eclipse endsThe eclipse is over.

What else can I see tonight?

During the eclipse, the Moon moves through the western part of the constellation Taurus. The Pleiades star cluster is to the upper right, and the Hyades cluster ― including the bright star Aldebaran, eye of the bull ― is in the lower left.

Why does the Moon turn red during a lunar eclipse?

Lunar Eclipse: View from the Moon
The landscape of the Moon, foreground, is reddened by sunlight filtered through Earth’s atmosphere. The Sun is obscured by the Earth, but the glow of its corona is visible. Credit: NASA’s Scientific Visualization Studio

The same phenomenon that makes our sky blue and our sunsets red causes the Moon to turn red during a lunar eclipse. It’s called Rayleigh scattering. Light travels in waves, and different colors of light have different physical properties. Blue light has a shorter wavelength and is scattered more easily by particles in Earth’s atmosphere than red light, which has a longer wavelength. Red light, on the other hand, travels more directly through the atmosphere. When the Sun is overhead, we see blue light throughout the sky. But when the Sun is setting, sunlight must pass through more atmosphere and travel farther before reaching our eyes. The blue light from the Sun scatters away, and longer-wavelength red, orange, and yellow light passes through.

During a lunar eclipse, the Moon turns red because the only sunlight reaching the Moon passes through Earth’s atmosphere. The more dust or clouds in Earth’s atmosphere during the eclipse, the redder the Moon will appear. It’s as if all the world’s sunrises and sunsets are projected onto the Moon.

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