An almost total lunar eclipse

On November 19, 2021 (late evening on the 18th in some time zones), the Moon passes into the Earth’s shadow and creates a partial lunar eclipse so deep that it can reasonably be called almost total. Credit: NASA‘s Scientific Visualization Studio

What is an “almost total” lunar eclipse?

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

How can I observe the eclipse?

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

Almost total lunar eclipse November 2021 world map

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

What can I expect to observe?

All times are in UTC on November 19, 2021.

6:02 Penumbral eclipse begins The moon enters the penumbra of the Earth, the outer part of the shadow. The moon begins to dim, but the effect is quite subtle.
7:19 Partial eclipse begins The moon begins to enter the Earth’s umbra and the partial eclipse begins. To the naked eye, when the Moon moves into the umbra, it appears that a bite is being taken out of the lunar disk. That part of the Moon inside the umbra will appear very dark.
8:45 Red color becomes visible
9:03 Eclipse top The peak of the eclipse occurs at 9:03 UTC. This is the best time to see the color red.
9:20 Red color is no longer visible Redness fades as less than 95% of the Moon is in the Earth’s umbra. It appears that a bite has been taken out of the opposite side of the Moon from earlier.
10:47 Partial eclipse ends The entire moon is in the earth’s penumbra, but again the attenuation is subtle.
12:04 Penumbral eclipse ends The 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 at the top right, and the Hyades cluster – including the bright star Aldebaran, the bull’s eye – is at the bottom left. Here are some more skywatching tips for the month of November.

Why does the moon turn red during a lunar eclipse?

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 more easily scattered by particles in the 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 in the entire sky. But when the sun goes down, sunlight has to pass through more atmosphere and travel further before it reaches our eyes. The blue light from the Sun spreads away, and red, orange and yellow light with longer wavelengths pass through.

Moon Landscape Red Corona Earth

The lunar landscape, the foreground, is red with sunlight filtered through the Earth’s atmosphere. The sun is hidden by the Earth, but the glow from its corona is visible. Credit: NASA’s Scientific Visualization Studio

During a lunar eclipse, the Moon turns red because the only sunlight that reaches the Moon passes through the Earth’s atmosphere. The more dust or clouds in the Earth’s atmosphere during the eclipse, the redder the Moon will look. It is as if all the sunrises and sunsets of the world are being projected on the Moon.

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