November’s Nearly Total Lunar Eclipse

This rare lunar event gave the moon reddish hues – and won’t happen again for 650 years!

For stargazers, the Moon is a perennial favorite. And if you looked up at the Moon in the early hours of Friday, Nov. 19, 2021, you were in for a stunning sight: a partial lunar eclipse blocked most of the light from the Moon, and turned the Moon as red as Mars!

The Moon - colored red because of a partial lunar eclipse - with a black sky
During November’s nearly total lunar eclipse, only a tiny sliver of the moon was illuminated – the rest of the Moon was cast in reddish hues. Photograph taken in southeast Pennsylvania.

The Partial (Nearly Total) Lunar Eclipse: Explained

Lunar eclipses occur when the lunar phase is a full moon, and when the Earth is exactly in between the Moon and Sun. When this happens, the Earth blocks sunlight from reaching the Moon (in other words, the Moon is in the Earth’s shadow). Since moonlight is mostly sunlight that gets reflected back to Earth, during a lunar eclipse the typically bright Moon will appear remarkably dimmer and fainter than usual.

In a total lunar eclipse, the entire Moon falls within the darkest part of Earth’s shadow, called the umbra. This eclipse was a partial lunar eclipse, meaning that there was an imperfect alignment of the Sun, Earth, and Moon – so the Moon only passed through part of the Earth’s umbra. However, NASA estimated up to 99.1% of the Moon was within Earth’s umbra – so it has often been described as a “nearly total” lunar eclipse.

Lastly, each lunar eclipse is visible from half of Earth. This particular partial lunar eclipse was easily visible from North America, and peaked at 9:03 UTC (4:03 EST).

What Makes This Partial Lunar Eclipse So Rare?

This partial lunar eclipse was historically rare since it lasted for a very long time (3 hours, 28 minutes, and 23 seconds). There hasn’t been a longer partial lunar eclipse since February 18, 1440, and there won’t be a longer partial lunar eclipse for another 648 years – until February 8, 2669!

Why Did the Moon Turn Red?

It’s the same phenomenon that makes our sky blue and our sunsets red, and it’s called Rayleigh scattering!

Different colors of light have different wavelengths. Blue light has a shorter wavelength whereas red light has a longer wavelength. Blue light is scattered more easily by particles in Earth’s atmosphere. Red light, however, can travel more directly through the atmosphere.

When the Sun is rising and setting, sunlight must pass through more of Earth’s atmosphere and travel farther before reaching our eyes. The blue light from the Sun scatters away, and colors with longer wavelengths (red, orange, and yellow) pass through. That is why sunrises and sunsets have reddish, orange, and yellow hues!

During a lunar eclipse, the Moon turns red because the only sunlight reaching the Moon passes through Earth’s atmosphere.

The best explanation for why the moon turns red during a lunar eclipse comes from Walter Lewin in this lecture video, starting at 1:09:24:

More Pictures of November’s Nearly Total Lunar Eclipse

I took several pictures of November 19th’s nearly total lunar eclipse, changing the exposure time to highlight either the reddish hues of the Moon (longer exposure time) or the tiny sliver of the Moon that was not within Earth’s umbra (shorter exposure time):

Did you see this partial lunar eclipse in the night sky? What other astronomical events have you seen? Let me know in the comments below!

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