Why Was The Lunar Eclipse So Dark?

While dark, the moon showed healthy color with a thick, yellow rim and shades of orange and deep red-brown. This photo was taken in Olympic National Park in Washington on September 27, 2015. Credit: Rick Klawitter
While dark, the moon never lacked for color with a thick, yellow rim and shades of orange and deep red-brown. This photo was taken in Olympic National Park in Washington on September 27, 2015. Credit: Rick Klawitter

If you saw the recent total lunar eclipse, you may have been struck by how dark it looked. I noticed something was up when I pointed my binoculars at the moon during totality. Normally, binoculars make the orange moon jump right out. Not this time. My 8x40s couldn’t pull much light from the lunar countenance. It was dim to the naked eye as well, especially the “top” or western two-thirds of the disk, the portion closer to the center of Earth’s shadow.

During a perigee eclipse, the moon passes more deeply into Earth's shadow compared to apogee. Moon distances not to scale and for illustration only. Credit: Bob King
During a perigee eclipse, the moon passes more deeply into Earth’s shadow compared to one that happens near apogee, when the moon is most distant from Earth. Moon distances not to scale and for illustration only. Credit: Bob King

You might recall it was a “Supermoon” or perigee moon at eclipse time, when our satellite is closest to Earth. Now it appears, its proximity may have played a role in the moon’s faintness. Here’s an observation from atmospheric scientist Richard Keen of the University of Colorado, who commented on the moon’s appearance in the blog a few days ago:

“I’m still collecting reports, but so far it appears the eclipse – predicted to be relatively dark due to the “Supermoon” that placed the moon deeper into the Earth’s shadow cone – may have been even a little bit darker.” This makes total sense upon reflection. Earth’s shadow narrows the farther back of the planet it goes. An apogee moon (farthest from Earth) would cross a narrower cone of shade closer to the border between shadow and sunshine. A moon nearer Earth would find the shadow roomier with its borders further from sunlight.

Spreading ash cloud from the Calbuco volcano photographed from the space station 250 miles up. Credit: NASA
Spreading ash cloud from the Calbuco volcano photographed from the space station 250 miles up in late April this year. Credit: NASA

Other factors may have also been at play including lingering aerosols — ash, dust, sulfuric acid droplets — from Chile’s Calbuco volcano, which erupted on April 22 this year. Although most of that earthly debris remained aloft in the southern hemisphere, some may have spread north. Sunlight has to pass through these light-absorbing minerals and chemicals on its way through the atmosphere and into the planet’s shadow. Less light means a darker moon during total eclipse.

The darkness of the moon during the recent eclipse may have been caused by a variety of natural and man-made factors. Credit: John Chumack
The darkness of the moon during the recent eclipse may have been caused by a variety of natural and man-made factors. Credit: John Chumack

Finally, another contributor to a fainter moon may lie closer to home. Forest fires. All spring and summer, they raged across the western states and Canadian provinces. Most of that smoke usually stays in the lower part of the atmosphere, but some may have found its way to the stratosphere, the very layer responsible for transmitting most of the sunlight that falls into Earth’s shadow and colors the moon.

Finally, a general rise in man-made sulfate pollution may also have contributed a share. Keen is taking yours and other observers’ eclipse observations to see how the moon’s faintness might be related to any or all of the factors we touched on.

4 Responses

  1. Bill Gucfa

    Hi Bob,

    Have you got an estimate of the umbral shadow magnitude on this eclipse?
    I’m wondering if it beat out the dark eclipse of December 30, 1963 (My first lunar eclipse) The temperature was only 4 degrees F. but the observation was well worth it!

    Thanks!

    Bill

    1. astrobob

      Bill,
      Richard Keen crunched the data and it now looks like the L value was 1.9. The predicted value had been 2.5, so it was a dark but not the darkest eclipse. Not like the ones in ’63 and ’82.

  2. Richard Keen

    Bill, were you in Philadelphia for the 1963 dark lunar eclipse? I was a teenager watching it, and it was also 4 degrees. A knuckle froze briefly to the cast iron telescope mount, which is the only time in my life I got frostbitten. But as you say, it was well worth the observation of such a unique event. I can even say it influenced my future career – 50+ years later in retirement, I’m still working on that eclipse!
    The 1963 eclipse had an average reported Danjon L of 0.2, the darkest eclipse since one in 1816 after Tambora went off. The ’63 eclipse (dark because of the Agung eruption in Indonesia) was 4th magnitude, more than 100 times dimmer than this week’s eclipse (about -1.2 magnitude), and 1000 times dimmer than the really bright lunar eclipse last April. The 1982 and 1992 eclipses following el Chichon and Pinatubo were about a magnitude brighter than the ’63 eclipse.

    1. Bill Gucfa

      Hey Richard,

      Actually, I wasn’t too far away. I was in Pawtucket, RI.
      My mother and sister chipped in and bought me a Christmas present that
      would change my life.
      It was a brand new 2.4 inch refractor. Believe it or not, I still have it!
      I’ll never forget that eclipse.
      Many thanks to you and Bob for the great information.

      Bill

Comments are closed.