The September 17, 2015 total lunar eclipse - the last of the recent tetrad of lunar eclipses over the past 17 months - was darker than expected. Several factors described below were in play. This photo was taken in Washington's Olympic National Park. Credit: Rick Klawitter

Why Was September’s Lunar Eclipse So Dark?

Article Updated: 23 Dec , 2015
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First off, a huge thank you to everyone who made and sent their Danjon scale estimate of the totally-eclipsed Moon’s brightness to Dr. Richard Keen, University of Colorado atmospheric scientist. Your data were crucial to his study of how aerosols in Earth’s atmosphere and other factors influence the Moon’s appearance.

Grateful for your help, Keen received a total of 28 observations from 7 different countries.

Graphs created by Dr. Richard Keen plotting Danjon L values submitted by Universe Today readers and others that compare expected values (top curve) with observed values. The Moon was about half as bright during totality as expected with L=1.9. Credit: Dr. Richard A. Keen

Graph created by Dr. Richard Keen plotting Danjon L values submitted by Universe Today readers and others that compare predicted values (top curve) with observed values. The Moon was about half as bright during totality as expected with L=1.9. Credit: Dr. Richard A. Keen

Using the Danjon information and estimates of the Moon’s brightness using the reverse binocular method, Keen crunched the data and concluded that the Moon was about 0.6 L (Danjon) units darker than expected and 0.4 magnitude dimmer, a brightness reduction of 33%. This agrees well with my own observation and possibly yours, too. No wonder so many stars sparkled near the Moon that night.

Depending on how clear the atmosphere is, the Moon's color can vary dramatically from one eclipse to another. The numbers, called the Danjon Scale, will help you estimate the color of Sunday night's eclipse. Credit: Bob King

Lunar eclipse brightness is rated on the Danjon scale where “0” equals a dark gray totality and “4” a bright, coppery yellow. Credit: Bob King

I think it’s safe to say, most of us expected a normal or even bright totality. So why was it dark? Several factors were at play — one to do with the Moon’s location in Earth’s shadow, the other with a volcanic eruption and a third with long-term, manmade pollution.

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

During a perigean 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’ll recall that the eclipse occurred during lunar perigee, when the Moon swings closest to Earth in its 27-day orbit. Being closer, it also tracked deeper into Earth’s umbra or inner shadow which narrows the farther back of the planet it goes. An apogean Moon (farthest from Earth) passes through a more tapered cone of darkness closer to the penumbra, where sunlight mixes with shadow. A Moon nearer Earth would find the umbral shadow roomier with the light-leaking penumbra further off in the distance.

Around midday on April 24, 2015, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this natural-color image of the ash and gas plume from Calbuco volcano in southern Chile. Credit: NASA

On April 24, 2015, NASA’s Terra satellite acquired this photo of the ash and gas plume from Calbuco volcano in southern Chile. Credit: NASA

But there’s more. Working independently, Steve Albers of NOAA and Brazilian astronomer Helio Vital suggested another reason: aerosols in the atmosphere. “Earth’s stratosphere is no longer completely clean of volcanic ashes,” said Vital in an e-mail communication. “In fact, lingering aerosols (ash, dust, sulfuric acid droplets) from the explosion of Calbuco five months ago may be to blame for that excessive darkening.”

With the lunar horizon in the foreground, the Earth passes in front of the Sun on September 27, 2015 in this simulation, revealing the red ring of sunrises and sunsets along the limb of the planet responsible for illuminating the Moon during the eclipse. The clarity of the stratosphere at eclipse time can greatly affect lunar brightness during totality. The Earth and Sun are in Virgo for observers on the Moon with the bright star Beta Virginis at top. Click to see the video. Credit: NASA's Scientific Visualization Studio

With the lunar horizon in the foreground, the Earth passes in front of the Sun on September 27, 2015 in this simulation, revealing the red ring of sunrises and sunsets along the limb of the planet responsible for illuminating the Moon during the eclipse. The clarity of the stratosphere at eclipse time can greatly affect lunar brightness during totality. The Earth and Sun are in Virgo for observers on the Moon with the bright star Beta Virginis at top. Click to see the video. Credit: NASA’s Scientific Visualization Studio

While much of the debris blasted into the stratosphere made for colorful sunsets in the southern hemisphere, some of that material has likely made its way to the northern hemisphere. Albers has noticed an increase in yellow and purple sunsets in his home town of Boulder in recent months, telltale signs of volcanic spew at play.

Forest fires that raged across the western states and Canadian provinces all spring and summer may also have contributed. 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.

Graph showing magnitude estimates of the Moon's brightness during totality using the reverse binocular method. The predicted magnitude was -1.7 (a little brighter than Sirius) vs. the observed -1.3. Credit: Dr. Richard A. Keen

Graph showing magnitude estimates of the Moon’s brightness during totality using the reverse binocular method. The predicted magnitude was -1.7 (a little brighter than Sirius) vs. the observed -1.3. Credit: Dr. Richard A. Keen

Sunlight has to pass through these light-absorbing minerals and chemicals on its way through the atmosphere and into Earth’s shadow. Less light means a darker moon during total eclipse. Coincidentally, much of the totally eclipsed Moon passed through the southern half of the umbra which “increased the effectiveness of the Calbuco aerosols (which are still more concentrated in the southern hemisphere than the northern) at dimming the light within the umbra,” writes Keen.

Oceanus Procellarum and Mare Imbrium are large, dark volcanic plains that contributed to the Moon's faintness and dark-hued totality. Credit: Bob King

Oceanus Procellarum and Mare Imbrium are large, dark volcanic plains that contributed to the Moon’s faintness and dark-hued totality. Credit: Bob King

It also so happened that the darkest part of the moon coincided with two vast, dark volcanic plains called Oceanus Procellarum (Ocean of Storms) and Mare Imbrium, artificially enhancing the overall gloom over the northern half of the Moon.

U.S. satellite-derived map of PM2.5 averaged over 2001-2006. Credit: Dalhousie University, Aaron van Donkelaar

U.S. satellite-derived map of PM2.5 (fine particulate matter which includes sulfates and soot) averaged over 2001-2006. Credit: Dalhousie University, Aaron van Donkelaar

Finally, the human hand may also have played a role in lunar color and brightness. The burning of coal and oil has caused a gradual increase in the amount of human-made sulfate aerosols in the atmosphere since the start of the industrial revolution. According to NASA, at current production levels, human-made sulfate aerosols are believed to outweigh the naturally produced sulfate aerosols. No surprise that the concentration of aerosols is highest in the northern hemisphere where most industrial activity is found.

Isn’t it fascinating that one blood-red Moon can tell us so much about the air we breathe? Thank you again for your participation!

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11 Responses

  1. Brian Sheen says:

    Thanks for the update, we came up with 2 on the Danjon scale so happy with that.
    One extra factor which affected our observation was that it had been dry with very little wind for several days so our atmosphere had more than its fair share of muck & bullets ie dust.

    Rosaeland Observatory.

  2. dorvinion says:

    What about forest fires?

    There were quite a few big ones this year in the Western US and Canada in August and Sept

    • Bob King says:

      Dorvinion,
      Thanks for the reminder! Yes, I had originally planned to include that possibility but forgot to add. My oversight. It’s in there now.

  3. UFOsMOTHER says:

    This is when I wish we had a permanent base on the Moon, What a view it would have been looking at Earth passing in front of the Sun on Eclipse day (and it would have lasted for hours) Also the 0.7L Danjon units darker than expected with all the other information matches exactly what we saw from Belgium so thanks Bob for the updates and your explanation, well done!

  4. Smokey says:

    There are several reasons why a data set such as this one is fun to do, but not usually very informative on the grand scale. Rather than pick it apart, I’ll just remind everyone gently that small amounts of subjective, inconsistent, non-verifiable data over the course of a single event are HARD to derive significant conclusions from, even if they can bat their proverbial eyelashes quite suggestively.

    None of it is to take away from anyone’s recording efforts or observations, and the data collected — unless totally fabricated, which I highly doubt! — is certainly “valid.” People saw what they saw, and they recorded it, which is a thing of awesome beauty! All I’m saying is that it’s difficult so say whether this “valid” sample is, at the same time, “significant.” If a scientist were to come along later and attempt to interpret these results, s/he might have a serious time of it trying to decide whether this event really WAS darker than it was expected to be, or (e.g.) whether the preceding hype, global atmospheric conditions & local light pollution produced a situation in which the eclipse was merely perceived to be darker than expected by those who observed it.

    Of course, if everyone had reported seeing the Moon catch fire and burn with a bright green flame during totality, THAT could probably have been considered “significant!” (The fact that only one or two of you out there saw that happen… yeah, sorry, not so much. ~_^)

  5. BlackWolfStanding says:

    Just a question: Anybody above or below the Arctic or Antarctic circle see this moon? It would be interesting to see. And what about the people on the ISS. Did they record the moon and was it significantly brighter from space? There are two times the Atmosphere plays a part in the darkness of the moon. When it bends the light to brighten up the moon or when direct observations through it. If the atmosphere played an important part then from space or from the polar regions, the moon should have been brighter even if by marginal proportions.

  6. BCstargazer says:

    Thank you Bob. It’s good to know that amateurs can play a (subdued) role in Astronomy.
    I agree with you: a combination of factors contributed to the dimness of Luna. Maybe it’s just my brain trying to find a pattern, but I noticed that during the last few pre-dawn rocket launches, high altitudes contrails were unusual in a spectacular way. Perhaps we could ask Dr. Kramer for his opinion on that detail.

  7. ntjiwaji says:

    The date in the caption of the first picture should be 27 September instead of 17.

  8. mikeu says:

    At the Ladd Observatory we measure zenith sky brightness using a SQM mounted on the roof. During totality the measurements were between 18.7 to 18.8 magnitude per square arc second, which is quite dark for Providence.

    There’s a graph at https://twitter.com/W9GYR/status/648525959214301184

Comments are closed.