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.
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.
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.
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.