Last September鈥檚 lunar eclipse was disappointingly dim. Now, it seems a Chilean volcano sucked some of the sparkle out of the spectacle.
During a lunar eclipse, the moon takes on a ruddy glow despite lying in Earth鈥檚 shadow. That鈥檚 because our atmosphere acts like a lens, refracting sunlight on to the moon.
The September 2015 eclipse, however, was widely felt to be dimmer than usual. Trained amateur astronomers from seven countries sent measurements of its colour to Richard Keen of the University of Colorado in Boulder, who crunched the data and concluded that the eclipse was 33 per cent less bright than expected.
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In a poster presented this week at the NOAA Earth System Research Laboratory in Boulder, Keen blamed this gloominess on the Calbuco volcano that erupted in April 2015. The eruption, last year鈥檚 largest, spewed ashes, dust and sulphuric acid droplets into the atmosphere.
Volcanic shadow
The volcanic pollution spread to the northern hemisphere, where it lingered for months, absorbing sunlight and dimming the eclipsed moon. 鈥淚t is like a single volcano on Earth has the ability to cast a shadow on the moon,鈥� Keen says.
The rays that illuminate the moon during eclipses mostly pass through the stratosphere, which stretches between about 10 and 50 kilometres above Earth鈥檚 surface. That means they are especially prone to being filtered out by volcanic particles, as these tend to settle higher in the atmosphere. Particles from pollution, fires or deserts rarely reach higher than the troposphere, the next layer down.
Keen has studied more than 30 lunar eclipses since 1960, and uses their brightness to gauge the dustiness of the stratosphere. Last September鈥檚 eclipse proved the first time Keen had seen changes to an eclipse from such a small volcano. 鈥淭he ability of this eclipse to identify such a relatively small eruption gives me confidence in using this technique,鈥� he says.
鈥淲e know that the moon鈥檚 surface temperature changes during an eclipse, and it鈥檚 really interesting to think that a single volcano on Earth can impact the temperature on the moon, even for a small amount of time,鈥� says of NASA鈥檚 Lunar Exploration Analysis Group. 鈥淭his goes to show that volcanoes have a global effect on the atmosphere and what goes into the atmosphere doesn鈥檛 just disappear straight away.鈥�
