Eclipsed Harvest Moon
Ken Tapping, 16th September, 2015
The next Full Moon, which will happen on the 27th, will be the “Harvest Moon”. For centuries this Full Moon has provided the extra light needed by farmers to get in the harvest. This year the Harvest Moon will have an additional attraction for sky watchers, although maybe not for farmers. It is going to be eclipsed by the Earth.
An eclipse of the Moon happens when its path around the Earth takes it through the Earth’s shadow. Since we only see the Moon and planets against the blackness of space because they are lit by the Sun, we might expect that as the sunlight is blocked by the Earth, the Moon will completely vanish, but something else is going on too. This is because our planet has an atmosphere. This bends some of the sunlight inward into the Earth’s shadow’ illuminating the otherwise darkened Moon. If we were standing on the Moon looking at the Earth instead of a bright blue marble we would see a black disc surrounded by a brightly glowing ring. It is what is going on in this glowing ring that makes every eclipse of the Moon different, but always spectacular and worth seeing
We are all familiar with the golds and reds of the sunset. These happen because when the Sun is low in the sky, the sunlight has to pass through a lot more atmosphere than when the Sun is high in the sky. Sunlight passing through the atmosphere is subject to scattering, where small particles, gas molecules, water vapour droplets and dust redirect the light out of the sunbeam away in random directions, weakening it. However, the extent to which this happens depends strongly on its wavelength – its colour. Blue light has a short wavelength and is much more strongly scattered than red light, which has a longer wavelength. The other colours: orange, yellow and green have intermediate wavelengths. The strongly scattered blue light makes our daytime sky blue. By the time the light from the setting Sun has found its way though many kilometres of atmosphere, most of the blue light has been removed, leaving the red and orange light, producing the beauty of sunsets.
By the time the atmosphere has directed sunlight towards the eclipsed Moon, that light has passed through about twice as much atmosphere as is the case with sunsets, so it is even redder; our eclipsed Moon will be red – like a glowing copper coin. However, there is more to the story. Volcanic eruptions, big sandstorms and human-generated pollution can put more and bigger particles into our atmosphere. This can make the scattering so severe that all colours are equally scattered. In an extreme case, instead of glowing copper-red, the eclipsed Moon will be a dim, ashy grey.
Years ago, there was an eclipse of the Moon while I was visiting the observatory in Algonquin Park, Ontario. The atmosphere was unusually murky, and the Moon was a dull grey, darker than the surrounding Milky Way, so that it looked like a hole. I was watching from inside a car because the mosquitoes and black flies were out in force. While I was watching the eclipse a fox and her cubs came out to play in front of the car, so I had two spectacles to watch.
On the 27th, at 20:11 EDT the Moon will start to enter the outer shadow. At 21:07 EDT the Moon will start to move into the full shadow. By 22:11 EDT the Moon will be fully in shadow. By that time (19:11 PDT) the Moon will have risen in Western Canada, and people in BC can start watching the show. The Moon will stay fully in shadow until 23:23 EDT (20:23 PDT), when it will start moving out. It will be out of full shadow by 00:27 EDT (On the 28th), which is 21:27 PDT (Still on the 27th). The Moon will leave the outer shadow at 01:22 EDT) 22:22 (PDT) and the show will be over.
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