The recent lunar eclipse was spectacular. From here on Earth, small telescopes showed the Earthsa国际传媒 shadow moving slowly across the Moonsa国际传媒 surface, crater by crater. That event was unique in that it was the first total lunar eclipse to be observed by the Blue Ghost spacecraft on the Moonsa国际传媒 surface. The image shows a red ring with a brilliant diamond, which was the Sun peeking past the edge of the Earth.
The Moon orbits the Earth at a slight angle to the plane in which our planet orbits the Sun. When the Moon is crossing that plane and happens to be on a line joining the Earth and Sun, the Earth can pass through the Moonsa国际传媒 shadow, giving us a solar eclipse, or the Moon can pass through the Earthsa国际传媒 shadow, giving us a lunar eclipse. Because the Earth is about four times the diameter of the Moon, our planetsa国际传媒 shadow is bigger, so lunar eclipses are more common than solar eclipses.
We are familiar with how lunar eclipses appear to us on the Earthsa国际传媒 surface. These always happen at the time of Full Moon, so before the eclipse begins we see our familiar Full Moon shining brightly in the sky. Then, as the Moon moves into the Earthsa国际传媒 shadow we see the edge of the dark shadow biting off one edge of the disc. Gradually more and more of the disc moves into the shadow, and then, after a while we see an edge of the disc brighten, as the Moon moves back out of the shadow.
However, during the period the Moon is totally in our planetsa国际传媒 shadow, something intriguing happens. Sometimes the lunar disc becomes a dull, ashy grey. On other occasions, it looks deep red, which is often referred to as a 鈥淏lood Moon鈥. It is easier to see what is going on by imagining we are observing the recent eclipse from the lunar surface, through the transparent dome covering the observation gallery of our imagined lunar base.
Initially we see the Sun in the sky with the Earth as a dark disc. Then, as the Sun moves behind the Earth, our planetsa国际传媒 atmosphere appears as a coloured ring surrounding the dark disc. The unhidden part of the Sun appears as a brilliant diamond, attached to the ring; then it vanishes behind the Earth leaving just that glowing ring. The colour of the light from that ring lights up the lunar surface around us with a blood red light.
When sunlight passes through a great thickness o the lowest part of our atmosphere, as it does at sunset, the shorter wavelengths of light, the blues and greens are scattered and absorbed, leaving the reds and yellows. These give us the beautiful colours of sunset. That light reaching us on the Moon has gone through an even bigger thickness of that sunset atmosphere, leaving us with just the red light.
Along with filtering out all colours other than red light, the atmosphere does one more thing to that light. It refracts it, bending it into the Earthsa国际传媒 shadow, so that although there is no direct sunlight hitting the Moon, it is illuminated with that red light, making the lunar disc have the colour of blood. Sometimes, when the atmosphere is particularly polluted, due to our activities or a volcanic eruption, the light passing through the atmosphere may be almost completely blocked, giving us an eclipsed lunar disc in a dull grey.
A long time ago I was at the observatory in the Algonquin Provincial Park, Ontario, and during an evening I was not working I went to a really dark area to watch a lunar eclipse. On that occasion the shadowed disc was so dark against the summer Milky Way it was like a hole in the sky. I was sweltering in a car with closed windows, because of the mosquitoes. At one point during the eclipse a family of foxes came out to play in front of the car, making the event particularly memorable.
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Venus now lies very low in the pre-dawn glow. After sunset Jupiter shines yellowish white high in the southwest and red Mars is high in the south. The Moon will be New on the 29th.
Ken Tapping is an astronomer with the National Research Councilsa国际传媒 Dominion Radio Astrophysical Observatory
