Well, the first ‘blood moon’ of 2014’s come and gone. I missed it – the night sky where I live was socked in with 10/10 overcast at an altitude of about 200 metres.
Still, I’ll have another chance on 8 October. And another on 4 April 2015. And a fourth on 28 September that year.
Although unusual, it’s not a unique occurrence to have four eclipses in quick succession. Technically they’re known as a tetrad.
The reason why eclipses are a bit erratic is interesting. A lunar eclipse is simple enough – the Moon passes through the shadow of the Earth. The reason lunar eclipses don’t happen every 27 days, as the Moon orbits the Earth, is because the Moon doesn’t always pass through the shadow when it’s ‘behind’ the Earth relative to the sun. It would if everything was lined up flat on the same plane – but it isn’t.
In fact, the Moon’s orbit is tilted relative to the ecliptic – the plane in which Earth and Sun orbit. The tilt varies between 4.99 and 5.30 degrees. The two points at which the orbit intersects the ecliptic are known as ‘nodes’, and they move around the Moon’s orbital path – technically, ‘precess’ – at a rate of 19.3549° annually.
For an eclipse to occur, the node (‘ascending’ or ‘descending’) has to coincide with the point where the Moon would pass through Earth’s shadow (which is on the ecliptic). That happens every 173.3 days. An eclipse is possible at that time, though again, the orbital mechanics don’t always mesh exactly. There are more factors than just ecliptic and orbital angle. Earth’s shadow has a dense part (umbra) and a less dense part (penumbra). Sometimes there is only a partial eclipse. Sometimes it’s total.
The interlocking mechanisms of orbital mechanics – the way Earth, Sun and Moon all move in a complex dance of planes, angles and distances – means we end up with circumstance where strings of lunar eclipses – like the current tetrad – cluster. Between 1600 and 1900, for instance, there were no tetrads. But this coming century, there will be 8 of them.
So why red? The answer is one of the reasons why science is so cool. If you were standing on the Moon during a lunar eclipse, you’d see the Earth as a dark circle rimmed with fire – the light of every sunset and sunrise happening on Earth, all at once.
It’s red because of Rayleigh scattering – the way that the atmosphere scatters particular frequencies of light. I won’t repeat the explanation here – check out my earlier post. Suffice to say, when sunlight passes through a horizontal thickness of atmosphere, the red wavelengths are what emerge – and those red light wavelengths refract into the shadow of Earth, lighting the Moon in blood-red hues.
So when you see a ‘blood moon’, what you’re actually seeing is the reflected light of every sunrise and sunset on Earth, all at once.
And that, my friends, is the really neat thing about those eclipses. Harbingers of doom? To me it’s cool science, on so many levels.
Copyright © Matthew Wright 2014
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