The science behind this year’s blood moons

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.

US Navy photo of a total lunar eclipse in 2004, by Photographer's Mate 2nd Class Scott Taylor. Public domain, via Wikipedia.
US Navy photo of a total lunar eclipse in 2004, by Photographer’s Mate 2nd Class Scott Taylor. Public domain, via Wikipedia.

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.

Colour photo of the Moon taken by the Galileo probe in 1990 - a view we never see from Earth. The - uh - 'dark side' is to the left, fully illuminated. NASA, JPL, public domain.
Colour photo of the Moon taken by the Galileo probe in 1990 – a view we never see from Earth. The – uh – ‘dark side’ is to the left, fully illuminated. NASA, JPL, public domain.

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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10 thoughts on “The science behind this year’s blood moons

    1. I was brought up with physics. Literally. For me the coolest thing about lunar eclisps is that the red is the reflected colour of every single sunrise and sunset on Earth. All at once.

  1. I have a lot to learn from you, interesting post and good explanation.
    I like the fact that i can see your ”shameless” plug a dozen times scrolling down your main page. I would probably do the same too so i’m not saying that i’m better.

    1. I said it was shameless. Actually, a fair number of visitors arrive on specific pages, so I’m adding it to each post while I’m running that particular promotion.

  2. Speaking of harbingers of doom, just yesterday I saw a Facebook comment from someone in some or other messianic group that this is the first of four before “Yeshua’s” return. I also believe in God, but that doesn’t mean I have to see mysterious signs in perfectly natural phenomena. It’s not as if we don’t have a perfectly rational explanation for what’s happening, but I’m running the risk of mounting a hobby horse here, so let me rather shut up.

    We also missed it due to cloudy skies, and I think at the time it happened the moon was a bit too low in the sky over here to observe it properly anyway. Let’s hope conditions are optimal for us both come the next one.

    1. I agree. Isaac Newton, whose theory basically describes the orbital mechanics behind these eclipses, certainly believed in God and was (at least at first) a devout Anglican.

      Yeah, we’ve got three chances with the weather for this tetrad…one of the must pan out!

    1. It IS cool. Here in NZ it was at a convenient time – just after dark. But inconveniently hidden by weather (sigh). There’ll be another one later this year! 🙂

  3. HI Matt-
    Susie sent me over from the party.

    Very cool explanation of the blood moon. I was disappointed to miss it here in Maine, USA because of overcast skies. I got up at 4 a.m. and the sky had this cool pinkish tinge but alas, clouds had rolled in. So I am marking October 8 on my calendar!

    1. Me too – it’s a wonderful sight & not to be missed in Earth’s otherwise slightly dull sky-scape. Thanks for popping over. Susie throws great blog parties!

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