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Total lunar eclipse – 15th June 2011

I am almost a little too excited about the lunar eclipse tonight. Why? Well it actually feels like a bit of a secret. A quick sweep earlier seemed that not that many people I know knew about it. Tonight, the sun, earth and moon will all be aligned – the Earth blocking the sun’s rays from hitting the moon.

You’ll notice the moon will appear a deep red colour this evening because even though the earth is acting as a massive solar shield, some light will still hit the moon by passing through the Earth’s atmosphere. Incoming light from the sun is refracted or bent and these refracted beams can still light up the moon which is only blocked from direct sunlight. When light is diffracted, more of the blue and green parts are scattered. That’s why sunsets are red too.

Tonight isn’t like any ‘normal’ lunar eclipse. The Moon will sink deeply into Earth’s shadow, passing almost directly through its centre. This means it’s going to be a dark eclipse. The darkest for almost 100 years and we’ll have to wait a further 47 years to witness similar. Totality lasts about 100 minutes – the longest for 11 years and almost 40 minutes longer than the one we had just before Christmas last year. On a scale which measures lunar eclipse brightness, the Danjon Scale (which ranges from L = 0 (nearly invisible) to L = 5 (bright coppery-red)), tonight will be a 4.8.

All this adds up to an amazing astronomical event.

The eclipse begins at 18.24 tonight. Totality actually starts at 20.22 and ends as 22.02. Unfortunately for us, it’s only going to be visible when the sun sets tonight at 21.19. If there are few clouds, you’ll be able to see the difference in the moon as soon as the sun sets. The Earth is currently far away from the sun in its orbit (aphelion) and the moon is relatively close to the earth in its orbit (perigee) so the eclipse will last longer than usual so there’s no excuse!

Now look south-east, pray for clear skies and enjoy the view!

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