Thursday, 24 June 2010

Partial Lunar Eclipse Tomorrow

Tomorrow (Saturday June 26th) at around 10:30pm, make sure you get outside to see if the Moon is visible. The weather forecast isn't promising but if you can see it, the Moon will be in partial lunar eclipse until around 12:30am. The best view will be around 11:30 - 11:50pm.

What will it look like?
Lunar eclipses only happen at full moon, so that's what you'll see first. Soon after 10:30pm you'll see a darkening and reddening in the bottom left area of the Moon. Over the next hour this spreads until about half the Moon's surface will hopefully look a really spooky red colour. By 1:00am it will have returned to normal.

Partial Lunar Eclipse
Simulation using free Stellarium software

What's happening?
The Moon is passing through the edge of the Earth's shadow, which is what makes half the Moon go dark. The interesting effect is the redness - that's caused by sunlight passing through and being refracted by the Earth's atmosphere. Blue light tends to scatter in our atmosphere, leaving the red wavelengths of light to continue towards the Moon.

This is the same effect that gives us a blue sky (from scattered sunlight) and red sunsets (from light travelling through more atmosphere, losing its blue light and leaving the red).

The first image below shows how our atmosphere splits up sunlight like a prism, and how the Moon becomes red as it goes through the eclipse. The second image shows an imaginary view from the Moon, with the Sun hidden behind Earth.

Lunar Eclipse

Image credit: Adapted by Dave Owen from public domain files at

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