A night in the shadow of the Earth

At about 3:30 in the morning (UK time) on the 21st February (that’s Thursday morning this week) astronomers will get the chance to witness the Moon completely immersed in the darkest part of the Earth’s shadow – an event astronomers call a total lunar eclipse. So here are my top ten facts and things you need to know to observe and understand what happens in this fascinating celestial event.

Planetary positions
Lunar eclipses can only happen when the Sun, Moon and Earth are exactly aligned. The Earth must be between the Sun and the Moon so that the Moon is full i.e. opposite the Sun in the sky. The Moon’s orbit is slightly angled with respect to the plane of the Earth’s orbit. For this reason the Moon also has to be in a special position (where it is aligned with the plane of the Earth’s orbit) otherwise we would get lunar eclipses at every Full Moon!

Wrap up warm
The golden rule of observational astronomy is make yourself comfortable. Lunar eclipses last hours and it can get pretty chilly under clear crisp dark skies. Make sure you wear lots of layers and have a warm drink and some food to hand. The worst thing, I find, is having to cut short a great observing session because you can’t feel your fingers!

Know your umbra from your penumbra
The Earth’s shadow is made of two parts. The outer lighter part is called the penumbra. As the Moon moves into the penumbra you begin to see the edge of the Moon dim and eventually get ‘eaten away’- this is called the partial phase. The centre of the Earth’s shadow is called the umbra and it is much darker. When the Moon starts to enter the umbra it rapidly begins to disappear until it is completely immersed, usually going a deep red colour – this is the ‘total’ phase.

Why does it go red?
The Moon goes red for the same reason the sky is blue. The Earth’s atmosphere scatters blue light. So as sunlight passes through our atmosphere most of the blue wavelengths are scattered away leaving predominantly red light to get through. This goes out into space and lights the Moon in a deep red glow.

But hang on if the Moon is in shadow why is it bathed in red light?
The simple answer is that the Earth’s atmosphere refracts (or bends) light around it so that even though the Moon is in the Earth’s shadow the bent red light is directed towards the darkened Moon.

Look out for the blue fringe
I only found out about this during the lunar eclipse on 3rd March last year. As light passes through Earth’s lower atmosphere it is reddened but as light passes through Earth’s upper atmosphere the ozone gas there absorbs light at red wavelengths. This can sometimes result in a noticeable blue fringe around the edge of the eclipsed Moon. I saw it during last years eclipse and you can see a slight hint of it in my image above.

How dark is the Moon?
How dark the Moon is during the total phase of the eclipse generally depends on how much dust and pollution there is in Earth’s atmosphere. You can estimate the darkness of the Moon using the Danjon scale. On the five-point Danjon scale 0 is a total phase where you can hardly see the Moon at all whilst 4 is a total phase where the Moon goes a bright coppery red.

Set your alarms!
February’s eclipse comes at a fairly unsociable time. Mid-eclipse when the Moon is in the darkest part of the Earth’s shadow is at about 3:30 in the morning. If you want to watch the whole eclipse you will have to start watching at about 00:40 on the 21st when the Moon begins to enter the Earth’s shadow. The Moon enters the umbra at about quarter to two and leaves it at about ten past five. Finally the Moon returns to normal as it completely leaves the Earth’s shadow at about quarter past six!

Grab a friend
Eclipses are definitely a social event so why not grab a few friends to share in the experience; even if it is just sitting in darkness waiting for completely cloudy UK skies to clear! ;-)

The great thing about lunar eclipses is that you don’t need any special equipment to observe them. If you want to, it’s useful to use a pair of binoculars to get a closer look but these are by no means a prerequisite.

Good luck and clear skies!

Above: The total lunar eclipse of 3rd March 2007; copyright Will Gater.

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