Imagine you are outside your house breathing some fresh air and enjoying the night view around you. You look up at the sky, hoping to catch a star or two, but what you saw instead was a reddish orangey full moon hanging in the sky. What happen to our Moon?!?
No fear, there is nothing wrong with our Moon – our Moon is as normal as it always is. It’s just passing through the Earth’s shadow, and we call that a lunar eclipse.
This is going to happen on Saturday (Dec 10), on a convenient time for us (in Malaysia). The Moon starts to enter the penumbral shadow at 7:33 pm and exit by 1:30 am the next day, with maximum eclipse at around 10:30 pm. This means that we no need to stay up late into the night to see it.
Credit: F. Espenak, NASA’s GSFC
For animation, click here. As you can see from the animation, as the Moon enters the Earth’s penumbra (P1 to U1), you may not observe any changes. The show really starts after 8:45 pm (U1), when the Moon starts to enter the umbra shadow. Look for the colour change (to reddish) as the Moon moves deeper and deeper into the shadow. Between 10:06 pm (U2) to 10:57 pm (U3) is what we called totality – this is the time when the whole Moon is in Earth’s umbra. After that, the Moon will slowly come out from the shadow, and by 1:30 am, you can pack and go to sleep.
Total Lunar Eclipse of 16 Jun 2011 taken outside my house. I did not managed to finish the whole sequence because the clouds rolled in. Credit: thChieh.
Why does the Moon turns reddish or orangey during totality? Shouldn’t it disappear as it enters the Earth’s shadow? The reason is our atmosphere. Take a look at the diagram below and it’ll explain everything.
If the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The presence of Earth’s atmosphere means that sunlight reaching the Moon must pass through a long and dense layer of air, where the light is scattered. Shorter wavelengths (blue) are more likely to be scattered, so by the time the light has passed through the atmosphere, the longer wavelengths (red) dominate. The scattering depends on the conditions/particles in our atmosphere, which in turns determine the colour of the totality Moon. Anything from bright orange to blood red is possible. If there has been a major volcanic eruption, for example, the atmosphere has so much dust that the shadow on the moon will appear dark throughout an eclipse.
What colour are we going to see this Saturday? It will be a surprise…
(When the eclipsed Moon is bright, the stratosphere is clear. On the other hand, a dark eclipse indicates a dusty stratosphere. There are atmospheric scientists out there who are studying lunar eclipses as a means of monitoring conditions in Earth’s upper atmosphere. How cool is that?)
But don’t just look at the red. A more not known colour is the turquoise blue. I’m not sure if it is visible to the naked eye, but I’m sure if you take a picture of it, you will see it (see picture below). This comes from light passing through the ozone layer, which absorbs red light and makes the passing light bluer. This can be seen as a soft blue fringe around the red core of Earth’s shadow. Start looking for the turquoise colour as the umbra eclipse begins (U1), it will be more obvious as the Moon moves into shadow, or when starts to come out of the shadow.
Total Lunar Eclipse of 16 Jun 2011 taken outside my house. The Moon had just fully entered the unbral shadow (U2). The top part is darker because it was deeper in the shadow; the bottom part is bluish due to the reason described above. Credit: thChieh.
If you are in an area without much light pollution, you can actually see the stars around the Moon during totality. Usually the bright full moon will drown all the stars around it, but during a totality, you can take picture of the full moon with the stars.
Go out and take a look at the Moon after dinner this Saturday, you won’t want to miss it, because this will be the last total eclipse until year 2014. Yes, you read it right. There will not be any total lunar eclipse for two whole years. So grab this last opportunity!