Lunar eclipses are fantastic to experience and, fortunately, they are a whole lot easier to observe than their solar counterparts. Unlike solar eclipses, you don’t need to be in a specific place to see them — so long as you can see the moon then you can enjoy the eclipse. Also unlike solar eclipses, you don’t need any special equipment to observe them.
Well, as luck would have it, in just a few hours, there will be a lunar eclipse for you to feast your eyes on. It takes place on Dec. 10, but if you miss it, the next one isn’t until 2014! Wait a minute, why no lunar eclipses until then? Well, it’s all about the orbit of the moon. Let me explain…
We see the moon because it reflects sunlight — turn the sun off with a magical stellar light switch and the moon would disappear. As it moves around the Earth, taking approximately a month to complete one orbit, we see differing portions of the illuminated half of the moon; from “new moon” (when we see none of the illuminated half) to “full moon” (when we see it all). Given that we see a full moon when the sun and moon are opposite to each other from the point of view of the Earth (when Earth is in the middle), you would expect a lunar eclipse every month. But due to the characteristics of the moon’s orbit, often an eclipse is not possible.
The orbit of the moon around the Earth is not along the same plane as the orbit of the Earth around the sun, instead it’s tilted by a mere 5 degrees. The moon’s orbit does cross the orbit of the Earth at two points called the “nodes” — which means that although the moon is opposite the sun in the sky, it’s not exactly opposite and is usually just above or just below the Earth’s orbit. For a total lunar eclipse to occur, the moon must lie at, or very near to, one of these nodes so that it passes through the shadow of the Earth. If it lies a little further away then we might see a partial lunar eclipse where just a portion of the moon passes through Earth’s shadow; even further away and no eclipse at all.
It’s complicated a little more by the gravity of the sun tugging at the moon and gradually shifting the nodes (known as the Saros Cycle) around the moon’s orbit which takes just over 18 years to complete one cycle. So, we see total lunar eclipses when the sun, Earth and moon are in a perfect line and beyond Dec. 10, this won’t happen until 2014. We will have lunar eclipses between now and then, but they will only be partial or even “penumbral” when the moon will just skirt through the lighter part of the Earth’s shadow.
Will you see it? Asia and Australia will have the best view of the eclipse. In the U.S., observers on the West Coast will have a better view before moonset. For more details on observing Saturday’s eclipse, check NASA’s eclipse page.