Wednesday’s total lunar eclipse was a treat, as we were treated to mostly clear skies and the moon was never more than a minute or two out of view as the clouds moved overhead. I set my digital camera on a tripod and proceeded to get several pictures of the event. It’s useless to try to get a video of the event, as it happens so slowly, but with good timing I managed to get one good pic every 5 minutes or so. I put them into a video, and to simulate a proper video I had each picture fade into the next over 5 seconds. This made a video of the eclipse that was about 60 times as fast as the real eclipse occurred:

As the eclipse progressed to totality, I edged up the exposure so that the area darkened by Earth’s shadow (called the “umbra”) became visible. It turns a haunting red color, for the same reason the sunset turns red. The Earth’s atmosphere refracts the red light from the sun and it lights up the darkened portions very dimly, although quite visible with the naked eye; I always imagined this would mean that, if you were on the moon at the time (it would be a solar eclipse there), you’d see the blackened Earth surrounded by a sunset-red halo. It must be beautiful. I had to go to bed, so I didn’t get anything after totality, but at that point it’s mostly the same thing in reverse.

The lunar eclipse has always been the stuff of history. According to legend, Christopher Columbus saved his life in the New World with a well-timed lunar eclipse, his foreknowledge of which convinced the locals that he had a special relationship with God. Also according to folklore, Columbus was a genuis who knew the Earth was round, where most everyone else, including his fearful crew, believed it was flat and they would fall off if they sailed too far. I don’t know about the eclipse legend, but the second claim is complete bogosity. It was widely known that the Earth was spherical at least since the 2nd Century BC, and the lunar eclipse is one big reason why.

The reason why we don’t have a lunar eclipse every month is because the moon’s orbit is tilted about 5 degrees with regards to Earth’s orbit around the Sun. So a total lunar eclipse only happens when the three bodies align at the time of the full moon. It doesn’t happen often, but wherever you are you should see one every few years or so. That’s enough to do several good observations in a single lifteime and share those observations with others.

One observation is that the curve of the Earth’s shadow is the same no matter where the moon is in the sky when the eclipse occurs. That can only occur if the Earth is a spheroid. Not only that, but you can also deduce the ratio of the size of the Earth to the size of the moon (which, by the way, is about 3.67:1). I’ve illustrated that with this picture:

Here, I’ve made the Earth (photo from NASA) its proper size relative to the moon using one of the eclipse shots I took (if you like this picture, click it for a 1024×768 version suitable as a wallpaper). Notice that it fits the curve of the shadow perfectly. Our ancient ancestors noticed that, too, and so they knew not only that the Earth was spherical, but also how big it was relative to the moon. Eratosthenes, after his brilliant calculation of the size of the Earth, could then deduce the size of the moon–both of which he got to within 1% of the correct answer (using the most generous length for the “stadium”).

Never underestimate how clever human beings are, or how much can be learned by just looking up and using your mind.