Today’s factismal: The technical term for an eclipse is syzygy.
Imagine, if you will, that tonight is a warm summer evening in Ancient Greece. You look up and are amazed that the Moon is slowly getting darker and turning a blood red. Being a philosopher, you immediately realize what is happening. The Earth, Moon, and Sun have become yoked together and so you shout out “syzygy!” (the ancient Greek word for “yoked together”). You have just witnessed an eclipse.
Eclipses are one of the more interesting natural phenomena. Not only did the eclipses of the Sun and Moon help ancient Greek philosophers to reason out how large the Solar system must be and the fact that the Earth is round, but they help show both how predictable and how uncertain the Solar System is. The predictable part is easy; once you know all of the factors affecting the orbits of the Earth and Moon, you can easily calculate when the next eclipse will happen and what type it will be. More than 3500 years ago (or about 800 years before that warm summer night in Greece) Babylonian astronomers were predicting eclipses.
The uncertain part is also easy once you stop and think about the problem. The Moon doesn’t orbit the Earth in a perfect circle; worse, the Moon’s orbit is tilted away from the Earth’s. As a result, sometimes the Moon is too high or too low or too far away to create an eclipse. That’s also why some eclipses are very short (like the one tomorrow morning; it will last just 4 minutes and 43 seconds) and some are very long (like the one in 1859 that lasted 1 hour, 46 minutes, and 27 seconds), and why some years we only see two lunar eclipses and some years we see five (1879).
That variation in the Moon’s orbit also determines what type of eclipse we’ll have. If the Moon is close to the Earth, then the Earth’s shadow completely covers the Moon and we get a total eclipse. If the Moon is a little farther out then the Earth’s shadow is smaller and only covers part of the Moon, giving us a partial eclipse. And if the Moon is farther out still, then we don’t get an eclipse at all! Or at least, not an umbral eclipse.
You see, the Earth’s shadow in space isn’t a perfect cone. Because the Sun is wide, the light from one side casts a slightly different shadow than the light from the other side. If you’ve ever looked at the shadows cast by light coming in from two nearby windows, you’ve seen this effect at work. The area in the middle is dark because no light from either window makes it into the room; this is the umbra (Latin for shadow). And the area directly in front of a window is bright because lots of light makes it in. In between the two regions is the penumbra (from the Latin for “nearly shadow”) where a little light from one window makes it in but no light from the other does. The same thing happens in space; when the Moon moves into the Earth’s penumbra, it gets about 10-30% less light and dims slightly. To an astronomer, that is the start of the eclipse even though a non-astronomer won’t call it an eclipse until the Moon moves into the umbra.
And now that you know what will happen, set your alarm clocks! Tomorrow’s eclipse will be short and sweet – don’t miss it!