It all seems like clockwork, and then, something unusual happens that seems to throw the orderly timing of the cosmos into chaos. We live in a world that seems so ordered; the Sun rises, goes across the sky and then sets. The Moon goes through its phases from new to full and back again. On a night when the moon rises full and beautiful, it starts to change, at first it is so subtle few notice it. But then, every so slowly, the moon begins to dim, and more alarming yet, it disappears.
One can only imagine how frightening the sight of a lunar eclipse must have been for our ancestors. Far more than us, they were in tune with the rhythms of the cosmos, the motions of the Sun, Moon and planets were the motions these people lived by. They told time by the daily passing of the Sun, or full moon to full moon gauged longer periods of time. And the the very stars marked the passing of seasons. The skies were orderly and dependable, except for when an eclipse happened. During that time, chaos reigned, and our ancestors prayed and begged for the Moon to be returned to the sky.
Eclipses have even influenced history, and even today there are those who still attach ancient superstitions to an eclipse. But such are the few, today we know what causes lunar eclipses. And although it may seem a magic show of shadow and light and a disappearing act by the Moon, we know how the "magic" works, but we can still appreciate the beauty.
There are actually several type of lunar eclipses, total, partial and penumbral. The upcoming eclipse is the best kind, a total lunar eclipse. This kind can only take place when the Earth passes directly in front of a full Moon, thus casting its shadow on the Moon's surface.
There are two parts to the Earth's shadow, the penumbra, and the umbra. The penumbra is the outer part of the shadow where sunlight is not completely blocked. The penumbral shadow only dims the Moon every so slightly, in fact unless you are in very dark skies, you may not notice this part of the eclipse at all. The umbra is the actual shadow created by the Earth. You will notice the Moon getting darker from the left side first. During the time when the entire Moon is in the umbra, it is said to be in totality.
Many people are surprised that the eclipsed moon is reddish but there is a reason. Some of the sunlight passes through the Earth's atmosphere and is bent around behind the Earth and towards the moon. The shorter wavelengths of light is scattered and only the longer orange and red wavelengths reach the moon. It is usually just enough light to cast a coppery red hue on the Moon.
There are times however, such as when there have been volcanic eruptions on the Earth, that the light is so scattered that almost no light reaches the moon and it may be so dark as to be not seen at all.
When the Moon is in totality, you will notice that the whole sky gets darker. You may not have realized just how bright a full moon is until it gets blocked out in an eclipse! Notice too that before the Moon started getting darker you could probably only see a few of the brightest stars in the sky, but during totality, you will see many more stars when they are not obscured by the Moon's light.
Totality can last for over an hour and a half and then gradually, the Moon will reappear, first a tiny sliver and soon as the full Moon it had been.
Now, we mentioned that there are other types of lunar eclipses. A partial lunar eclipse is when only part of the Moon travels through the umbral shadow of the Earth. Depending on how much of the Moon passes through, you may or may not notice this type of eclipse. A penumbral eclipse is when the Moon passes only through the penumbral shadow of the Earth. During a penumbral eclipse, you would likely not notice any darkening of the Moon unless you were in very dark skies and were looking for it! Therefore, we don't recommend watching penumbral eclipses.