As you may have known, there was a total lunar eclipse several hours ago. The moon was already totally eclipsed by the time it rose over the United States, but was still a beautiful sight. There will be plenty of high-quality photographs available online, I’m sure, so let me share with you some rather bad photographs I took. I was not planning to take any pictures, but it was such a beautiful occurrence that I felt compelled to try. It is not easy to photograph a partly glowing object at night, let me tell you. In the first photograph, perhaps a quarter of the moon is illuminated. (The glow makes it appear more illuminated in the photograph.)
Lunar eclipse. Credit: Darmok.
In the second photograph, the moon is minutes away from emerging from Earth’s shadow. There is only a small “bite” missing. Again, the glow overwhelmed my camera.
Lunar eclipse, almost over. Credit: Darmok.
Phases of the moon. Source: Wikipedia.
The moon orbits Earth once every 29.5 days—hence the origin of the word month. Of course, half of it is lit and half is in darkness at any time, just like Earth, but we see different parts of the moon as it orbits us. When it’s between Earth and the sun, the dark side faces us and we call it a new moon. As it orbits, we see more of the lit side, until when it is on the side of Earth opposite the sun, we see the fully lit side and it is a full moon. Incidentally, this also means that specific phases correlate with specific times on Earth. For instance, since the full moon just mentioned is on the opposite side of Earth from the sun, that means it is over Earth’s night side, and is the highest at midnight. It is not visible at all to the day side of Earth. (See this Wikipedia illustration for more.)
So how do eclipses fit in? When the moon passes between the Earth and the sun, it casts a shadow on Earth, and the part of Earth in the shadow sees the moon pass in front of the sun and cause a (solar) eclipse. Similarly, when Earth is between the sun and the moon, as it was for this event, the moon passes through Earth’s shadow and we see a lunar eclipse.
You might think that there should be two eclipses every month—every new moon should cause a solar eclipse and every full moon should cause a lunar eclipse. But the moon’s orbit is inclined very slightly (around 5°) relative to our orbit around the sun. As a result, the moon usually passes just above or just below the sun. Only occasionally will an alignment occur and an eclipse be possible. Therefore, solar eclipses will always occur during new moons, and lunar eclipses, as this one was, will occur during full moons; however, the converse is not necessarily true.
Update: New Scientist has an article on the eclipse, including some photographs.