New Century opens with a Lunar Eclipse

     The night sky rings in the new century by staging a total lunar eclipse in January. On the night of January 20]/21st, the full moon will do a disappearing act. You will want to catch this one because the US wont see another total lunar eclipse for three years.

     A lunar eclipse occurs when the moon passes through the Earth's shadow, with Earth positioned between the Sun and Moon. This lineup also means that the Moon is full. That sounds like very little requirements for an eclipse, so why don't we have a lunar eclipse every month?

    To understand why we do not have a lunar eclipse every time we have a full moon, imagine the Earth as a marble orbiting around the Sun, a basketball, on a tabletop. The plane of the Moon's orbit does not lie flat on the tabletop, but instead is tilted about five degrees. This would mean that as the Moon goes around the Earth, sometimes it is above the tabletop and sometimes it is below.
The lunar orbit intersects the Earth's orbit twice each month. These points of intersection are called nodes.

     For a total lunar eclipse to occur, the moon must be full and must also be at a node point in its orbit. Sometimes the full moon is very close to a node and we have a partial lunar eclipse. When this happens a portion of the Moon passes through the Earth's shadow. Partial eclipses can result in anywhere from a sliver to most of the Moon darkened.

     This month's eclipse is a total lunar eclipse. The first thing to look for as the eclipse begins around 9PM, is a subtle darkening around the eastern edge. It is a gradual dimming as the Moon begins to pass through the penumbra, the outer part of the Earth's shadow. It usually takes about thirty minutes before enough of the Moon is dimmed that you will notice it easily.

     Around 10PM, the Moon begins to pass through the umbra, the darkest part of the Earth's shadow. This is the part of the eclipse where the Moon actually seems to disappear. Totality, the point in the eclipse where the Moon is completely within the Earth's shadow, begins at 11:05PM. Totality will last until 12:22AM, when the Moon will begin to reverse its disappearing act.

     During totality, the Moon does not simply just get dark, it may turn any number of colors from grey to orange to blood-red. Just what color the Moon actually gets during totality depends on two factors. First, it depends on how deeply into the umbra the Moon passes. The second factor, usually the more important, is the weather on those places of the Earth where it is sunrise or sunset when the Moon is being eclipsed.

     That second factor, the weather where sunrise and sunset are, may sound pretty strange, but it is important. Any light hitting the Moon once the Earth blocks off direct sunlight is light bent around Earth by its atmosphere. To some degree the Earth's atmosphere acts like a spherical lens, focusing light into the shadows where the Moon is. When the air is clear along the edge of the Earth
between the day and nighttime side: then the Moon can still be pretty bright even during totality. If it is cloudy in those areas, or if there is a lot of volcanic dust in the air (such as was the case during the eruption of Mt St Helens in the early 1980's) the Moon can look dark and colored during the eclipse. A lot of dust in the atmosphere can cause the Moon's color to range in oranges and reds.

     If you have binoculars or a small telescope, an eclipse can really be fun. Watch craters get gobbled up by the shadows as the eclipse progresses. The growing shadows can really highlight crater details otherwise difficult to observe. Our ancestors found lunar eclipses a frightening and mysterious event because they did not know what was happening. We now know exactly what is happening, but many still feel and eerie sense of wonder watching this celestial spectacle.