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Several key factors must be correct for a total lunar eclipse. First, we need a full moon. In October, the full moon occurs on the 27/28th. This is the point at which the moon is directly opposite the sun in the sky and its face is totally illuminated. Lunar eclipses only occur when the moon is full, just as solar eclipses only occur when the moon is new. But if that is the case, why don?t we have a lunar eclipse every month? The answer lies in the moon?s orbit.
The moon’s orbit is tilted relative to the Earth-Sun plane, by about five degrees. Because of its inclined orbit, the moon and sun’s paths intersect only occasionally. These intersection points are called nodes. Only when both the moon and the sun are at the nodes, will we see an eclipse. In the case of a lunar eclipse, the moon passes through the Earth?s shadow in space and the sunlight is blocked.
This year’s eclipse is mainly visible from the Americas. Observers in the United States will get to enjoy the October 2004 eclipse in its entirety, while those in Europe and Western Africa will see the eclipse begin just as the moon is setting below the horizon. For observers on the west coast the eclipse begins just after the sun sets, during bright twilight. The moon will begin darkening at about 0114 UTC October 28th (9:14 EDT Oct. 27th), and may look like a bite has been taken out of it. An hour later, the moon will have almost completely disappeared, but wait a few minutes and you will notice the moon turn copper colored. This is when the moon begins to enter the umbra, or darker inner shadow of our planet. This total phase will last from 0223 to 0345 UT. The moon will leave earth?s shadow an hour after that.
The color of a lunar eclipse varies drastically from one eclipse to the next. Because this year’s eclipse occurs so close to Halloween, one might suspect that the moon would turn a bright, haunting orange, or even a deep blood red. But why does the moon turn orange at all? The answer is that some of the light from the sun is bent around and through Earth’s atmosphere. The color is dependant on the amount of pollutants or smoke is in earth’s atmosphere. Years in which volcanoes have erupted have yielded particularly dark eclipses. This month’s activity of Mt. St. Helens could release enough particulate matter into the atmosphere to color the moon nicely.
Keep in mind that because of the earth’s rotation and the moon’s orbit, this will be the last total lunar eclipse visible anywhere in the world until March 2007. This may lead many observers to attempt to capture the event with a camera. The best advice for first time eclipse photographers is take lots of exposures without the flash. However, with most camera lenses, the moon is only going to appear as a small dot on the final print. A telephoto lens or a telescope will provide can enlarge the size; but a 2,000mm lens (or a telescope with a 2,000mm focal length) is needed to fill a 35mm frame. Digital cameras can be used with a telescope if you simply point the camera through the telescopes eyepiece. The advantage of digital photography is that you can see your results right away.
So this October, don’t let the ghosts and goblins keep you indoors as Halloween approaches. Get out there and enjoy the last total lunar eclipse until 2007.
Here is some additional information about the eclipse from NASA.
Rod Kennedy is a technician and education outreach coordinator at the Casper Planetarium, Wyoming?s first planetarium. He received his Chemistry degree from the University of Northern Colorado, and has been interested in astronomy for 10 years.