Earth Overshadowing Moon Makes Skywatchers See Red

A while ago I wrote about a blue moon, a rare phenomenon during which the moon takes on the color of a robin's egg. If you're more partial to red, the moon should be draped in your favorite color soon as it shows the effects of a lunar eclipse.

Hans Nielsen, a Geophysical Institute professor who teaches an astronomy course, said Alaskans can expect to see a coppery moon briefly after it rises on September 26.

The moon's sudden appearance in the hue of a newly minted penny is caused by the earth, which casts a circular shadow that the moon orbits through once or twice a year. This shadow, known as the umbra, is like a black circle in space. At about 230,000 miles from the earth, where the moon orbits, the umbra has a diameter of 5,700 miles. Since the ball of the moon is 2,160 miles in diameter, it fits into the dark umbra for hours at a time during a lunar eclipse.

Because the moon is passing through the earth's shadow, you might expect the moon to be black, rather than red, during a lunar eclipse. Sometimes the eclipsed moon does dress in black (for a reason described later), but moon viewers have most often seen the lunar-eclipsed moon as deep red, brownish, rust red, brick red, or orange.

The moon appears red during a lunar eclipse because red-orange light is good at penetrating the earth's atmosphere, a 30-mile thick layer of gases. All colors of light from the sun are deflected around the earth by gas molecules in the atmosphere, but reds and oranges are the only colors that survive the long trip through the atmosphere without being scattered away. The same process creates pretty sunsets. During a lunar eclipse, reddish light that continues around the earth projects on the moon when it's in the umbra. If an astronaut was standing on the moon during a lunar eclipse, he or she would see a circular red sunset rimming the dark sphere of the earth.

Volcanoes occasionally blacken the moon during lunar eclipses. After Krakatoa erupted in 1883, tiny ash particles clogged the upper atmosphere. Those aerosols blocked the journey of red light through the atmosphere, and people saw a dark moon during the next lunar eclipse. The same effect was noticed after El Chichon volcano erupted in southern Mexico in 1982.

On September 26th, the moon will be in the earth's shadow for about an hour and 10 minutes, but we won't be able to see if for that long, Nielsen said. The moonrise in Alaska, the Yukon, and other areas of the far northwest will only allow viewing of the moon in the umbra for a few minutes just after the moon crawls above the eastern horizon. In Fairbanks, the moon rises at 7:26 p.m.; in Anchorage, it rises at 7:18 p.m.

When the moon passes through the umbra and is once again hit by direct sunlight, the fuzzy outline of the earth will appear. The curved shadow is the same one that made Aristotle guess that the earth was a sphere many centuries before most other people believed it.

If you somehow find yourself not looking at the eastern horizon around dinnertime on September 26, or if clouds deny you the chance, other lunar eclipses will occur on September 16, 1997, and in January and July of 2000. With a little luck, we'll all get to see red on September 26.