Window on Nature
By Lowell & Kaye Christie, F47246
See the moon in a different light during a total lunar eclipse this month.
Wolf moon, storm moon, harvest moon, hunter’s moon. The Earth’s lunar neighbor goes by many names as it crosses the sky in shades of whitish-gray. But sometimes, depending on the season or the specific atmospheric conditions, the moon may take on unexpected shades. Normally it requires a bit of luck to see one of the moon’s unusual colors. But near the end of August, if you’re willing to skip a little sleep, you can watch the moon turn red.
Approximately once a month (every 29 ½ days) as the moon circles the Earth, it ends up opposite the sun. We know these bright nights as full moons. But since the moon’s orbit differs by about five degrees from that of the sun, every now and then one of these full-moon nights turns into a special celestial show.
Two to four times each year, when the Earth, moon, and sun align, the Earth’s shadow passes across the face of the moon to create an eclipse. In the early-morning hours of August 28 we’ll have one of the longest total lunar eclipses we’ve seen in years.
There are actually three types of eclipses, depending on how much of the sun’s light is blocked by the Earth. The first type is when the sun’s light is only partially obstructed. These are called penumbral eclipses and go almost unnoticed, since there is just a slight darkening of the moon.
The second type creates a partial eclipse where only some of the moon actually moves into the full shadow of the Earth. A partial eclipse shows a circular bite being taken out of the moon, the size determined by how much of the moon passes into the shadow area. This circular shape may have been the first indication to early astronomers that the Earth was round.
This month we will have a total eclipse, the third type, as the moon falls completely into the Earth’s shadow for approximately 90 minutes. With the absence of sunlight you might expect the moon to disappear, but it doesn’t. Instead it changes color.
The color of sky objects is greatly affected by the condition and thickness of the Earth’s atmosphere. As light travels through air, the difference in size between tiny air particles and the length of light waves causes a scattering of the colors. Each colored wavelength, being a different size, is affected in a different way. It turns out that the blue end of the spectrum is scattered more effectively than the red.
When you look upward, peering through a relatively thin layer of air, the sky appears blue. But when you look toward the horizon, there is more air to scatter the blue rays and you’ll see a higher proportion of the red spectrum. In the evening, that intensifies the spectacular shades of sunset.
At least that’s what happens under normal conditions. Once in a blue moon, circumstances change.
We’ve heard people talk about a blue moon, meaning a seldom-occurring event, for many years, but only recently learned that sometimes the moon truly can turn blue. However, the most common use of the term “blue moon” refers to the second full moon in a month, an infrequent event that most recently occurred on June 30 of this year. But in 1883 the explosion of Indonesia’s Krakatoa volcano changed the atmosphere and also made the moon appear blue.
The eruption filled the air with tiny particles about one micron wide (one-millionth of a meter). This reversed the normal situation, because these particles were just the right size to scatter red light waves, leaving the blue end of the spectrum more apparent. So for quite some time, people watching the night sky saw a blue satellite orbiting the earth.
This can and has happened again, since this size particle can be created by massive forest fires or, as we’ve observed, volcanic eruptions. According to an article about blue moons on NASA’s Web site, people saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico, and following the eruptions of Mount St. Helens in 1980 and Mount Pinatubo in 1991.
But unless we have a volcanic incident sometime during the next few weeks, it’s unlikely that we’ll see a blue moon this month. So let’s get back to the question of why the moon doesn’t just disappear during a lunar eclipse.
As sunlight travels through the atmosphere at the Earth’s surface, it bends slightly. You can see this effect while watching the sun set in the evening. As it sinks below the western horizon, the sun seems to flatten out rather than remain round. This distortion is caused by the light rays bending downward.
Now imagine standing on the moon during a total eclipse. As you gazed at the dark side of the Earth, even though all the sun’s rays were blocked, you would see a ring of sunset all around the edges of the planet. It’s caused by the bending waves of light following the curvature of the Earth. Depending upon the cloud cover and the dust particles in the Earth’s atmosphere, the scattering of the blue part of the spectrum would cause the muted color to display as red.
Of course, you don’t have to travel to the moon to see the color change. From Earth, the first part of the eclipse will show the moon in its normal colors, but as the eclipse reaches totality, the moon will likely turn a subtle shade of red.
This month’s eclipse is the second total eclipse in 2007. The first one, on March 3, was visible only on the East Coast. The upcoming eclipse can be seen throughout North America, but people living west of the Rocky Mountains will be able to view the entire event. Here on the West Coast the moon will start to disappear at 1:51 a.m. Pacific Daylight Time, with totality occurring an hour later. After 90 minutes the process will reverse as the moon begins to move out of the Earth’s shadow.
Unfortunately, for those living in the far eastern part of North America, the moon will set and the sun will rise before the show is complete. But these folks shouldn’t be too upset. They had the March eclipse all to themselves and will have another chance to see a full lunar eclipse on February 21, 2008. But Westerners, you’d better not miss this one, because the next visible total lunar eclipse won’t occur again until December 21, 2010.
For more information about upcoming lunar eclipses and viewing times, visit the U.S. Naval Observatory’s Lunar Eclipse Computer Web page at http://aa.usno.navy.mil/data/docs/LunarEclipse.html.