Sky-Watchers Await Total Lunar Eclipse on Thursday
for National Geographic News
|May 12, 2003|
On the night of May 15 the full moon will slip into Earth's shadow and darken to an orange-reddish glow during the first of four total lunar eclipses to occur over the course of the next 17 months.
The celestial show promises to capture the attention of both amateur and professional astronomers and remind them of Earth's place within the cosmos.
"The lunar eclipse can be appreciated and celebrated as an event which vividly illustrates our connection and place among the planets in the solar system," said Fred Espenak, NASA's eclipse expert at the Goddard Space Flight Center in Greenbelt, Maryland.
The eclipse will be visible to sky-watchers throughout the Americas, Europe, and Africa. It officially begins Thursday night at 9:05 p.m. ET when the moon enters the outer portion of the Earth's shadow, known as the penumbra.
Casual observers will notice a marked change about an hour later at 10:03 p.m. when the eastern edge of the moon enters the inner, darker part of Earth's shadow, called the umbra.
Totality, as the phase when the entire moon is inside the umbra is known, begins at 11:14 p.m. ET and will last for 53 minutes, until 12:07 a.m. ET. The moon will fully exit the Earth's umbra at 1:17 a.m. and make last contact with the penumbra at 2:15 a.m.
The most recent total lunar eclipse occurred on January 9, 2001. The next chance to catch a total lunar eclipse will be November 9, followed by total lunar eclipses on May 4 and October 28 in 2004. After this run, the moon will not completely slip into Earth's umbra until March 3, 2007.
During a total solar eclipse, the new moon passes in front of the sun and momentarily casts day into the darkness of night. But during a total lunar eclipse, the moon remains at least partially lit during the event.
Even though Earth blocks the moon from direct sunlight during an eclipse, some sunlight is refracted, or bent, by the Earth's atmosphere and illuminates the moon. The atmosphere scatters most of short wavelengths of lightblue, green, and yellowout of the refracted light so that primarily the orange and red rays reach the moon, said Espenak.
"The more dust the atmosphere has, the more scattering takes place and the redder, and darker, the moon appears," he said.
Since there have not been any major volcanic eruptions or extensive forest fires recently, astronomers believe the atmosphere is relatively clear of the type of particles that could cause a deep-red eclipse.
Byron Soulsby, an eclipse expert at the Theodore Lunar Observatory in Canberra, Australia, anticipates a bright orange or red during this event.
"Because the moon is near the edge of the inner shadow of the Earth, the umbra, the moon will probably be reasonably bright on its Northern edgethe upper edge in the northern hemisphere where the eclipse will be best seenand grading to deeper red towards its center and lower edge," said Soulsby.
Espenak said that lunar eclipses are a great spectacle to get children interested in science and astronomy but that the events themselves are only of minor scientific value. A few people, however, do use lunar eclipse observations in their ongoing research.
For example, Richard Keen, an atmospheric scientist at the University of Colorado at Boulder, uses reports of lunar eclipse brightness as part of an ongoing research project to calculate a history of the optical thickness of volcanic dust layers.
Sulfur dioxide and other gases from volcanic eruptions spew up into the stratosphere, which lies 10 to 30 miles (16 to 48 kilometers) above the ground, and blocks a portion of the sunlight that is normally refracted to the moon.
Knowing the thickness of this volcanic dust layer helps climatologists understand the effect volcanoes have on climate and helps vulcanologists estimate the total amount of material ejected by an eruption.
To measure the brightness of the lunar eclipse, Keen suggests viewing the moon through reversed (turned the wrong way) binoculars with one eye and comparing the image to stars of known magnitude seen with the naked eye.
"The moon, even during a total lunar eclipse, generally appears much bigger and brighter than the stars, and its size and brightness need to be reduced before direct comparisons can be made," Keen writes in an article submitted to several astronomy publications about his research.
The estimated magnitude of the reduced moon can then be adjusted by a factor depending on the magnification of the binoculars, which yields the actual magnitude of the moon.
Another way to estimate the brightness of a lunar eclipse is to use the so-called Danjon scale, which was created by the late French astronomer André-Louis Danjon.
The four-point scale for evaluating the moon's luminosity during totality ranges from 0, a very dark eclipse that renders the moon nearly invisible, to 4, a very bright copper-red or orange.
"Although total eclipses are only of minor scientific value, they are remarkably beautiful events which can be seen without expensive equipment," said Espenak.
Unlike solar eclipses, a lunar eclipse is safe to look at with the naked eye. No special filters or glasses are required to block out harmful rays from the sun. Astronomers suggest looking at the moon through binoculars and telescopes to enhance the moon colors.
The only thing that is critical for viewing a lunar eclipse is a view unobstructed by tall buildings, trees, or mountains.
At the time the event begins in the U.S. and Canada, the moon will be low in the southeastern sky. The moon will rise higher as the eclipse unfolds and will settle in the south-southwestern sky as it ends in North America.
Observers in Europe, the Middle East, and most of Africa will be able to watch the early stages of the eclipse, but the moon will set before the eclipse ends. The eclipse will not be visible in Alaska or extreme northwestern Canada. Observers in Hawaii will only be able to catch the final stages.
Michael Reynolds, president of the Chabot Space and Science Center in Oakland, California, notes: "What's kind of cool about a lunar eclipse is you don't have to travel. You can step out in the backyard and see it."
Lunar brightness observations can be sent to Richard Keen: Richard.Keen@Colorado.EDU
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