Lunar Eclipse Saturday "Should Be Quite Colorful"

John Roach
for National Geographic News

November 7, 2003

On Saturday night the full moon will slip into Earth's shadow and darken to an orange-reddish glow, giving sky-watchers their second chance this year to catch an astronomer's delight: a total lunar eclipse.

The celestial show will be visible throughout most of the Americas, Europe, Africa, and western and southern Asia, giving up to three billion people—weather permitting—a rewarding view of the cosmic display.

"It should be quite colorful, whitish on one pole to more copper-red on the other," said Michael Reynolds, an author and eclipse coordinator for the Association of Lunar and Planetary Observers in Jacksonville, Florida.

The spectacle officially begins at 5:15 p.m. ET on Saturday when the full moon makes contact with the outer portion of Earth's shadow, called the penumbra. Most observers will not notice any changes until 6:32 p.m. when the moon makes contact with Earth's dark inner shadow, known as the umbra.

Totality, the phase when the Earth's shadow completely immerses the moon, begins at 8:08 p.m. and lasts for 25 minutes, making it a relatively short total lunar eclipse. The moon exits the umbra at 10:04 p.m. and the whole event comes to an end at 11:22 p.m.

While viewers in the eastern United States, Europe, and Africa can watch the whole show, viewers along the west coast of North America will witness the moon rise partially eclipsed. Viewers in western Asia, meanwhile, will watch the partially-eclipsed moon set.

Short and Bright

Total lunar eclipses occur when the moon passes through the Earth's shadow. Because the projection of the Earth's shadow reaching the moon is much larger than the diameter of the moon itself, the moon does not have to move through the center of the shadow to be totally eclipsed.

For this particular eclipse, the moon will pass through the southern edge of Earth's umbral shadow.

Unlike a center-shadow lunar eclipse, Saturday's event will be shorter, brighter and/or more colorful, Reynolds said. "Since this is just barely total, both should apply for this eclipse."

During total lunar eclipses, the moon does not go completely dark. While the Earth stands between the Sun and moon during totality, a certain amount of light is bent, or refracted, by Earth's atmosphere and illuminates the moon.

Fred Espenak, an eclipse expert at NASA's Goddard Space Flight Center in Greenbelt, Maryland, likens the effect to sitting at the edge of a pool or lake and putting a stick in the water.

The stick "appears to bend," he said. "That is due to refraction. As light passes through the water it is refracted. The same thing happens to light waves in the atmosphere; they are bent in the direction of the moon."

The atmosphere scatters most of the short wavelengths of light—blue, green, and yellow—out of the refracted light leaving primarily orange and red wavelengths to reach the moon, thus its orange-reddish glow.

Fire Effect?

The more dust that is in the air—whether from volcanic eruptions, forest fires, or the build up of pollution—the more refracted light scatters. As a result, less light reaches the moon.

Espenak said the moon during this particular eclipse could be darker than originally anticipated, owing to an unexpected increase of particulate matter in the atmosphere from the forest fires that recently scorched nearly 800,000 acres (325,000 hectares) of land in California.

"It depends on how much dust these fires in California [threw] up in the atmosphere," he said.

Reynolds said he does not expect the fires to darken the eclipse owing to the time span between the fires and the eclipse.

Last in a Series

"The most unusual thing about this particular eclipse is that it is the last total eclipse in the Saros series that began in 1769 and recurred every 18 years and 11 days," said Fred Espenak.

A Saros series is a natural rhythm in the occurrence of similar eclipses. Each series typically lasts for 12 or 13 centuries and contains a total of 70 or more penumbral, partial, and total eclipses. At any given time there are approximately 40 different Saros series in progress.

Saturday's eclipse is the last total eclipse of Saros 126. The first total eclipse in the series was June 19, 1769. The series began with a partial eclipse on July 8, 1219 and will end with a partial eclipse on August 30, 2490.