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July 26, 1999: This Wednesday, July 28, sky watchers in
the Americas, east Asia and the Pacific have a chance to view
the last lunar eclipse before the year 2000 when the full moon
passes through Earth's umbral shadow for over 2 hours. This eclipse
will be a partial one. At maximum approximately 40% of the Moon
will be covered by the darkest part of Earth's shadow giving
the moon an eerie, orange-colored glow.
Nevertheless, mission scientists are cautiously optimistic. "Lunar Prospector has gone through a couple of eclipses similar to this one and experienced no difficulties," says Lisa Chu-Thielbar, the Lunar Prospector Mission Office Outreach Coordinator. "However, this eclipse is near the end of Prospector's intended lifetime so the operations team is justifiably cautious about turning systems off and on and surviving eclipses. Will Lunar Prospector experience any unusual difficulties on July 28? We hope not!" If the eclipse on July 28 poses a hazard to Lunar Prospector, why schedule the crash three days after the eclipse rather than before? According to Dr. David Goldstein of the University of Texas at Austin, there are good scientific reasons to wait, including the simple fact that the spacecraft will fly right over the impact site on the July 31. Also, Goldstein and his colleagues want to observe the crash after the full moon on July 28 to take advantage of better solar illumination around the lunar limb. "We expect to only be able to see gas that is over a warm sunlit surface and above the limb of the Moon," he explains. "During a full moon, the terrain just behind the visible limb is dark. That's not optimal because if the ground is dark and cold it will tend to trap and re-freeze gases liberated by Prospector's impact. Waiting until after the full moon will improve the prospects for a longer-lasting exosphere - an extremely thin local atmosphere - which the astronomy team led by Dr. Edwin Barker of UT Austin hopes to observe." For more information about plans to observe the impact, including tips for amateur astronomers, see our July 21, 1999 headline Bracing for Impact. Above: The July 28, 1999 partial
lunar eclipse will be visible from the Americas, east Asia, Australia
and the Pacific. Image adapted from material at Fred
Espanek's eclipse web site. By the light of the shadowy Moon...The Moon will enter the outskirts of Earth's shadow early
in the morning of July 28 at 8:56 UT (1:56 a.m. PDT). The darkening
of the Moon's southern hemisphere will accelerate at 10:22 UT
(3:22 a.m. PDT), and the eclipse maximum will occur at 11:34
UT (4:34 a.m. PDT). In North America the best viewing sites will
be along the west coast. Observers on the eastern seaboard will
be able to see just the beginning of the eclipse at dawn. [See
Fred Espanek's eclipse
visibility map for more information.] Above: A diagram from Fred
Espanek's eclipse web site showing the progression of the
July 28, 1999 partial lunar eclipse. Before entering the main
umbra, or cone of shadow, the Moon must pass through the penumbra,
or zone of partial shadow, which exists because the sun is a
disk and not a point source of light. Wednesday's eclipse is
partial -- only 40% of the Moon will be darkened by Earth's umbral
shadow. Even during a total lunar eclipse, when the Moon passes
entirely through the umbral shadow, the Moon is not completely
dark. That's because sunlight is refracted as it passes through
Earth's atmosphere and bent toward the lunar surface. The depth
of shadow and the orange hue of the lunar surface during an eclipse
are influenced by the amount of dust or ash in the earth's upper
atmosphere. Eclipses following volcanic eruptions on Earth can
appear especially dark and coppery-colored. |
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Bracing for Impact -- Astronomers prepare to observe the crash of Lunar Prospector on July 31, 1999. Includes observing hints for amateurs, July 21, 1999, NASA Science News Destined for a Watery Grave -- NASA scientists have decided to send Lunar Prospector crashing into the Moon's south pole in search of water, June 4, 1999, NASA Science News Zeroing in on Lunar Ice -- Astronomers explore the Lunar Prospector crash site using radar, June 4, 1999, NASA Space Science News Lunar Prospector set to make science "splash" -- NASA/Ames press release NASA Press Release (3 September 1998) -- announcing enhanced estimate of quantity of water on the Moon NASA Press Release (5 March 1998) -- announcing the detection of ice on the Moon Lunar Prospector Home Page -- from NASA/Ames Ice on the Moon -- informative article about lunar water -- where it is and how to find it. Lunar Prospects -- Astronomy Picture of the Day, Sep. 18, 1998 Impact Moon -- Astronomy Picture of the Day, Mar. 26, 1999 The Nine Planets: the Moon -- from SEDS |
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