| Chasing Shadows |
EasyNews, UseNet made Ea .. |
| ::darkshadows:: (blood@thirsty.net) |
2008/02/14 22:13 |
Chasing Shadows
Edna DeVore
Director of Education and Public Outreach, SETI Institute
SPACE.com Thu Feb 14, 6:15 AM ET
On Wednesday, February 20, we're in for a celestial treat. There's a
total lunar eclipse visible from North and South America as well as
Europe and parts of Africa. It's a great excuse to spend time
out-of-doors watching the Moon drift into the shadow of the Earth.
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Lunar eclipses occur when the Moon and Sun are opposite each other in
the sky, and the Earth is in between. The full moon also occurs when
the Moon and Sun are opposite each other in the sky. However, most
months, the Moon's orbit carries it above or below the Earth-Sun line,
the full Moon misses the Earth's shadow, and there is no lunar
eclipse. In a typical year, we can see between 0 and 3 total eclipses
of the Moon. There are also partial and penumbral eclipses when the
Moon passes through just a part of the shadow of the Earth, and dims
very little. If you want to plan ahead, there's a comprehensive list
of lunar eclipses at NASA's eclipse home page.
What will happen? As the Moon orbits Earth, it will cross into the
Earth's shadow. At first, the Moon enters the penumbra, a partially
shaded area that surrounds the darker umbra, or central shadow. The
Moon dims only a little in the penumbra. It's hard to see any
difference. Once the Moon begins to enter the umbra, the shadowed moon
darkens to a dull red or red-brown glow. It doesn't get completely
dark because light from the Sun is bent by Earth's atmosphere with the
red light bent the most. Thus, when the Moon is deep in the Earth's
shadow, reddish light illuminates its surface.
What can you see? Beyond the color change, look at the shape of
Earth's shadow on the Moon. Like your own shadow, Earth's shadow
reveals its shape. More than two millennia ago, the Greeks reasoned
that the Earth was round because they always saw this curve during
lunar eclipses. Columbus didn't have to prove that the Earth was
round; it was well-understood thousands of years earlier.
Where can you watch the eclipse? There's no need to go to a major
observatory to see a lunar eclipse. It's visible from anywhere that
you can see the Moon. If you would like to join with other people for
an evening to share the experience, many amateur astronomy clubs will
host lunar eclipse parties, and they welcome visitors. It's fun to
join a group of dedicated amateur astronomers who can answer your
questions while you watch the eclipse.
The Night Sky Network is a national network of amateur astronomy
clubs. You can locate a club near you, and find out if they are
planning a "moon party" on the 20th. Likely, they will have telescopes
available for viewing the darkened moon and other celestial treats.
They may also be conducting short explanatory demonstrations related
to eclipses from "Shadows and Silhouettes," a kit of materials
developed by the Astronomical Society of the Pacific in collaboration
with SETI Institute and Lawrence Hall of Science. You can learn about
lunar and solar eclipses, phases of the moon, and transits via these
activities.
"Shadows and Silhouettes" was sponsored by NASA's Kepler Mission which
will seek Earth-size planets in orbit about distant stars by looking
for transits. A transit occurs when a planet crosses between the Earth
and a distant star, blocking a small part of the star's light.
Basically, we see the transit because we are in the shadow of that
distant planet. This dip in the star's brightness reveals the presence
of the planet, and repeated observations of subsequent transits
confirm the discovery. The Kepler Mission launches in 2009 to begin
observations that may reveal other Earths in our galaxy.
So, simple shadows reveal much about the shape of the Earth, and,
possibly, the presence of other Earths around distant stars.
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