Total lunar eclipse Thursday night!
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From: Peter Langston <psl>
Date: Thu, 26 Sep 96 01:09:42 -0700
Subject: Total lunar eclipse Thursday night!
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Thursday 9/26 is the last lunar eclipse visible in North America this
century. Starts at 5:12, full coverage at 7:19, totality continues
until 8:29, and eclipse ends at 10:36 PM, PDT
North America readies for lunar show Total eclipse.
May shed light on Earth's ozone (UPI)
The 20th century's last total lunar eclipse is set to occur in late
September. One of the more impressive lunar eclipses of recent decades will
be visible from North America the night of Sept. 26-27. It's the last
"total" lunar eclipse - one in which the entire moon switches off, like a
lamp - visible from North America until the year 2000.
On Thursday evening, Earth will pass between the sun and moon. Result:
Earth's shadow will fall on the moon, bathing it in an orange-red light.
In ancient times, soothsayers interpreted a lunar eclipse as an omen of the
world's end. Primitive people banged pots and chanted to the skies,
desperate to scare away the monster that was supposedly devouring the moon.
Astronomers no longer associate lunar eclipses with the world's end.
But in recent years, iconoclastic researchers have exploited lunar eclipses
to assess modern "end of the world" fears.
One astronomer is studying lunar eclipses to detect the deterioration of
Earth's ozone layer, the portion of the stratosphere that shields us from
cancer-causing solar radiation. Ozone degradation, he says, might alter the
color and size of Earth's shadow, or "umbra," on the moon.
Other scientists are measuring how much light Earth reflects onto the
moon - "Earthshine" - in hopes of answering a controversial question: Is
Earth's climate warming? They used a lunar eclipse to calibrate their
So far, neither experiment has yielded conclusive results. The Sept.
26 eclipse starts at 8:12 p.m. ET. If skies are clear, sharp-eyed
observers - especially those with binoculars or telescopes - will see a
vague shadow begin to nibble at the edge of the full moon. The shadow will
grow until 10:19 p.m., when "totality" starts. That's when the moon is
almost invisibly dark, save for - in most eclipses - a copper-red glow.
The eclipsed moon turns red for the same reason that sunsets and sunrises
are red: because sunlight passing through the atmosphere bends, or refracts,
and shifts toward the red end of the spectrum. Those refracted rays bathe
the darkened moon in a red glow.
Totality ends at 11:29 p.m.; the moon will slowly regain its brightness.
By 1:36 a.m., it will be a full moon again. Astronomers call it an
impressive eclipse because totality is unusually long - one hour and 10
minutes. Why does one eclipse paint the moon a bright blood red, while
another merely dabs it in watercolor orange?
"Imagine what it would look like if you were standing on the moon during
a lunar eclipse," says astronomer Michael Bennett of the Astronomical
Society of the Pacific. "First you'd see the sun going behind Earth. As the
sun disappears ... Earth would be surrounded by a very bright red ring. And
that ring would be so bright that it would actually light up the lunar
landscape around you.
"That would be the light of all Earth's sunsets and sunrises hitting the
moon all at once."
On the West Coast, the moon rises that evening at 6:31 p.m. PT. That's
79 minutes after the eclipse starts and 48 minutes before totality. For
that reason, residents of California, Oregon and Washington will see the
moon rise "like it has already had a bite bitten out of it," Bennett says.
Lunar eclipses can be "really beautiful," recalls astronomer Susan Cannon
of the Calgary Science Center in Alberta, Canada. "I remember one I saw in
Manitoba - it was just a stunner. The moon was just blood red. .... I
couldn't believe it."
But during another less dramatic lunar eclipse, she adds, "the moon just
got sort of 'orangey.' "
That casual recollection reflects a larger scientific mystery: Why are
some eclipses spectacular and others less so? Why does one eclipse paint
the moon a bright blood red, while another merely dabs it in watercolor
And why, in rare eclipses, does Earth cast so dark a shadow that the moon
almost disappears - an ominous black sphere against the star-sprinkled sky?
Earth's atmosphere is always changing. Sometimes it's cloudy; sometimes
it's packed with particles from volcanic eruptions. For these and other
reasons, the degree of refraction varies from eclipse to eclipse. And so,
as a result, does the degree of lunar reddening.
"If you have a major volcanic eruption like El Chicon or Mount Pinatubo,
that can actually inject particles into the stratosphere. There's no
condensation up there, so they can stay up there a long time," says John
Westfall, a professor at San Francisco State University and president of
the Association of Lunar and Planetary Observers. While the eclipsed moon
doesn't foretell the end of the world, it may reflect - literally - its
Volcanic dust can make the umbra look especially dark, Westfall says.
This implies that knowing Earth's weather and volcanic conditions at any
given moment should allow astronomers to predict the exact size and color
of the umbra.
Yet they can't. Their predictions are always just a little bit off. Once
again, the reason probably lies in our atmosphere; it's almost certainly
more complex than their computer models assume.
Past lunar eclipses
For astronomers, lunar eclipse anomalies are an old headache. In 1702,
the French astronomer Philippe de la Hire noticed something odd: The umbra
is slightly bigger than it "should" be. Simple geometry and our knowledge
of the distances to the moon and sun - about 240,000 miles and 93 million
miles, respectively - indicate the umbra should be a certain size during an
eclipse. Yet, de la Hire complained, it's usually 2 percent wider than the
For decades, amateur astronomers with powerful telescopes have timed the
exact moments that the umbra enters and exits lunar craters. Based on
thousands of these "crater timings," they conclude that de la Hire was
right: The umbra is 2 percent bigger than it should be.
One possible explanation is that a layer of meteoritic dust floats 74
miles to 93 miles high. That dust might extend the umbra's shadow.
But the meteoritic explanation doesn't satisfy astrophysicist Erich
Karkoschka. Writing in the September issue of Sky & Telescope, he points
out that the umbra isn't always 2 percent bigger; rather, the size
discrepancy varies from eclipse to eclipse. That variation implies that a
more complicated atmospheric effect is at work. That effect may involve the
much-publicized ozone layer.
The moon and the ozone
In the last decade or so, environmentalists have grown concerned because
industrial pollutants called chlorofluorocarbons (CFCs) rise into the
stratosphere and trigger chemical reactions that damage the ozone layer.
Without the ozone layer, Earth would be bathed in deadly solar ultraviolet
radiation; the skin cancer rate would skyrocket. The worst ozone
deterioration is in Antarctica, site of the infamous "ozone hole."
Karkoschka has created an atmospheric computer model that includes the
ozone layer. The model shows that during an eclipse, when Antarctica is in
the right position for sunlight to pass through its atmosphere en route to
the moon, the ozone hole should refract sunlight in a way that makes the
umbra bigger than normal. When sunlight passes through more temperate
latitudes, where the ozone is denser, the umbra should appear smaller.
Unfortunately, sunlight won't refract through the Antarctic atmosphere
during a lunar eclipse until Nov. 9, 2003. So there's no way to test
Karkoschka's theory until then - unless ozone levels plummet in temperate
latitudes before then.
Still, if Karkoschka is right, then those ancient soothsayers may have
been on the right track. While the eclipsed moon doesn't foretell the end
of the world, it may reflect - literally - its environmental decay.
© 1996 Peter Langston