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Get Ready for Saturday: 10
Interesting Lunar Eclipse Facts
By Robert Roy Britt
Senior Science Writer
The second chance to see a total
lunar eclipse in 2003 comes this weekend for most
residents of North America and Europe. Two more in 2004
will round out this current spate of lunar
disappearances. Then there will be a nearly 3-year
drought.
What's going on? What causes an
eclipse, and why do they occur on an irregular schedule?
Unpredictability is at the heart of
eclipse lore. For the ancients, a lunar eclipse was a
haunting event. Some believed a dragon of the sky was
eating the Moon, flooding it with blood before consuming
it entirely. Of course it always came back, and that
fueled more speculation.
Here are 10 facts about the dark
history of lunar eclipses and how they really work.
1) Earth gets in the way.
The cause of lunar eclipses is
simple: Earth gets in the way.
The Moon is a visible beacon
because it reflects sunlight. The Moon makes no light of
its own. Earth always casts a shadow into space, and
every now and then everything aligns just right so that
the shadow falls on the Moon.
Lunar eclipses can only occur at
Full Moon, that time each month when the Moon is directly
opposite the Earth in relation to the Sun. It's like you,
representing the Earth, standing in front of a bright
light and casting a shadow on a child.
This week's total lunar eclipse
occurs Saturday evening, Nov. 8 in North and South
America and early Sunday morning for Europe and Africa.
Next: So why don't they occur every
month?
2) Why lunar eclipses don't
occur every month
Since lunar eclipses occur always
at Full Moon, it makes sense to ask why each Full Moon
does not generate an eclipse.
Eclipses are relatively rare
because the plane in which the Moon orbits around Earth
is tilted 5 degrees compared to the plane of
Earth's travels around the Sun, a
plane that astronomers call the ecliptic.
To visualize, think of commingling
Hula Hoops floating on the surface of a pool, and push
one down so that half of it is below the surface and half
above.
When the Moon gets into the
ecliptic --right at the surface of the pool -- during its
full phase, then a lunar eclipse occurs. (The word
"ecliptic" stems from the word "eclipse.")
The geometry of any eclipse -- the
relative positions of the Sun, Earth and Moon -- is
eventually repeated during a set of complex cycles that
each last just more than 18 years. This Saros cycle, as
the whole thing is called, is behind the bunching of
eclipses, too. Astronomers have
figured it out and can predict eclipse timing and
circumstances far into future.
Next: So just how often do they
occur?
3) Lunar eclipses are frequent,
relatively speaking
Though you might not have logged
many lunar eclipses in your life, they are common
compared to solar eclipses, at least in one sense.
Solar eclipses are fairly numerous,
generally two to five per year, but the area on the
ground covered by totality is only a few tens of miles
(kilometers) wide, so it's rare to be in the path of a
total solar eclipse. In any given location on Earth, a
total solar eclipse happens only once every 360 years.
Lunar eclipses are less frequent,
but total lunar eclipses are visible everywhere that it
is nighttime as the event takes place -- essentially half
the globe.
Any given location can experience
up to three lunar eclipses per year, as last happened in
1982. Some years there are none.
Next: What would we see from the
Moon?
4) Lunar eclipses visible from
the Moon, too
When Earth experiences a total
lunar eclipse, things get interesting on the Moon, too.
If you were there, on the side
facing Earth, the home planet would block out the Sun.
The Sun's light would not completely disappear, however.
Earth would be ringed by light scattered through its
atmosphere.
SPACE.com's Night Sky Columnist,
Joe Rao, puts it this way, "The Sun would be hidden
behind a dark Earth outlined by a brilliant red ring
consisting of all the world's sunrises and sunsets."
The light refracted by all these
sunrises and sunsets can fall on the Moon, giving it a
red glow instead of it completely
disappearing.
Each eclipse is different, however,
and some yield little of this reddening effect.
Next: Ancient lore - A bite out of
the Moon
5) A bite out of the Moon
For ancient people, eclipses were
odd and inexplicable, even terrifying. Some cultures saw
lunar eclipses as signs of celestial wrath that portended
famine or disease.
The Chinese word for eclipse is
chih, which means "to eat." One can imagine that the
bloody cast of the Moon in some eclipses only added to
the fear of what was going on. Even into the 19th
Century, the Chinese navy fired cannons to scare off the
dragon they imagined was eating the Moon.
Next: Myths persist today
6) Myths persist today
Myths die hard. In Japan, some
people still cover wells to avoid being poisoned by the
disease of the Moon during an eclipse.
Native residents of Arctic regions are
known to turn over their utensils to avoid contamination.
In other cultures, people yell at
the Moon during an eclipse, or they bang pots or even
shoot into the air.
7) Stonehenge may have predicted
eclipses
The mysterious arrangement of
boulders in England has long been associated with
celestial meaning. But scientists have struggled to
figure out what the meaning was for the folks who erected
the rocks about 3,500 years ago.
One purpose may have been to
predict lunar eclipses.
In the year 2000, a researcher who
had been studying Stonehenge for two decades stumbled
upon the observation that if a person placed stone
markers at strategic locations atop 19 columns in the
array, a known 47-month cycle of lunar eclipses would
become apparent.
8) Eclipses changed history
When the Moon disappeared in 413
BC, Athenians saw it as a bad omen and delayed their
planned retreat from the Sicilian city of Syracuse, where
they had fought for two years in the Peloponnesian War.
The Syracusans used the delay as an
opportunity to break the siege, contributing -- some
believe -- to the fall of Greek civilization.
Christopher Columbus actually used
an eclipse knowingly to perhaps alter history.
Stranded in Jamaica in 1503, on his
fourth voyage, Columbus and his crew were wearing out
their welcome with the natives, who were feeding them.
Columbus knew a lunar eclipse was coming, so he
"predicted" the Moon's disappearance. The natives begged
him to bring it back and, of course, he did, in due time.
9) Eclipse time limits
Columbus knew that lunar eclipses
don't go on forever. Astronomers say none can last more
than 3 hours and 40 minutes. Totality cannot run more
than 1 hour and 40 minutes.
The period of totality for the Nov.
8-9 eclipse will be just 25 minutes.
An eclipse on May 4, 2004 -- best
visible in Europe -- will have a period of totality of 1
hour and 16 minutes.
Why the difference? Earth's shadow
is cone-shaped. Envision a slice of it, a two-dimensional
circle through which the Moon can pass. Longer stretches
of totality mean the Moon is traversing the center of the
circle. Shorter total eclipses occur when the Moon's path
is nearer the top or the bottom of the shadow.
10) The Moon's Name
Full Moons have all sorts of names
that have been passed down from ancient (and sometimes
more modern) cultures. Many of the names common in North
America come from Native American traditions, others were
imported from Europe.
November's Full Moon is called the
Beaver Moon by others (since, presumably, beavers are
actively preparing for winter). There are other names.
And in this sense, every eclipse
has a name, too. You can, by another tradition, call this
the Frosty eclipse. Weather, permitting, you'll get to
see it and, if tradition is any guide, you'll want to
dress warmly.
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