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Oct - Nov 99
Astronomical
Feature of the Month
ECLIPSES
Eclipse! Just saying the word seems to invoke
feelings of mystery and magic in everyone’s mind. Indeed, seeing one is a rare
and special treat. Any one spot on Earth will experience a total solar eclipse on the
average of only once every 350 years (which makes the next two totalities even more
amazing - in just 18 months TWO total solar eclipses will cross the heart of Africa and
Madagascar and end in the Indian Ocean!) So to see a "totality", you have to be
in just the right spot on the earth. And "just" is the right word! When you look
up at the Sun and 
Moon, they appear to be about the same size. The Sun is
actually 400 times larger than the Moon, but by an amazing coincidence, it is also 400
times further away, reducing its apparent size to the same as the Moon’s. The
match is so good that the "path of totality" is never more than 167 miles in
diameter, and is usually less. Sometimes the Moon is too far away to block 
the
entire Sun out and a "ring" of Sun surrounds it. This is called a
"annular" eclipse. An eclipse proceeds in what is called "contacts."
First contact is when the disk of the moon touches the outer edge of the Sun for the first
time. Second contact occurs when the Moon’s disk touches the inner edge of the Sun,
completely blocking it off and commencing the "totality" portion of the eclipse.
Third contact happens when the Sun comes back out from behind the edge of the Moon, and
fourth contact occurs when the disk of the Moon completely uncovers the Sun. Often just
before second contact and just after 
third contact, a beautiful
phenomenon called a "diamond ring" happens. The Moon is not perfectly round, nor
is the Sun. A diamond ring forms as small portions of the Sun peek out from behind the
Moon. During totality, the sky darkens and many stars and planets become visible in the
sky, as do solar prominences and the solar corona. The view is often described as
ethereal, mystical, and even divine. No one who sees a totality ever forgets it.
International Association of Astronomical Artists