What Does a Solar Eclipse Look Like?
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If you are in the path of totality the eclipse begins with
a partial phase in which the Moon gradually covers more and
more of the Sun. This typically lasts for about an hour
until the Moon completely covers the Sun and the total eclipse
begins. The duration of totality can be as short as a few seconds,
or as long as about 8 minutes. An Eclipse Tree
As totality approaches the sky becomes dark and a twilight that
can only be described as eerie begins to descend. The last flash of
light from the surface of the Sun as it disappears from view behind the Moon gives the
appearance of a diamond ring and is called, appropriately, the diamond ring effect (image
at right).
During Totality
As totality begins, the solar corona (extended outer
atmosphere of the Sun) blazes into view. The corona is a
million times fainter than the photosphere (surface)
of the Sun. Only when the eclipse is total can the
corona be seen. At this point the sky
is sufficiently dark that planets and brighter stars are
visible, and if the Sun is active one can typically see
the Sun's chromosphere, just above the photosphere,
marked with solar prominences and flares
around the limb of the Moon, seen as bright pink
(see image at left).
Predicting Eclipses
Two different cycles of the Moon determine the pattern of eclipses over
time. First, a solar eclipse may occur only at a new Moon, as the
Moon passes between the Earth and the Sun, casting its shadow toward the
Earth. The other cycle involves the gradual shift in orientation of the Moon's
orbit. The Moon's orbit is tilted at about 5o to the Earth's orbit.
The Moon usually passes
above or below the Sun as seen by Earth.
Only when these two cycles are favorably combined can a solar eclipse occur.
When the new Moon is near one of its nodes a solar eclipse occurs. (The nodes are the two points where the Moon's orbit intersects the plane of the Earth's orbit, the ecliptic.) The nodes of the lunar orbit are gradually shifting their orientation in space, giving us proper alignment about every 346.6 days, called an eclipse year.
Approximately every 6585.3 days, these two cycles are in the same phase again, and we begin a new eclipse cycle season. This cycle, termed the Saros cycle, was known to ancient people by comparing dates of earlier eclipses, even though they knew nothing about the relative motions of the Moon, Earth and Sun!