One of the
most remarkable coincidences found in nature is the fact that
the Moon and the Sun both appear the same size as seen from Earth.
The Moon, a small, cold, and dark body is only 3,500 km in diameter
while the sun, a self-luminous, gaseous giant, is 1,400,000 km
across. The coincidence arises from the fact that although the
sun is 400 times larger than the Moon, it is also 400 times farther
from Earth. A direct consequence of this fortuitous geometry is
that during a total solar eclipse, the Moon occludes the Sun with
a nearly perfect fit.
The fundamental
basis of the solar eclipse is the alignment of the Sun, Moon and
Earth such that some region of Earth passes through the Moon's
shadow. This shadow is composed of two parts: the outer or penumbral
shadow and the inner or umbral shadow. From within the penumbra,
only part of the sun is obscured. In contrast, the dark, central
umbra is the shadow of complete or total eclipse. During a total
eclipse, the umbra sweeps across Earth from west to east and the
course it travels is called the path of totality. Anyone standing
within this zone will see the Sun completely obscured by the moon
for as long as 7 1/2 minutes. At this time, the solar corona is
visible as a halo about the moon and the landscape takes on the
appearance of an eerie twilight. Outside the path of totality
but still within the penumbra, a partial eclipse is seen. The
path of the umbra is rarely more than 300 km wide, while that
of the penumbra is about 7,000 km wide. Sometimes the umbral shadow
misses Earth entirely and only a partial eclipse occurs.
