This is a diagram of a typical solar eclipse. During a total solar eclipse, the umbra reaches the Earth. During an annular eclipse, it does not. An eclipse occurs when the Moon passes in the path of the Sun and Earth.
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An eclipse of the Sun occurs when the Earth passes through the Moon's shadow.
A total eclipse of the Sun takes place only during a new moon, when
the Moon is directly between the Sun and the Earth and is positioned
just right to cast a shadow on the Earth.
When a total eclipse does occur,
the Moon's shadow covers only a small portion of the Earth, where the eclipse
is visible. As the Moon moves in its orbit (at 1 km/s), the position of
the shadow changes, so total solar eclipses usually only last a minute or two in a given location.
In ancient times, people were frightened by solar eclipses (even back then
people realized that the Sun was essential to life on Earth).
Now eclipses are of great interest to the public and to solar astronomers.
Eclipses provide an opportunity to view the Sun's outer atmosphere,
the solar corona.
Because eclipses occur infrequently, solar astronomers have built special instruments called coronagraphs to
view the Sun. Coronagraphs block out ("occult") the light coming
from the photosphere, allowing the light scattered from the corona to be viewed.
If you ever get to view a solar eclipse, make sure to never look at the Sun directly! Always use one of these safe techniques.
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