The surface of the Sun, called the photosphere, is at a temperature of
about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only
by comparison with the surrounding regions). Sunspots can be very large, as
much as 50,000 km in diameter. Sunspots are caused by complicated and not
very well understood interactions with the Sun's magnetic field.

A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona,
extends millions of kilometers into space but is visible only during a
total solar eclipse (left). Temperatures in the corona are over 1,000,000
K.

It just happens that the Moon and the Sun appear the same size in the sky
as viewed from the Earth. And since the Moon orbits the Earth in
approximately the same plane as the Earth's orbit around the Sun sometimes
the Moon comes directly between the Earth and the Sun. This is called a
solar eclipse; if the alignment is slighly imperfect then the Moon covers
only part of the Sun's disk and the event is called a partial eclipse. When
it lines up perfectly the entire solar disk is blocked and it is called a
total eclipse of the Sun. Partial eclipses are visible over a wide area of
the Earth but the region from which a total eclipse is visible, called the
path of totality, is very narrow, just a few kilometers (though it is
usually thousands of kilometers long). Eclipses of the Sun happen once or
twice a year. If you stay home, you're likely to see a partial eclipse
several times per decade. But since the path of totality is so small it is
very unlikely that it will cross you home. So people often travel half way
around the world just to see a total solar eclipse. To stand in the shadow
of the Moon is an awesome experience. For a few precious minutes it gets
dark in the middle of the day. The stars come out. The animals and birds
think it's time to sleep. And you can see the solar corona. It is well
worth a major journey.

