A solar eclipse occurs when the Moon, during its monthly trip around
the Earth, happens to line up exactly between the Earth and the Sun, so that it casts a shadow on the Earth. But
because of the Sun's large diameter, the shadow consists of two regions. The innermost cone of total darkness is called the umbra
(latin for "shadow"),
and it is projected in the center. Anyone in this central area will observe the total eclipse
, because the Sun will be temporarily obscured by the Moon. The outer shadow is partially illuminated
by the Sun, and is called the penumbra
. Anyone in this region will see the partial eclipse
, since the Sun is only partially obscured by the Moon. (Unfortunately, the terms "umbra" and "penumbra" are also
used to describe regions of sunspots (see the glossary
) so be careful to avoid confusion!)
(Image credit: Norm Sleep)
The glory of a solar eclipse comes from the dramatic view of
the Sun's corona, or outer atmosphere, which we can see only
when the brilliant solar disk is blocked by the Moon.
The corona is not just light shining from around the disk.
It is actually the outermost layer of the solar atmosphere.
Although the gas is very sparse, it is extraordinarily hot
(800,000 to 3,000,000 K), even hotter than the surface of the Sun!
The corona shows up as pearly white streamers, and their shape
is determined by the Sun's current
magnetic fields. Thus every eclipse will be unique and
beautiful in its own way.
If an eclipse occurs when the Moon lines up between the Earth and the
Sun, shouldn't there be an eclipse every month?
Solar eclipses do occur at New Moon, but not at every New Moon.
Most often the Moon passes a little higher or a little
lower than the Sun. There is a solar eclipse once or twice a year,
when the Moon's and Sun's positions exactly line up.
How to observe a solar eclipse
When there is a solar eclipse, it is only visible from a small
area of the Earth. It's unlikely that, during your lifetime,
you will ever have a total solar eclipse right where you live.
However, many people travel long ways to experience a total
solar eclipse. Or, if you're lucky, you might be able to see a
partial solar eclipse (one where the Moon doesn't quite cover
all the Sun's disk) nearby someday.
You can safely observe a TOTALLY eclipsed Sun with your eyes.
However, for observing the beginning and ending of an eclipse,
or for a partial eclipse, you will need a
an appropriate type of welder's glass,
or special Mylar glasses to observe the Sun safely.
The July 22, 2009 Solar Eclipse
Image Credit: Dr. Andrew Sinclair. Source: http://eclipse.gsfc.nasa.gov.
Eclipse Predictions by Fred Espenak, NASA/GSFC
News and Press Releases
Watch the July 22, 2009 Eclipse On the Web:
Live Eclipse 2009 (Japan)
National Astronomical Observatory of the
Chinese Academy of Sciences
Images of the 22 July, 2009 Solar Eclipse
Eclipse Video Clips (visualastronomy.com)
NASA's 3D animation
Satellite images of eclipse shadow
Magnetohydrodynamic prediction of the corona
Predictions of the coronal shape
Lunar limb profile
Other Web Sites on the July 2009 Eclipse
Solar Eclipse Activities
If you have a SID monitor, be sure to read:
The Ionosphere during a Total Solar Eclipse