One consequence of the Moon's orbit about the Earth is that the Moon
can shadow the
Sun's light as viewed from the Earth, or the Moon can pass through the shadow
cast by the
Earth. The former is called a solar eclipse and the later is called a
lunar eclipse. The small tilt of the Moon's orbit with respect to the
plane of the ecliptic and the small eccentricity of the lunar orbit make such
eclipses much less common than they would be otherwise, but partial
or total eclipses are actually rather frequent.
Frequency of Eclipses
there will be 18 solar eclipses from 1996-2020 for which the eclipse
be total on some part of the Earth's surface.
perception that eclipses
are infrequent is because the observation of a total
eclipse from a given point on the surface of the Earth is not a common
occurrence. For example, it will be two decades before the next total
solar eclipse visible in North America occurs.
The next total solar eclipse
will be on
August 11, 1999, with the path of totality crossing the North
Atlantic, Europe, the Middle East, and India.
In this section we
consider solar eclipses and in the next we discuss lunar eclipses.
Geometry of Solar Eclipses
The geometry associated with solar eclipses is illustrated in the following
figure (which, like most figures in this and the next section,
is illustrative and not to scale).
Geometry of solar eclipses
The shadow cast by the Moon can be divided by geometry into the completely
shadowed umbra and the partially shadowed penumbra.
Types of Solar Eclipses
The preceding figure
allows three general classes of solar eclipses (as observed from any particular
point on the Earth) to be defined:
As illustrated in the figure, in a total eclipse the surface of the Sun is
completely blocked by the Moon, in a partial eclipse it is only partially
blocked, and in an annular eclipse the eclipse is partial, but such that the
apparent diameter of the Moon can be seen completely against the (larger)
apparent diameter of the Sun.
Total Solar Eclipses occur when the umbra of the Moon's shadow touches
a region on the surface of the Earth.
Partial Solar Eclipses occur when the penumbra of the Moon's shadow
passes over a region on the Earth's surface.
Annular Solar Eclipses occur when a region on the Earth's surface is
in line with the umbra, but the distances are such that the tip of the umbra
does not reach the Earth's surface.
A given solar eclipse may be all three of the above for different observers.
For example, in the path of totality (the track of the umbra on the
Earth's surface) the eclipse will be total, in a band on either side of the
path of totality the shadow cast by the penumbra leads to a partial eclipse,
and in some eclipses the path of totality extends into a path associated with
an annular eclipse because for that part of the path the umbra does not reach
the Earth's surface.
Total Solar Eclipses
A total solar eclipse requires the umbra of the Moon's shadow to touch the
surface of the Earth. Because of the relative sizes of the Moon and Sun and
relative distances from Earth, the path of totality is usually very narrow
(hundreds of kilometers across). The following figure illustrates the path of
totality produced by the umbra of the Moon's shadow. (We do not show the
penumbra, which will produce a partial eclipse in a much larger region
on either side of the path of
totality; we also illustrate in this figure the umbra of the Earth's shadow,
which will be responsible for total lunar eclipses to be discussed in the next
Solar eclipse (not to scale)
As noted above, the images that we show in discussing eclipses are
illustrative but not drawn to scale. The true
relative sizes of the Sun and Earth
and Moon, and their distances, are very different than in the above figure.
Animations of Solar Eclipses
Here are three animations that illustrate observations in a solar eclipse. The
generally the case of a total solar eclipse; the next two
are simulated views of two recent solar eclipses from unusual vantage
points, one from the Moon and one from the Sun (these last two were constructed
using the program
In these last two simulations, the shadow cast on the Earth is the penumbra,
which can cover a region thousands of kilometers in diameter. If the eclipse
is total, the path of totality traced out by the umbra is much narrower.
Appearance of a Total Solar Eclipse
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, depending on the details.
As totality approaches the sky becomes dark and a twilight that can only be
described as eerie begins to descend. Just before totality waves of
shadow rushing rapidly from horizon to horizon may be visible. In the
final instants before totality light shining through valleys in the Moon's
surface gives the impression of beads on the periphery of the Moon (a
phenomenon called Bailey's Beads). 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).
, the solar
(extended outer atmosphere of the Sun)
blazes into view. The corona is a million times fainter than the surface of the
Sun; thus only when the eclipse is total can it be seen; if even a tiny
fraction of the solar surface is still visible it drowns out the light of the
corona. 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 solar
flares around the limb of the Moon, even without a telescope
(see image at left).
The period of totality ends when the motion of the Moon begins to uncover the
surface of the Sun, and the eclipse proceeds through partial phases for
approximately an hour until the Sun is once again completely uncovered.
Here is a
movie of the 1994 total solar eclipse (3.1 MB MPEG;
Source; here is a
QuickTime version, but note that it is 15 MB in length).
A partial solar eclipse is interesting; a total solar eclipse is awe-inspiring
in the literal meaning of the phrase.
If you have an opportunity to observe a total solar eclipse, don't miss it! It
is an experience that you will never forget.
Patterns of Eclipses
Because solar eclipses are the result of periodic motion of the Moon about the
Earth, there are regularities in the timing of eclipses that give cycles of
related eclipses. These cycles were known and used to predict eclipses long
before there was a detailed
scientific understanding of what causes eclipses. For example, the
ancient Babylonians understood one such set of cycles called the
Saros, and were able to predict eclipses based on this knowledge.
Here is a link to a
discussion of such cycles
and regularities in eclipse patterns.
Solar Eclipse Resources
Here are some resources for those interested in keeping track of eclipses.