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Solar Eclipses
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
For example there will be 18 solar eclipses from
1996-2020 for which the eclipse will be total on some part of the Earth's
surface. The common 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.
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).
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| 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:
- 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.
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.
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 their 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 section.)
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| 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 first demonstrates 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
Starry Night).
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).
 As
totality begins , the solar
corona
(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 prominences and
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.
Lunar Eclipses
As we have noted in the preceding section, the Earth casts a shadow that the
Moon can pass through. When this happens we say that a
lunar eclipse
occurs. Just as for solar eclipses, lunar eclipses can be
partial or total, depending on whether the light of the Sun is partially or
completely blocked from reaching the Moon. The following figure illustrates a
total lunar eclipse with the Moon lying in the umbra of the Earth's shadow.
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| A Lunar Eclipse |
During a total lunar eclipse the Moon takes on a dark red color because it is
being lighted slightly by sunlight passing through the Earth's atmosphere and
this light has the blue component preferentially scattered out (this is also why
the sky appears blue from the surface of the Earth), leaving faint reddish light
to illuminate the Moon during the eclipse. Here is an
animation of a lunar eclipse.
Upcoming eclipses of the Moon (and Sun) may be found in this
table of eclipses.
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Eclipses