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.

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:
  1. Total Solar Eclipses occur when the umbra of the Moon's shadow touches a region on the surface of the Earth.
  2. Partial Solar Eclipses occur when the penumbra of the Moon's shadow passes over a region on the Earth's surface.
  3. 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.)

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.

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