INTRODUCTION:

A solar eclipse occurs when the moon passes between the Sun and the Earth so that the Sun is fully or partially covered. This can only happen during a new moon, when the Sun and Moon are in conjunction as seen from the Earth. At least two and up to five solar eclipses can occur each year on Earth, with between zero and two of them being total eclipses. Total solar eclipses are nevertheless rare at any location because during each eclipse totality exists only along a narrow corridor in the relatively tiny area of the Moon's umbra.

A total solar eclipse is a spectacular natural phenomenon and many people travel to remote locations to observe one. The 1999 total eclipse in Europe helped to increase public awareness of the phenomenon, as illustrated by the number of journeys made specifically to witness the 2005 annular eclipse and the 2006 total eclipse. The recent solar eclipse of January 26, 2009, was an annular eclipse (see below), while the solar eclipse of July 22, 2009 was a total solar eclipse.

In ancient times, and in some cultures today, solar eclipses have been attributed to supernatural causes. Total solar eclipses can be frightening for people who are unaware of their astronomical explanation, as the Sun seems to disappear in the middle of the day and the sky darkens in a matter of minutes.

TYPES OF ECLIPSE:

There are four types of solar eclipses:

A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark silhouette of the Moon, and the much fainter corona is visible. During any one eclipse, totality is visible only from at most a narrow track on the surface of the Earth.

An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon.

A hybrid eclipse (also called annular/total eclipse) transitions between a total and annular eclipse. At some points on the surface of the Earth it is visible as a total eclipse, whereas at others it is annular. Hybrid eclipses are comparatively rare.

A partial eclipse occurs when the Sun and Moon are not exactly in line and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside of the track of an annular or total eclipse. However, some eclipses can only be seen as a partial eclipse, because the umbra never intersects the Earth's surface, passing above or below the earth's polar regions.

Effects During a Total Solar Eclipse:

A total solar eclipse begins almost unnoticeably. As the Moon starts its passage across the face of the Sun, a small "bite" appears on the western edge of the Sun. Gradually, as more and more of the Sun disappears, an interesting effect can be seen: the tiny spots of light shining through the leaves of a tree, for example, show up on the ground as crescent images of the slowly vanishing Sun.

As the partial phase progresses for about an hour, there is little hint of the approaching darkness. But in the last few minutes before totality, daylight fades very quickly.

While a small crescent of the sun remains in the sky, a curious eclipse phenomenon is often observed. Thin wavy lines of alternating light and dark can be seen moving and undulating in parallel on plain light-colored surfaces. These so-called shadow bands are the result of sunlight being distorted by irregularities in the Earth's atmosphere, and are best observed on an open floor or wall.

As the narrow crescent of the Sun finally begins to disappear, tiny specks of light remain visible for a few seconds more. These points of light are spaced irregularly around the disappearing edge of the Sun, forming the appearance of a string of beads around the dark disk of the Moon. These lights are known as Baily's beads, named after Francis Baily, the 18th century English amateur astronomer who was the first to draw attention to them. The beads are actually the last few rays of sunlight shining through valleys on the edge of the Moon.

Baily's beads make their brief appearance up to 15 seconds before totality. When a single point of sunlight remains, a beautiful "diamond ring" effect is created against the outline of the Moon. This final sparkling instant signals the arrival of the moon's shadow. The last ray of sunlight vanishes and totality begins.

Suddenly the sky above is dark. The Moon's shadow, racing along the Earth at speeds up to several thousand miles per hour, brings a swift and dramatic nighttime effect. The sky near the horizon still appears bright, and this distant scattered light produces a slight reddish glow and unusual shadow effects. This daytime darkness is not quite as black as at night. But its startling onset and unearthly appearance combine to create a unique visual ambience.

In the center of this darkened sky hangs the featured spectacle of the eclipse -- the corona of the Sun. This pearly white crown of light shines in all directions around the darkened solar disk. A million times fainter than the Sun itself, the full glory of the corona is visible only during a total solar eclipse. Wispy plumes and streamers of coronal light reach out distances up to several diameters of the Sun before they fade into darkness.
Against the backdrop of the white corona and the black disk of the Moon, two colorful effects are usually seen. First is the light from the Sun's lower atmosphere, the chromosphere.

For a few seconds both after the beginning and before the end of totality, this pinkish glow appears at the edge of the Moon. Also often visible are several solar prominences. These red cloudlike appendages arch above the surface, reach a maximum height of nearly one-third the diameter of the Sun itself.

This marvelous view of the Sun clearly commands the center of attention during totality. But there are other sights to see as well. Because the direct light of the Sun is blocked, some of the brighter stars and planets become visible. Sometimes a total solar eclipse reveals a small comet on its path near the Sun.

The darkness of totality resembles nighttime, and plants and animals react accordingly. Birds stop singing and may go to roost. Daytime flower blossoms begin to close as if for the night. Bees become disoriented and stop flying. The temperature drops in the coolness of the Moon's shadow. All of Nature seems still and quiet for this brief moment of daytime darkness.

And then the shadow passes. A bright speck of sunlight flashes into view at the western edge of the Sun as the corona disappears. Totality has ended. The same events that preceded totality now occur in reverse order and on the opposite side of the Sun. Baily's beads appear, followed by a thin crescent of the Sun. Daylight returns as more and more of the Sun is gradually uncovered by the passing Moon.

Finally the complete disk of the Sun is restored. The eclipse is over. The Moon continues in its orbit around the Earth, casting its shadow off into the vastness of space. Nothing tangible remains of the eclipse except some photographs and scientific data. Yet the memory of the experience is permanent -- the fleeting beauty of the corona etched into the mind's eye by the sheer grandeur of the event. There is simply nothing else like it.

APPEARENCE 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.