All about Solar Eclipses

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"All about Solar Eclipses"
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Nothing there is beyond hope,

Nothing that can be sworn impossible,

Nothing wonderful, since Zeus,

Father of the Olympians,

Made night from mid-day,

Hiding the light of the shining Sun,

And sore fear came upon men

- Archilochus (ca. 680-645 BC)

Total solar eclipses struck wonder and terror into the hearts of ancient observers. The Greeks named the phenomenon "eclipse," based on their word for "abandonment." Wherever people worshipped the Sun as a life-giving god, the apparent "eating" of the Sun by the Moon was seen as an ominous event.

Since eclipses begin almost unnoticeably with the Moon moving placidly toward the Sun and then beginning to take a small "bite," these events would have taken almost all ancient people by fearful surprise. Many would bang drums and shake noisemakers to ward off evil spirits during total eclipses.

But some advanced civilizations took astronomy seriously and made detailed records of solar eclipses some 4,000 years ago. These people were rarely taken by surprise. Since eclipses were considered signs from heaven foretelling the future of the emperor and the empire, accurate predictions of their future appearances were considered a matter of highest concern, a matter of state. Failure to make accurate predictions cost some astronomers their lives.

The Chinese really did believe that a dragon eats the Sun during totality. Their word for eclipse literally means "to eat." They also believed the dragon eats the Moon during lunar eclipses.

Greeks were so fearful of the gods that they actually ended a war during an eclipse in deference to their gods' anger.

Europeans too feared eclipses. They saw them as omens, foretelling the death of monarchs and the ending of kingdoms and empires. In his epic poem, "Paradise Lost," Milton did not neglect the depiction of this primal fear:

As when the Sun, new risen,

Looks through the horizontal misty air,

Shorn of his beams, or from behind the Moon,

In dim eclipse, disastrous twilight sheds

On half the nations, and with fear of change

Perplexes monarchs

As astronomers over the centuries busied themselves by refining their measurements of planetary orbits and other celestial events, it became clear that solar eclipses were natural events. There was no eating involved at all. Nor were eclipses portents of anything related to human affairs, except perhaps human fear.

The Moon is about 2,160 miles in diameter. It orbits the Earth every 29.5 days. A solar eclipse occurs whenever the Moon moves between the Earth and the Sun, casting a shadow on the surface of the Earth. This doesn't happen every month because the Moon's orbit tilts at about 5 degrees with respect to Earth's orbit around the Sun. This is why we see the Moon rise in a more northerly or southerly portion of the sky at different times of the year.

At least twice a year, the Sun, Moon and Earth line up so that some portion of the Earth is shadowed. That portion covered by the Moon's faint outer shadow, the penumbra, experiences a partial eclipse. That portion fully covered by the Moon's dark inner shadow, the umbra, experiences a total eclipse.

Astronomers must engage in careful study of the comparative motions of the Earth and the Moon in order to calculate where total solar eclipses will take place in the years to come. The umbral shadow races over the Earth for 10,000 miles, but this path of totality is only 100 miles wide. The chance of any specific place on Earth experiencing totality during any given eclipse is quite slim. Some areas haven't had a total solar eclipse for hundreds of years.

The Moon completely covers the Sun for only a few minutes during an eclipse. Sunsets usually refract light towards the reds. That's why humans are used to seeing those colors in the twilight sky. But as the Moon begins to edge over the Sun, the sky begins to take on an unusual, unearthly, dark blue tinge. This is why "plein air" artists eagerly ready their canvases, brushes, and palettes to capture that strange light well before totality.

Twilight comes in the middle of the day. The temperature begins to drop and so does the amount of radiation reaching the skin. Flower petals begin to close. Birds stop singing. Bees stop flying. Herd animals head for shelter. Wild animals hide. They are all preparing for a night that will end in mere minutes.

When the Moon almost covers the Sun, leaving only a crescent of light, some eerie effects occur, effects artists and photographers eagerly await. The leaves of a tree will fracture the shadow thrown by the eclipse into thousands of tiny eclipse images on a sidewalk or a driveway nearby. Wavy patterns of interference lines will shimmer on light-colored surfaces such as houses and office buildings. They undulate in response to distortions in the atmosphere. These shadow bands look very much like the wavy lines that can be seen at the bottom of a swimming pool during a sunny day.

Sharp-eyed astronomer Francis Baily drew attention to irregularities on the Moon's surface when he observed and described tiny specks of light remaining visible during the last 15 seconds before totality. This string of beads, "Baily's Beads," is the last bit of sunlight shining through valleys between lunar mountains. If anyone had still believed the Moon was smooth in the 18th century, Baily's observations proved them wrong.

Then the Moon covers the Sun, leaving visible only a halo of superheated plasma, the solar corona. Before the development of telescopes that could be adjusted to block the Sun's rays, creating artificial eclipses on demand, astronomers had to travel long distances to observe the corona during total eclipses. Normally, direct sunlight washes out the corona. Now, Earth-based and orbiting telescopes can record the ebbing and flaring of coronal energy output for hours at a time.

The types of solar eclipses vary with the Moon's distance from the Earth, which ranges from 221,000 to 252,000 miles. As the distance changes, the Moon's apparent size as seen on Earth varies. This means the Moon's apparent size isn't always large enough to cover the Sun, which means any solar eclipse occurring during the far side of the Moon's orbit will not result in totality anywhere on Earth. The best eclipse that can be observed at that point is one that permits a bright ring of sunlight to shine through. This is called an annular eclipse.

During some rare solar eclipses, the slight differences in the curvature of the Earth play an unusual role. The Earth is a bit wider at the equator than it is at the poles. So, if the apparent size of the Moon is large enough to just barely covers the Sun "here" as the umbra races across the Earth's surface, it may not be enough to cover the Sun "there" in the umbral path. The resulting mixed-type eclipse will next occur in 2013.

After totality, the Moon slowly releases its shadowy grip on the Sun's rays. A bit of sunlight flashes out and the corona disappears. Baily's Beads once again appear, then a thin crescent with the eerie effects described above, and then full daylight returns. Flowers open, birds chirp, animals come out of hiding. Life returns to normal. Only the memory of a deep feeling of awe, paintings carefully rendered, and photographs quickly taken remain as evidence of one of nature's most spectacular events: a total solar eclipse.

This video shows a total solar eclipse occurring over Turkey. People are standing in the remains of an ancient Greek theater with their gear watching the eclipse.

A beautiful time-lapse video of a solar eclipse may be seen here.

Watching partial and total solar eclipses is dangerous. Protect your eyes. Use gear recommended here.

More about this author: Sally Morem

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