Millions and millions of people have observed solar eclipses during centuries and millennia past. Many of those people believed that the Sun disappeared because it was being eaten by a dragon, by a wolf or serpent. Or even a black pig.

While most of us in this 21st Century world have advanced beyond such beliefs, it is also true that many people do not fully understand how eclipses occur. Perhaps least understood are so-called partial or annular eclipses, of the type coming June 10.

Because the orbit of the Moon is tilted about five degrees off from the plane of the Earth's orbit around the Sun, most of the time the New Moon will seem to pass either just above or just below the Sun in our sky, and no eclipse results. If, however, the New Moon lies at or close to that point where its path in the sky and that of the Sun appear to intersect (called a "node") then a solar eclipse results.

A total eclipse of the Sun takes place when the Moon casts its dark conical shadow, called the umbra (from Latin for "shade") onto the Earth's surface. The umbra is the central core of the shadow and is uniformly dark. From within the umbra the Sun will appear completely blotted out by the Moon.

Although the Sun is about 400 times larger than the Moon, the Moon in turn is about 400 times closer to the Earth than the Sun. Call it a fortuitous circumstance or just plain dumb luck, but because of this size/distance combination, both the Moon and Sun appear virtually the same size in our sky -- roughly one half of a degree in width.

The umbra is at most 170 miles (270 kilometers) wide where it touches Earth. It can travel a third of a way around the Earth in just a few hours. So a total eclipse can only be seen from a narrow band called "the path of totality."

It was mentioned above that a total eclipse of the Sun occurs about once every 18 months. However, for a specific geographic location, total solar eclipses are few and far between. In fact, any given spot on the Earth's surface will play host to a total solar eclipse on an average of once every 375 years.

So unless you are exceedingly lucky and happen to live where the path of a total eclipse is expected to pass sometime in the near future, it is likely that you will need to travel in order to witness a total eclipse of the Sun.

As it happens, there is one scheduled for later this year, on December 4. The regions that will be touched by the Moon's umbral shadow include parts of southern Africa, the Indian Ocean and southern Australia.

Total solar eclipses also seem to have a perverse habit of occasionally passing over inaccessible parts of the world. You might want to keep this in mind should you miss out on this December's event, for the next one after that is scheduled for November 23, 2003. (Antarctica anyone?)

Surrounding the umbra is the penumbra (Latin, paene-umbra, "almost shadow"), also conical but much larger (5,000 miles, or 8,000 kilometers) in diameter. The penumbra is simply the half-shadow that lies outside every deep shadow, whether it's cast by the Moon or a house. It is the umbra's outer shell and wherever the penumbra falls, a partial eclipse will occur.

For most people along the path of the partial eclipse, there will be no noticeable change in overall sky brightness. But folks in the parts of the South and West, where between 70 and 80 percent of the Sun is blocked, may see a hint of twilight. Locations include San Francisco, Los Angeles and Phoenix.