How do astronomers predict solar eclipses and How often does a total eclipse occur?

“The day was turned to night” reads an inscription on a Babylonian tablet, describing the total eclipse of the sun of July 31, 1062 B.C.

Eclipses were momentous and terrifying events in ancient times; the Babylonians took them to be signs from the gods. They are less mysterious to us in the twentieth century, now that we understand our solar system better and can even predict when an eclipse will occur.

Nevertheless, a total solar eclipse is still a startling event: the black circle of the moon slides in front of the sun and seems to swallow up its light, although the fiery corona of the sun still burns around its edges like a halo. The earth becomes unnaturally dark and quiet for several minutes as people, birds, and animals look up in puzzlement or fear at this strange interruption of their light and warmth.

When the moon, as it orbits Earth, passes between Earth and the sun, it totally obstructs some region’s view of the sun, somewhere in the world. This phenomenon doesn’t happen every time the moon orbits Earth ( every month) as one might expect, because the moon’s orbit of Earth is tilted in a different plane from Earth’s orbit of the sun.

The orbits cross only once every six months in what are called the eclipse seasons. Each eclipse season is a month long, during which time an eclipse occurs somewhere on the earth when the moon lines up between an observer and the sun. For an eclipse to be total, with the sun completely blocked, two more conditions must be fulfilled:

The moon must be in that part of its elliptical orbit where it passes close enough to Earth that it appears to obscure the sun completely, even though the sun is really much larger. When the moon is not sufficiently close, an annular eclipse results (annulus is Latin for “ring”), with the yellow perimeter of the sun showing clearly around the shadow, instead of just the thin, ghostly corona of the total eclipse.

The other criterion for a total eclipse is that the moon’s shadow pass across the center of the sun; partial eclipses result in geographical regions where all of the sun is not covered and the moon seems only to be taking a bite out of it. Since the moon is so much smaller than the sun, eclipses affect only certain areas of the world at a time. A total eclipse happens somewhere on the earth only every 2 or 3 years. A given spot can experience a total eclipse only once every 360 years.

How can anyone tell this? Stargazing is probably the oldest scientific pursuit. In the process of observing the points of light in the heavens, how they seemed to move in relation to one another, man has come up with many different explanations for the system that would produce what he sees. Four thousand years ago the people of Stonehenge used their boulders as an immense astronomical timer, telling the seasons by the positions of stars sighted along different rocks; some scientists think the stones were used to predict eclipses. Ancient Chinese civilizations were certainly able to predict them, without necessarily understanding them.

From the earth, the stars seem fixed relative to each other, whereas the planets are independent, moving at different speeds, seeming to double back on themselves as they traverse the sky, forming looped trajectories over the course of a few months.

Planets is from the Greek word planetes, or “wanderers.” Until the sixteenth century, mankind assumed that Earth was the center of the universe; then the Polish churchman Nicolaus Copernicus came up with the simplest explanation for what he saw. Since Copernicus’s model fits in with other knowledge we have acquired since his time, we accept it as fact: the moon indeed orbits Earth, and the planets all orbit the sun. We have refined his model over the centuries, so that it better suits our purpose of understanding and predicting astronomical events.

Now, the calculation of eclipses for American scientists is carried on with computers in the U.S. Naval Observatory in Washington, D.C. Into those computers is fed the latest information gleaned from instruments planted on the surface of the moon by the Apollo astronauts; we know better than anyone in history the shape of the moon and its distance from us at different times.

Identifying which lunar mountains and valleys will lie along the perimeter of the moon at the time of an eclipse tells us the shape of the shadow it will cast, and therefore who will see the eclipse and when. The technology of prediction is still being refined; we are even now as much as 5 miles off at predicting the “edge of totality”, the borderlines of the areas on the earth where the eclipse is total, but our timing of the period of totality is now correct to within a second or two. Considering the sizes of the bodies involved, such accuracy is impressive.

The last total eclipse in the United States was on February 26, 1979, along an arc shaped swath of territory in the Northwest extending into Canada. The next will be in Alaska in 1990, and in Hawaii in 1991. An annular eclipse will take place in the southeastern United States in 1984.

Why, finally, do scientists bother to keep such careful track of eclipses? It would probably make us nervous if they didn’t, but there are also many practical reasons: to find out more about the sun, the stars, the climate on the earth, and even energy production.

During total eclipses, and only then, we can observe and photograph the solar corona. The corona is the faint outer region of the sun. Its light is too weak to be seen during the day, since the blue of the sky itself is much brighter; and at night, obviously, the sun is obscured from us by the earth. But during a total eclipse, the lunar shadow covers the main light generating area of the sun, so its rays do not scatter off the air molecules in front of us to make a bright blue sky. Instead, the sky is dark, and the glowing gases of the corona appear around the edge of the moon.

Sunspots, “weather” on the surface of the sun, seem to have an effect on weather on the earth, and we can watch those tempests raging in the corona better when the full blaze of the sun does not overload our instruments. Our observation of the gases of the sun’s corona is helping our efforts to make energy from a new source, which does not begin in Arabian oil wells or leave vast quantities of deadly radioactive waste behind: atomic fusion. In fusion, energy is released when two atoms are joined. It is cleaner and more powerful than conventional nuclear power, which is based on fission or the splitting of atoms.

Fusion occurs only under incredibly high temperatures such as those on the sun (11,000 degrees Fahrenheit and hotter), where gases are compressed by the attraction of the sun’s powerful magnetic field. Scientists in the United States and the Soviet Union are trying to make fusion occur by compressing gases in a man made magnetic field inside a huge, doughnut shaped reactor called a tokamak.

During total eclipses, energy researchers analyze the corona to understand better how gases behave under extreme pressure, to improve the design of fusion reactors.