Celestial Coincidence

Solar eclipses are such dramatic and powerful events that they have even stopped wars. The total solar eclipse of May 28, 585 b.c., occurred while the Lydians and the Medes were at war. (Ancient Lydia was in what is now western Turkey; ancient Media was in what is now Iran and Azerbaijan.) According to the Greek historian Herodotus, the combatants were so frightened by the sudden, unexpected darkness that they stopped fighting and made peace.

Just how is our tiny Moon able to line up perfectly with our huge star and block out the entire surface of the Sun's disk? The answer lies in celestial coincidence: For a matter of minutes during a total solar eclipse the Sun and the Moon appear to be the same size in the sky. The Sun is actually 400 times larger than the Moon, but because it is also 400 times farther away it appears to take up the same amount of space in the sky that the Moon takes up. (If we lived on Mars we would never experience a total solar eclipse, because that planet's tiny moons are too small for their distance from Mars to hide the Sun completely.) This size-distance relationship does not hold constant, however, because the Moon does not have a perfectly circular orbit and so its distance from us, and therefore its corresponding apparent size, varies slightly. A total solar eclipse takes place only when the Moon lies relatively close to Earth and blocks all of the Sun's visible surface from view. What becomes visible at this time is the pearly white glow of our star's outer atmosphere, the corona.

A second condition for a total solar eclipse to take place is that the new Moon must pass directly between the Earth and the Sun. Easy! Doesn't that happen every month? No. Because its orbit is tilted, the Moon usually passes above or below the level of the Sun in the sky each month. But on those special occasions when the Earth, Moon, and Sun are in a direct line we have a total solar eclipse.

Partial, Annular, and Total-ly Awesome!

The Moon always casts a shadow in space—a cone-shaped wedge of darkness that points away from the Sun. In the near miracle of a total solar eclipse the cone hits the Earth. This shadow forms an almost perfect ellipse—a sort of squashed circle—on the Earth's curved surface. Over a period of several hours the elliptical shadow crosses the Earth, making a long, narrow path thousands of kilometers long, though up to only a few hundred kilometers wide. Only from within this path of totality do we see a total solar eclipse. If we are in front of or behind the ellipse, or off to its side, we can look “around” the Moon to see part of the Sun. In such an instance we see a partial eclipse, or the partial phases of a total solar eclipse.

Often the shadow cone doesn't hit the Earth. Instead it passes above the planet without making contact even when the Earth, Moon, and Sun are in line. This happens when the Moon is farther than average away from the Earth, and the Earth is closer than average to the Sun. At these times the Moon's disk appears too small to block the Sun completely from us. If we observe the Sun at this time (Always be sure to use a protective filter when looking toward the Sun) we see a ring of ordinary sunlight around the dark disk of the Moon. Because “anulus” is the Latin word for “ring,” an eclipse of this type is known as an annular eclipse.

Annular eclipses occur as often as total eclipses: one about every 18 months. Counting all types of solar eclipses, including partial eclipses, there are at least two and up to five each year. But only during total solar eclipses does the sky darken dramatically and the outer parts of the Sun's atmosphere appear. For each place on Earth a total solar eclipse occurs overhead only about once every 360 years—more often near the equator and less often near the poles. The next such eclipse to cross North America won't occur until August 21, 2017. But now that jet travel is easy, many hundreds of dedicated eclipse observers, including me, travel around the globe to see all the total solar eclipses.

Eclipse Computers

Being able to predict when the sky will go dark because of an eclipse is reassuring, helps in making travel plans (see sidebar), and has come in handy over the centuries. The ancient peoples who built Stonehenge, a giant set of standing stones in southern England, aligned the stones 4,000 years ago so that pairs of them pointed to positions where the Sun and Moon rose at special times of the year. Some scientists have suggested that Stonehenge was also an “eclipse computer,” but it isn't clear whether this giant observatory could predict eclipses. Independently, more than 500 years ago the Mayan people of Central America, observed the Sun, Moon, and planets and could reliably predict when eclipses might occur. But despite these early attempts at prediction, clouds could hide an eclipse; or an expected eclipse could take place on the other side of the world, so early observers could not always verify the accuracy of their predictions.

Our current ability to predict eclipses is based on Isaac Newton's 17th-century explanation of the law of gravity. (Newton's law of gravity shows how all bodies in space affect one another in a way that is proportional to their masses and the distance between them.) Newton's work allows scientists to use equations to predict how celestial bodies interact. Edmond Halley, who is more famous for his work on comets than on eclipses, used Newton's ideas to become the first to predict in detail the path an eclipse would take across the Earth's surface. He provided maps, in advance, of the path of the eclipse of 1715 that crossed England. Equations that can be solved to find the paths of eclipses were worked out by Friedrich Bessel in Prussia (now Germany) in the 19th century. Today, we use computers to solve these equations.

Now You See It, Now You Don't

An eclipse travels across the Earth's surface at several thousand kilometers per hour. If the eclipse's path is near the Earth's equator, the Earth's rotation keeps up with most of this speed, leaving the eclipse to appear to travel at only about 1,600 kilometers per hour. Once, a high-speed airplane succeeded in keeping up with an eclipse for about an hour: In 1973 a Concorde supersonic plane flew across Africa in the path of an eclipse. Ordinary airplanes can't keep pace with an eclipse, but they can enable observers to get above clouds or water vapor in the Earth's atmosphere to get a better view of an eclipse. Even so, it is usually more interesting and exciting to observe an eclipse from the ground, where you can feel the air grow still and the sky grow darker and darker.

Every few decades an eclipse crosses directly over a major observatory, making detailed scientific observation of the event easy. In 1991 the path of totality crossed the Mauna Kea Observatory in Hawaii, the site of some of the world's largest telescopes. But usually scientists wanting to study eclipses have to carry their instruments to the site in advance, and set up camp within the area of the path of totality. Imagine all the preparation and hardship it took for scientists to ride private railway cars into Siberia for the 1936 eclipse there! Traveling by jet plane to Timbuktu (for the 1973 eclipse) and Papua New Guinea (for the 1984 eclipse), was easy by comparison.

Activity

  1. What is the path of totality?
  2. Today we can predict when lunar and solar eclipses will occur. Imagine you were living in the time and place of the Lydian and Medes. They were so surprised by the eclipse that they ended their war. The next total solar eclipse will not occur for a few more years. What do you think it would be like to witness the event? What would it be like to be watch it quickly grow dark outside? Write a few sentences about what it might be like to witness a solar eclipse.