Pearly white streamers of the solar corona blaze across the darkened sky during a total solar eclipse. Normally hidden in the Sun's glare, the outer atmosphere of our home star is visible to our eyes only when the Moon blots out the solar disk.

that eclipses recur in patterns after an interval called the "Saros cycle ," which lasts 18 years, 11 days, and 8 hours. Combining that cycle with Earth's rotation allows astronomers to forecast the exact paths and durations of eclipses far into the future. Such cycles are common in the universe because the simple physical laws that govern motions often lead to repeating patterns.

However, ancient astronomers explained those patterns in a manner very different from the way we explain them today. In their conception of the universe, Earth sat still at the center while everything else rotated around it. This was a perfectly natural assumption. After all, our feet are planted firmly on the ground. We have no sense of whizzing through space or of spinning on an axis at hundreds of miles per hour. In this "geocentric " framework, the Sun, Moon, planets, and stars surrounded Earth on a nested set of spheres . Some of the motions were easy to describe. For example, the stars shone from the outermost shell, which an unseen deity rotated at a steady pace.

Accounting for the motions of the planets required more complicated schemes. Their wanderings included zigzags and loops that simple rotation on invisible spheres could not explain. In the second century A.D., the Greek scientist Ptolemy devised an elaborate model of the heavens in which the planets traveled on small circles, called "epicycles ," while also moving around Earth on their larger spheres. This reproduced the curlicues of planetary motion among the stars, although never exactly as astronomers observed them. Even so, Ptolemy 's model reigned for nearly 1,500 years in the absence of further advances.

In the sixteenth century, the Polish astronomer Nicolaus Copernicus put forth the next serious model of the solar system, one in which all planets, including Earth, moved around the Sun. (The Greek scientist Aristarchus of Samos is said to have proposed this in the third century B.C., but those writings have not survived. ) Copernicus still thought the planets orbited in perfect circles, which isn't quite true. Further, his ideas weren't broadly accepted until long after his death. Even so, they prepared the way for the man considered by many to be the first modern scientist: Galileo Galilei of Italy.

Galileo was the first person to use a telescope for astronomy. In 1610 he found four points of light along a line on both sides of Jupiter. The points seemed to move (continued)