Question

Today, astronomers use computers and mathematical models to determine when eclipses and other celestial events will occur. But people have been predicting and observing eclipses for centuries, far before the advent of technology. Ancient civilizations around the world independently discovered that similar eclipses happen about once every 18 years. That’s how long it takes for the sun, moon, and Earth to return to the same relative positions. This period of time is known as a saros cycle. The word “saros” likely comes from a Greek word that means “to sweep.” It may be a reference to the way eclipses appear to sweep across the sky. A single saros cycle can continue uninterrupted for more than a thousand years. Eventually, a new one will begin with slightly different timing, and the 18-year intervals will start again.

It’s estimated that a total solar eclipse is visible only about once every 370 years in any given location. So how did ancient people, who rarely traveled long distances, figure out this pattern? It’s because the saros cycle applies to both solar and lunar eclipses. Lunar eclipses occur much more frequently, and they’re visible from anywhere in the world at nighttime.

Which of the following best describes a saros cycle?

A. A rare type of lunar eclipse during which the moon appears green

B. A period of about 18 years that’s used to predict eclipses

C. A mathematical model used by modern-day astronomers

D. A never-ending cycle of eclipses that occur every 370 years

Answer 1

B. A period of about 18 years that’s used to predict eclipses