One Universe: At Home in the Cosmos

Motion | Pages 12-13 | See Linked Version

A Cosmic Clock

If you look toward the North Star on a clear evening on January 1, you see constellations such as Cassiopeia (left panel, top), the Little Dipper, and the Big Dipper. Six hours later (center panel), the constellations have rotated one-quarter of the way around the sky, due to Earth's spin on its axis. After 12 hours (right panel), our planet has completed half of its daily 24-hour rotation and the constellations have traveled halfway around the sky. Similarly, the constellations appear to circle the North Star over the course of a year as Earth revolves around the Sun. Thus the sky looks the same on July 1 at 6:30 p.m., halfway through the year, as it does on January 2 at 6:30 a.m., halfway through the day. Observations of these and other recurring patterns in the cosmos led to the calendars we use today.

January 1, 6:30 p.m.

January 2, 12:30 a.m.
April 1, 6:30 p.m.

January 2, 6:30 a.m.
July 1, 6:30 p.m.

west each day. At night the stars moved in the same direction, apparently revolving around a single point in the northern sky. By tracking the positions of the Sun and other stars for thousands of days, these earliest astronomers found that they followed predictable paths with repeating cycles. Skywatchers also monitored the locations of many patterns of stars, or constellations. When certain constellations appeared exactly at sunrise or sunset, the astronomers could determine how long daylight would last, how warm or cold it would get, and how many more days they could count on such weather.

This evolving awareness of the flow of time, patterns in the heavens, and the change of seasons led to the first agricultural societies and civilizations. The stars served as harbingers of many key events in those societies. The cyclical nature of those events led to the creation of the calendar, the system by which we still organize time today. For example, ancient Egyptian farmers knew that when the bright star Sirius rose in the east just ahead of the Sun, it was time for the Nile's annual flood. The warmest weather in the Northern Hemisphere also occurred at the same time. Since Sirius is called the "Dog Star" for its position in the constellation Canis Major (the great dog), we still refer to those warm weeks as the dog days of summer.

Other remnants of early gazing at the sky persist in modern society. The Babylonians divided the annual solar cycle into months, based on a narrow band of 12 constellations through which the Sun traveled during the year. As each star pattern in turn appeared at the sunrise horizon, the Babylonians knew that one-twelfth of the year had passed. Today, this "zodiac" still provides fodder for mildly amusing astrological predictions about our lives on the daily comics pages. Other cultures, notably in China and the Middle East, divided the year into different but equally accurate segments based on the changing shape and position of the Moon. The lunar calendar still dictates the timing of such events as Easter, Passover, Ramadan, and the Chinese New Year.

Ancient astronomers also excelled in their studies of eclipses--alignments with the Sun in which the Moon casts a dark shadow upon Earth or vice versa (page 16). A special cosmic coincidence allows the Moon to blot out the Sun. Although the Sun is 400 times larger than the Moon, it is also 400 times farther away from Earth. The two bodies thus appear almost exactly the same size in the sky, a situation that occurs nowhere else in the solar system. As a result, Earth is the only planet that experiences solar eclipses in such a spectacular fashion. The Moon's shadow sweeps across the landscape at more than 1,000 miles per hour, casting a brief pall of night in the middle of the day. Temperatures drop, birds stop singing, blossoms close, and nature itself seems to pause. An otherworldly glow surrounds the Moon's black disk. That's the corona, the million-degree atmosphere of the Sun that we usually can't see.

Total solar eclipses happen every couple of years, seemingly over random parts of the globe. But observers who tracked the motions of the Moon and the Sun realized (continued)