moons of the planet Jupiter, although it has since been determined from experiments on the surface of the earth. For a long time, an astronomer could foretell on what night and hour an eclipse of a given satellite would occur but he could not make the predicted minute agree with the actual time of eclipse. During the seventeenth century, Roemer, a Danish astronomer, discovered that the eclipses come 8 minutes and 18 seconds early when the earth is nearest to Jupiter and 8 minutes and 18 seconds late when it is on the opposite side of its orbit. He thus was the first to suspect that light could not flash instantaneously across the 186,000,000 miles which is the diameter of the earth's orbit. After it was found that light required 8 minutes and 8 seconds to come from the sun to the earth, it was a simple matter to find the space that it would move over in a year. This number, 63,368 radii of the earth's orbit or about six million million miles, has been since taken as a handy unit of measure in estimating the distances to the stars. Thus a light year is the distance that light will travel in a year at the rate of 186,000 miles a second. An astronomer expressing the distance of a star in terms of this unit would say that Alpha Centauri is 4.26 light years distant instead of 25,000,000,000,000 miles, and 4.26 years would represent the time that it takes light to come from that star to the earth.
Although it takes light only about 8 minutes to reach the earth from the sun and about 4 years from the nearest star, it requires only one second from the moon, which is nearer than the sun, and 47 years from the North Star, which is a long way from being our nearest star. Sirius, our largest star, is distant only 8½ light years, but Vega and Capella, which rank just below it, are respectively three and four times farther away. Some stars are so distant that it requires thousands of years for their light to reach us. According to the calculations of Kapteyn, only one-eighth of the 6200 stars whose distance was measured are nearer than 100 light
[141]
