—Callimachus, 240 B.C.
Such severe storms were so common on the Mediterranean when they glittered in the eastern sky during their early rising in October, that their appearance was a signal for the closing of navigation. After the stars had made their final disappearance in the west, the people held a festival to celebrate their great relief and joy. The Kids were certainly not a popular star group with the ancient seaman's wife, for literature pictures her as clasping her hands and gazing fearfully out to sea as the winds and waves swirled and leaped under the influence of the "mad stars" and imperiled the safety of her dear ones.
The beautiful first magnitude star Capella may be seen any month of the year except July. It lies below and to the north of Perseus (three bright stars dangling below the Big W) and may be sighted in a straight line from the top of the bowl of the Big Dipper.
In the early evening on February 5th it lies exactly on the meridian halfway between Orion and the North Star, while in August it is low in the northeast and in June low in the northwest. The spectroscope has shown that Capella really consists of two great suns of nearly equal brilliance. These two stars lie so close together that there is not much more than half the distance between our earth and sun between them,—and they revolve around this common center of gravity in only 104.2 days!
As they revolve around this point the lines in the spectroscope periodically split, thus proving the double nature of the star. In the present state of our knowledge about eight stars out of twenty are binaries or multiples.
Short as seems the period of revolution of Capella and its companion compared to Sirius and its companion, which is about 50 years, or the couple of couples composing Castor, which is about 900 years, there are stars so close that they complete a revolution in a few hours. The very atmospheres of one pair of suns, δ Cephei, perhaps intertwine like lovers' arms, for they whirl about their common center of gravity in only 4½ hours!
The distance of Capella from the earth has been estimated as being about 34 light years. A light year is the distance that light would travel in one year at the rate of 186,000 miles a second. This is about six million million miles! Thus the twin suns in the system of Capella could have rolled off their tracks and exploded into atoms 29 years ago and yet we would know nothing of this catastrophe for 5 years to come.
The fact that light takes a certain amount of time to travel and is not instantaneous, was first discovered by watching the little 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.