Page:Popular Science Monthly Volume 5.djvu/442

426 till after the year 1840. The same is to be said of Alpha in Centaur, observed in 1832 and in 1839 on the Cape of Good Hope by Henderson and Maclear; this is the nearest to us of all the stars.

There are two ways of determining these parallaxes. The first is, to compare together the positions observed at intervals of six months; the other, to discover an apparent motion in a star (as compared with a motionless star situated at a far greater distance than that which is studied): this apparent motion being due to the perspective produced by the annual revolution of the earth in its orbit. This is the method mostly employed now. Galileo, in his "Dialogues;" Gregory, in the "Proceedings of the Royal Society" (1675); Huyghens, in his "Cosmotheoros," published in 1695; Condorcet, in his "Éloge of Roemer," in 1773; and William Herschel, in 1781, have described both methods. Hooke, Flamstead, Cassini, Bradley, Robert Long, Herschel, Piazzi, and Brinkley, strove, from 1674 to 1820, to determine the small quantity of the apparent movement of the brightest stars, which used to be regarded as the nearest; but their efforts were fruitless, owing to the inconsiderable amount of this motion. There was need of instruments of the utmost precision, a rigid spirit of observation, and an indomitable patience, in order to get at trustworthy results.

Since 1840 the attention of astronomers has been oftentimes directed to this investigation, and thousands of calculations have been made. With great difficulty astronomers have succeeded in determining the parallaxes of a few stars. But the inevitable errors of observation often involve the results in obscurity. Let the reader only bear in mind that there is not one star that is sufficiently near to give us a parallax of one second! A second is the dimension to which would be reduced a circle one metre (3 ft. 3.37 in.) in diameter carried away to a distance of 206 kilometres (127.72 miles) from the eye. This appears to be less than nothing: it is equal to the thickness of a hair stretched at the distance from the eye of 20 metres (74 feet). The apparent annual movement of a star, whose distance can be known, is performed altogether within this infinitesimal space. For an observer on the star that is nearest to us, this hair would conceal the whole space between the earth and the sun.

As no star offers a parallax of one second, it follows that the nearest of the stars is distant from earth no less than 206,265 times 92,000,000 miles. The space which surrounds the planetary system is void of stars to that distance at least.

The star which is nearest to us, Alpha of Centaur, has a parallax of 0."91. Its distance from earth is 226,400 times the radius of the earth's orbit, or 21,000,000,000,000 miles. This is our neighbor star, and its distance is probably the minimum distance between star and star—21,000,000,000,000 miles. Each of these stars shines with its own light—is a sun like our own.