XXVII THE MOTIONS OF THE HEAVENLY BODIES
About three hundred and twenty years ago Giordano Bruno was burned at the stake for his audacity in believing in the existence of other worlds. A few decades later the famous astronomer Galileo was forced to publicly recant his belief that the earth moved. Yet the truth could not long be suppressed by such means, and since those dark days man's advance in knowledge has been so rapid that it seems to us today in this wonderful age of scientific discovery almost inconceivable that man ever believed that the earth, a tiny planet of a vast solar system, was "the hub of the universe," the fixed and immovable center about which revolved all the heavenly bodies. Very reluctantly, however, and with bitter feeling, but in the light of overwhelming evidence man finally gave up his long-cherished idea of terrestrial importance, and when finally forced to move his fixed center of the universe, he moved it only so far as the comparatively nearby sun.
This center he then regarded as fixed in space and also held to his belief that the stars, set in an imaginary celestial sphere, were immovable in space as well, and all at the same distance from the sun. So, scarcely two hundred years ago we find that the astronomer Bradley was endeavoring to measure this common distance of the "fixed stars." Though he failed in this attempt he made the important discovery that the observed positions of the stars are not their true positions, owing to the fact that the velocity of light is not infinite but takes a definite finite interval of time to travel a given distance. As a result the stars always appear displaced in the direction of the earth's motion around the sun, the amount of the displacement depending upon the velocity of the earth in its orbit and the velocity of light. This "aberration of light," as it is called, furnished additional proof that the earth revolves about the sun and was one more nail driven into the coffin of the old Ptolemaic theory that the earth was the center of the universe. Bradley also discovered that the positions of the stars were affected by the wabbling of the earth's axis, called its "nutation."
Although in the days of Bradley neither the methods of observation nor the instruments were sufficiently accurate to show the minute shifts in the positions of the stars that reveal the individual motions of the stars and the distances of those nearest to us, yet the discovery of the two large displacements in the positions of all the stars, due to the aberration of light and the nodding of the earth's axis were of the greatest value, for they were a necessary step in the direction of the precise measurements of modern times. It is only through measurements of the greatest refinement and accuracy that it is possible to detect the motions and distances of the stars and to discover the wonderful truths about the nature and structure of the universe that they are revealing to us today.
After unsuccessful attempts extending over several centuries the distance of one of the nearest stars, the faint 61 Cygni, as it is catalogued, was finally determined by the astronomer Bessel in the year 1838.
This star is about ten light-years distant from the earth, which places it about six hundred and thirty thousand times farther away from us than the sun; that is, we would have to travel six hundred and thirty thousand times the distance from the earth to the sun to reach this very close stellar neighbor, 61 Cygni. The nearest of all the stars, Alpha Centauri, is over two hundred and seventy thousand times the distance from the earth to the sun. It is, therefore, little wonder that the early astronomers believed that the stars were fixed in space since even the nearest is so far away that, viewed from opposite points in the earth's orbit, its apparent change in position due to our actual change in position of 186,000,000 miles, amounts to only one and a half seconds of arc. Two stars separated by one hundred and sixty times this angular distance might possibly be glimpsed as two distinct stars by a person with good eyesight, though to most of us they would appear as one star. Upon the measurement of such minute angles depended a knowledge of the distances of the nearest stars.
It is to Sir William Herschel that we owe the discovery, more than a hundred years ago, of the motion of the sun through the universe. From the consideration of a long series of observations of the positions of the stars this famous astronomer discovered that the stars in the direction of the constellation Hercules were separated by much greater angular distances than the stars diametrically opposite in the heavens. In other words, the stars were spreading apart in one portion of the heavens and crowding together in the opposite direction and he rightly interpreted this to mean that the sun was moving in the direction of the constellation of Hercules. It was not until the spectroscope was applied to the study of the heavens in the latter part of the nineteenth century that the amount of this motion of the sun was found to be about twelve and a half miles per second, or four times the distance from the earth to the sun in a year.
It is to Sir William Herschel that we owe also the discovery of binary systems of stars in which two stars swing around a point between them called their center of gravity.