After the laws of the motions of the planets had been determined, it was only natural that men should ask themselves what forces were concerned in these motions. The ancient philosophers had proposed the idea that the sun attracts all heavenly bodies, but the suggestion had not been accepted by the world at large. However, after the discoveries of Kepler, the English, philosopher Newton advanced the theory that there is in the universe an attractive force which influences all matter, beyond the limits of known space. He further proved that the intensity of this force varies directly with the product of the attractive masses, and inversely, with the square of the distances between them—that is, the greater the bodies the greater the attraction; the greater the distance between them, the smaller the attraction. This law of gravitation has been verified by repeated experiments, and, taken in connection with the astronomical theories of Copernicus and Kepler, has made celestial mechanics what they are today.
By the aid of the law of gravitation, many astronomical predictions have been fulfilled. Among the most famous is the following incident:
In the early part of the last century, astronomers noticed that the motions of the planet Uranus did not agree with those derived from calculations based upon the law of gravitation. About 1846, two investigators, M. Leverrier, of France, and Mr. Adams of England, stated, as their opinion, that the discordance between theory and observation in the case of the motions of Uranus, was due to the attraction of a planet, not yet known, and they calculated by means of the law of gravitation, the size and orbit of the unknown planet. In the fall of 1846, this planet was actually discovered and named Neptune. It was found to harmonize with the predictions made by the astronomers before its discovery.
During the days of Newton, the question was raised if the celestial bodies outside of the solar system obey the law of gravitation. Among the stars, there are some which are called double stars, and which consist of two stars so near to each other that the telescope alone can separate them to the eye. In 1803, after twenty years of observation, William Herschel discovered that some of these couples were revolving around each other with various angular velocities. The son of William Herschel continued this work, and many years later, he discovered that the laws of motion of these double stars are the same as those that prevail in the the solar system.[A] This result indicated not only the universality of the law of gravitation, but also the probability that all heavenly bodies are in motion.
[Footnote A: History of the Inductive Sciences, Whewell, 3rd ed. Vol.
I, pp. 467-469.]
[Sidenote: The invention of the spectroscope laid the foundation of the new astronomy.]
Then, early in the nineteenth century, a new method of research began to be developed, which was destined to form a new science of astronomy. It had long been known that white light when passed through a glass prism is broken into a colored spectrum, with colors similar to those observed in the rainbow. Now it was discovered that when white light passes through vapors of certain composition, dark lines appear in the spectrum, and that the position of the lines varies with the chemical composition of the vapors. By the application of these principles, it was shown, towards the middle of the last century, that the chemical composition of the heavenly bodies may be determined. Later,it was discovered that by noting the positions of the dark lines in the spectrum, it could be known when a star or any heavenly body is moving, as also the direction and amount of its motion. These unexpected discoveries led to a study of the heavens from the spectroscopic point of view, which has resulted in a marvelous advance in the science of astronomy.
[Sidenote: All heavenly bodies are in motion.]
It has been determined that all heavenly bodies are in motion, and that their velocities are great compared with our ordinary conceptions of motion. Most of the stars move at the rate of about seven miles per second, though some have a velocity of forty-five miles, or more, per second. Many stars, formerly thought to be single, have been resolved into two or more components. The rings of Saturn have been proved to consist of small bodies revolving about the planet in obedience to Kepler's law.[A] Clusters of stars have been found that move through space as one body, as possible counterparts of the planetary system.[B] It has been demonstrated, further, that the sun itself, with its planets, is moving through space at a very rapid rate. Professor Simon Newcomb, perhaps the greatest astronomer of the day, says, "The sun, and the whole solar system with it, have been speeding their way toward the star of which I speak (Alpha Lyrae) on a journey of which we know neither the beginning nor the end. During every clock-beat through which humanity has existed, it has moved on this journey by an amount which we cannot specify more exactly than to say that it is probably between five and nine miles per second. The conclusion seems unavoidable that a number of stars are moving with a speed such that the attraction of all the bodies of the universe could never stop them."[C] In brief, the new astronomy holds that all heavenly bodies are in motion, and that the planetary system is but a small cluster of stars among the host of heaven. Further, it has weighed the stars, measured the intensity of their light, and determined their chemical composition, and it affirms that there are suns in the heavens, far excelling our sun in size and lustre, though built of approximately the same elements.
[Footnote A: See C. G. Abbott, Report of Smithsonian Institution, for 1901, pp. 153-155.]