Laplace’s theory was powerfully supported by Herschel’s observations of the various nebulæ in the heavens. But, with the supposed resolution of the various nebulæ after the erection of the Rosse reflector in 1845, the evidence in favour of the nebular theory seemed to be greatly reduced. In 1864, however, the discovery of the gaseous nebulæ, by means of the spectroscope, gave further support to the theory. Powerful aid was lent to the nebular hypothesis by the famous German physicist, Hermann Ludwig Ferdinand von Helmholtz (1821-1894), in 1854, in his theory of the maintenance of the Sun’s heat. Many theories had been already advanced to account for this. After the discovery of the conservation of energy, Julius Robert Mayer, one of the discoverers, put forward the theory that the Solar heat was sustained by the inflow of meteorites from space, and this idea was developed in 1854 by Sir William Thomson, now Lord Kelvin (born 1824), but it was soon apparent that the supply of meteors required to sustain the Solar heat was such as would have increased the mass of the Sun very considerably. Accordingly the hypothesis was partially abandoned, and was succeeded by that of Helmholtz, who pointed out that the radiation of the Sun’s heat was the result of its contraction through cooling. The rate was then estimated at 380 feet yearly, or a second of arc in 6000 years. This theory was at once generally accepted. It assumes the Sun to be still contracting, and therefore, on going backwards in imagination, we reach a period when the Sun must have been much larger than now, and, in fact, extended beyond the orbit of Neptune.
Several objections to Laplace’s nebular theory were urged by various investigators. Among these was the retrograde motions of the satellites of Uranus and Neptune, and the extremely rapid revolution of the inner satellite of Mars. Other objections were urged by Babinet, Kirkwood, and others, and at length a sweeping reform of the nebular theory was proposed by Faye in 1884, in his work, ‘Sur l’Origine du Monde.’ Faye put forward the idea that all the planets interior to the orbit of Uranus were formed inside the solar nebula, while Uranus and Neptune came into existence after the development of the Sun was far advanced. But the objections to Faye’s theory are formidable, and the hypothesis has not been accepted.
A popular exposition of the nebular theory was given in 1901 in Ball’s work on ‘The Earth’s Beginning.’ He exhaustively discusses the whole question, and explains the retrograde motion of the satellites of Uranus and Neptune as due to the fact that the planes of the orbits of the satellites will eventually be brought to coincide with the ecliptic. These motions, says Ball, do not disprove the nebular theory. “They rather illustrate the fact that the great evolution which has wrought the Solar System into its present form has not finished its work: it is still in progress.”
The theory that the Sun’s heat was maintained by meteors, was extended by Proctor in 1870 to explain the growth of the planets through meteoric aggregation as well as nebular condensation. Certainly the theory, as developed by Proctor, accounted fairly well for the various features of the Solar System; but the highest development of the meteoritic theory is due to Lockyer, who published his views in 1890, in his work, ‘The Meteoritic Hypothesis.’ Lockyer claims that his views are merely extensions of Schiaparelli’s ideas regarding the concentration of celestial matter. He considered the chief nebular line to be identical with the remnant of the magnesium fluting, which is conspicuous in cometic and meteoric spectra; but Huggins and Keeler, with more powerful instruments, disproved the supposed coincidence. Lockyer considers that “all self-luminous bodies in the celestial space are composed either of swarms of meteorites or of masses of meteoric vapour produced by heat. The heat is brought about by the condensation of meteor swarms, due to gravity, the vapour being finally condensed into a solid globe.”
Lockyer divided the stars into seven groups, according to temperature, the order of evolution being from red stars through a division of second-type stars to Sirian stars, regarded as the hottest stars; through a second division of solar stars to fourth-type stars. In fact, the theory aspires to give a complete explanation of all celestial phenomena, from meteors to nebulæ. Newcomb, however, considers that the objections to the theory are insuperable, and his opinion is shared by the majority of astronomers, many of whom, however, consider that there are elements of truth in the theory; but Lockyer undoubtedly carried his ideas to an extravagant extent.
Lockyer’s evolutionary order of the stars is not supported by Vogel. Zöllner suggested in 1865 that yellow and red stars are simply white stars in a further stage of cooling; but Angström showed that atmospheric composition is a safer criterion of age than colour. Vogel’s classification, first published in 1874, and further developed in 1895, is from the standpoint of evolution. He considers Orion stars and Sirian stars to be the youngest orbs. Solar stars are considered by Vogel to have wasted much of their store of radiation, and red stars are viewed as “effete suns, hastening rapidly down the road to final extinction.” He considers stars of Secchi’s fourth type to be also dying suns, both types representing alternative roads for stars of the Solar type in their decline into dark stars. This view is supported by Dunér, and is distinctly confirmed by Hale’s observations with the Yerkes telescope. Vogel’s views, in fact, are generally accepted among astronomers. The nebular theory, modified by subsequent research, seems destined to hold its own against all attacks.
Distinctly supplementary to the nebular theory are the remarkable researches, commenced in 1879, by Sir George Howard Darwin (born 1845), son of Charles Darwin the great biologist. George Howard Darwin was born in 1845, at Downe in Kent, was educated at Cambridge, and studied for the law; but in 1873 he returned to Cambridge, where he became Plumian Professor of Astronomy in 1883. In 1879 he communicated to the Royal Society the first of his papers on tidal friction, which were summed up in his book on ‘The Tides,’ published in 1898. He finds that the tides act upon the Earth as a brake does upon a machine,—they tend to retard its rotation. Consequently, the day is growing longer, the Moon’s orbit is becoming enlarged, and its period of revolution is being lengthened.
At present the day is about twenty-four hours long, and the month about twenty-seven days. The day, however, will be lengthened at a more rapid rate than the month, and in the remote future the day and month will both last fifty-five of our present days. The Moon will revolve round the Earth in the same period that the Earth rotates on its axis, and the two bodies will perform their circuit round the Sun as if united by a bar.
Not only can we foresee the future of the Earth-Moon System, but we can also read the past. According to Darwin’s theory, the Earth, in the remote past, was probably rotating on its axis in a very short period, between three and five hours. The Moon must then have been much nearer us than it is now, and was probably revolving round its primary in the same period that the Earth took to rotate on its axis. The two globes, then gaseous, must have been revolving almost in actual contact. Had the month been even a second shorter than the day, the Moon must inevitably have fallen back on the Earth. As it was, the condition of affairs could not endure. The condition of the Moon resembled that of an egg balanced on its point. The Moon must either recede from the Earth or fall back upon it. The solar tide here interfered, and caused the Moon to recede from its primary until it reached its present distance of 239,000 miles.
The fact that the Earth and Moon were almost in contact suggests that they were probably in contact. In other words, the Moon originally formed part of the Earth, which, in consequence of its short-rotation period, and probably also owing to the interference of the solar tide, split into two portions, and the smaller of these now forms the Moon. It is likely that the matter now forming the Moon was detached from the Earth in separate particles. Just as the tides raised by the Moon tend to retard the motion of the Earth, so the Earth tides raised in the Moon have already done their work. The Moon now rotates on its axis in the same time as it revolves round the Earth. Part of the evolution of the Earth-Moon system is completed. Schiaparelli’s discovery that the rotation periods of both Venus and Mercury coincide with their times of revolution is distinctly confirmatory of Darwin’s theory.