320. A new light has been thrown on the possible development of the earth and moon by Professor G. H. Darwin’s study of the effects of tidal friction (cf. § 287 and §§ 292, 293). Since the tides increase the length of the day and month and gradually repel the moon from the earth, it follows that in the past the moon was nearer to the earth than now, and that tidal action was consequently much greater. Following out this clue. Professor Darwin found, by a series of elaborate calculations published in 1879-81, strong evidence of a past time when the moon was close to the earth, revolving round it in the same time in which the earth rotated on its axis, which was then a little over two hours. The two bodies, in fact, were moving as if they were connected; it is difficult to avoid the probable inference that at an earlier stage the two really were one, and that the moon is in reality a fragment of the earth driven off from it by the too-rapid spinning of the earth, or otherwise.
Professor Darwin has also examined the possibility of explaining in a similar way the formation of the satellites of the other planets and of the planets themselves from the sun, but the circumstances of the moon-earth system turn out to be exceptional, and tidal influence has been less effective in other cases, though it gives a satisfactory explanation of certain peculiarities of the planets and their satellites. More recently (1892) Dr. See has applied a somewhat similar line of reasoning to explain by means of tidal action the development of double stars from an earlier nebulous condition.
Speaking generally, we may say that the outcome of the 19th century study of the problem of the early history of the solar system has been to discredit the details of Laplace’s hypothesis in a variety of ways, but to establish on a firmer basis the general view that the solar system has been formed by some process of condensation out of an earlier very diffused mass bearing a general resemblance to one of the nebulae which the telescope shews us, and that stars other than the sun are not unlikely to have been formed in a somewhat similar way; and, further, the theory of tidal friction supplements this general but vague theory, by giving a rational account of a process which seems to have been the predominant factor in the development of the system formed by our own earth and moon, and to have had at any rate an important influence in a number of other cases.
AUTHORITIES AND BOOKS FOR STUDENTS.
I. General.
I have made great use throughout of R. Wolf’s Geschichte der Astronomie, and of the six volumes of Delambre’s Histoire de l’Astronomie (Ancienne, 2 vols.; du Moyen Age, 1 vol.; Moderne, 2 vols.; du Dixhuitième Siècle, 1 vol.). I shall subsequently refer to these books simply as Wolf and Delambre respectively. I have used less often the astronomical sections of Whewell’s History of the Inductive Sciences (referred to as Whewell), and I am indebted—chiefly for dates and references—to the histories of mathematics written respectively by Marie, W. W. R. Ball, and Cajori, to Poggendorff’s Handwörterbuch der Exacten Wissenschaften, and to articles in various biographical dictionaries, encyclopaedias, and scientific journals. Of general treatises on astronomy Newcomb’s Popular Astronomy, Young’s General Astronomy, and Proctor’s Old and New Astronomy have been the most useful for my purposes.
It is difficult to make a selection among the very large number of books on astronomy which are adapted to the general reader. For students who wish for an introductory account of astronomy the Astronomer Royal’s Primer of Astronomy may be recommended; Young’s Elements of Astronomy is a little more advanced, and Sir R. S. Ball’s Story of the Heavens, Newcomb’s Popular Astronomy, and Proctor’s Old and New Astronomy enter into the subject in much greater detail. Young’s General Astronomy may also be recommended to those who are not afraid of a little mathematics. There are also three modern English books dealing generally with the history of astronomy, in all of which the biographical element is much more prominent than in this book: viz. Sir R. S. Ball’s Great Astronomers, Lodge’s Pioneers of Science, and Morton’s Heroes of Science: Astronomers.
II. Special Periods.
Chapters I. and II.—In addition to the general histories quoted above—especially Wolf—I have made most use of Tannery’s Recherches sur l’Histoire de l’Astronomie Ancienne and of several biographical articles (chiefly by De Morgan) in Smith’s Dictionary of Classical Biography and Mythology. Ideler’s Chronologische Untersuchungen, Hankel’s Geschichte der Mathematik im Alterthum und Mittelalter, G. C. Lewis’s Astronomy of the Ancients, and Epping & Strassmaier’s Astronomisches aus Babylon have also been used to some extent. Unfortunately my attention was only called to Susemihl’s Geschichte der Griechischen Litteratur in der Alexandriner Zeit when most of my book was in proof, and I have consequently been able to make but little use of it.