ASTRONOMY AND HISTORY.* WE are apt to think of astronomy
as altogether concerned with the problems of space. It is in this aspect, of course, that it is presented in such a book as the first volume of Professor Newcomb's " projected series having the double purpose of developing the elements of Practical and Theoretical Astronomy for the special student of the subject, and of serving as a handbook of convenient reference for the use of the working astronomer in applying methods and formulae." This work is too technical for review in our columns, and we need only say that, for the purpose of the astronomer, it fully comes up to the expectations raised by Professor Newcomb's great reputation. We intend rather to deal with another aspect of astronomy —its bearing upon certain problems in cosmic or human history—as illustrated by the other books lying before us. In gazing upon the beautiful spectacle which the star-strewn
• (1) A Compendilim of Spherical Astronomy. By Simon Newcomb. London : Macmillan and Co. [128. 6d. net.' —(2) Lectures on the Method of Science. Edited by T. B. Strong. Oxford at the Clarendon Press. [7s. 6d. net] — (3) Stonehenge and other British Stone Monuments. By Sir Norman Lockyer. I ondon : Macmillan and Co. 110s. net. J
heavens present on a clear night,. we are looking into " the dark backward and abysm of time," as well as trying to penetrate the infinite depths of space. Whilst we see Sirius as it was eight years ago, and Aldebaran as it was within the last thirty years, and the sun and planets as they are practically at the moment of gaze, we see Arcturus by light which started on its journey when George III. was King, and the New Star in Perseus by light which left it whilst the Spanish Armada was getting ready to sail.
. But there is another way in which the ethereal vibrations that we call light transmit the history of the stars from which they emanate. This is lucidly explained by Dr. A. H. Fison in one of the very interesting and suggestive Lectures on the Method of Science which the Dean of Christ Church has edited. These lectures were delivered at the Oxford Summer Meeting last year, and in this handy form they should do much to teach the ordinary reader what science claims to be and how its operations are conducted. Dr. Fison deals with the evolution of double stars. Our own sun happens to be a single star, separated from its nearest neighbour by an in- credible number of miles. But many stars are really double, or multiple, consisting of two or more stars like the sun in comparatively close neighbourhood and in physical association, revolving about one another—or rather, about their common centre of gravity—in definite orbits. Very often the members of such a system are of different colours—we know orange and green, chrome yellow and sea-water blue, sea-green and lilac pairs—and the possible inhabitants of planets illuminated by such twin suns must be subject to chromatic effects that would drive an artist frantic with despair. Many of these binary systems are so far away that no telescope can separate their components. But the spectroscope reveals their nature, because it enables us to tell whether a star is moving towards us or away from us by the consequent shift iu the spectral lines, analogous to the sudden change in the pitch of a locomotive's whistle as it tears past the platform on which the observer is standing. In some cases these lines not only shift, but are duplicated periodically; and a little thought will show that this can only happen when the star under examination consists of a pair of stars, each of which in turn approaches and recedes from us,—i.e., is moving in such an orbit that the earth lies nearly in its plane. Some of these double stars, again, vary in light, and this is explained by the fact that during their revolution one of them periodically eclipses the other. Now the cycle of light-changes shown by a bright star known as $ Lyrae is such that we can only regard it as consisting of two stars which are separated by a distance leas than the diameter of the larger one. Dr. Fison shows that this unusual state of things corresponds to the earliest stage in the history of our own moon, as Sir George Darwin has deduced it from his study of the retarding influence of the tidal brake. In /3 Lyrae we are witnessing the birth of a double star, and watching a process strictly analogous to that which, many millions of years ago, gave us our satellite. This is a marvellous instance of the way in which astronomy enables us to study the history of an invisible phenomenon,— invisible to the physical eye, with all its instrumental aide, but quite clear to the mental eye of science.
Another way in which astronomy throws light upon history is explained in Sir Norman Lodger's interesting study of Stonehenge and other megalithic monuments in the British Islands, which forms a sequel to the same author's book on The Dawn of Astronomy. Following the same line of thought which he formerly applied to certain Egyptian temples, Sir Norman Lockyer is able to show that Stonehenge was originally built to serve the purpose of a primitive observa- tory. It was built for the use of the astronomer-priests who, perhaps, brought the wisdom of the East to this country many centuries before its very name had emerged into the light of history. The sun and star worship which forms the basis of so many types of primitive religion was closely inter- woven with the practical need of fixed dates in the year for the use of the agriculturist, who possessed no calendar to tell him when to sow his seed and when to expect his harvest. To this day the Pleiades are known as the " hoeing-stars " in South Africa, and take the place of a farming calendar to the Solomon Islanders, just as their midnight culmination fixed the beginning of the feast of Isis at Busiris, and regulated the fifty-two-year cycle of the ancient Mexicans. It is now proved, from a comparative study of prehistoric monuments and ancient temples all over the world, that one of the first uses of these edifices was to enable the astronomer- priests to fix such dates with ease and certainty. Every one knows that the sun rises at a different point on the horizon daily throughout the year, and that the time of star-rising varies night by night. These simple facts sufficed primitive man for the determination of his calendar. Some of the Egyptian temples were carefully oriented so that a bright star, like Sirius or Canopus, was visible down the avenue which served as the tube of a telescope at a particular hour on a given night in the year. By far the greater number of stone circles, of which Stonehenge is the best- known example, were so oriented that the rising sun on Midsummer morning flashed its rays directly upon the central altar ; in other words, their axis was carefully directed to the point on the horizon over which the sun rose on the longest day in the year. There are many other instances of different orientations, each corresponding to the fixed points in some ancient calendar. But Stonehenge was certainly a solar temple and observatory. Now comes in the bearing of this fact upon the history of its building. The point of Mid- summer sunrise is not always the same. The precession of the equinoxes—or the fact that the celestial pole is travelling round a vast circle, which it takes about twenty- five thousand years to complete—causes a secular change in this point. At the present day the axis of Stone- henge is not directed to the Midsummer sunrise, but to a point nearly one degree away from it. Now we know the rate at which the place of Midsummer sunrise changes in consequence of precession, and it is a comparatively simple calculation to determine the date at which the sun must have risen on Midsummer Day in a line with the avenue at Stone- henge. The conclusion is that this remarkable edifice must have been erected about 1680 B.C., the possible error being not greater than two hundred years on either side of this date. Thus astronomy tells us that Stonehenge must have been built somewhere between 1900 and 1500 B.C., if the assumption of its purpose be correct. The general evidence from similar temples all over the world hardly leaves room for doubt on that head, and thus we obtain what may safely be called the first definite fact in British history, carrying it back more than fifteen centuries before our islands first came into contact with recorded civilisation. It is clear that the people who could build Stonehenge for such a purpose were far removed from mere savagery. This is no inconsiderable achievement in historical research, and the details of Sir Norman Lockyer's fascinating work will be studied with pleasure by all thoughtful readers.
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