LECTURE III.

THEORIES FOR THE EXPLANATION OF THE MOTIONS OF THE HEAVENLY BODIES.

F in tracing the career of astronomy in the primitive ages of the world, we have been left to pursue our way dimly, through cloud and darkness; if regrets rise up, that time has swept into oblivion the names and country of the early discoverers; in one reflection there is some compensation: while the bright and enduring truths which they wrested from nature have descended to us, their errors, whatever they may have been, are for ever buried with their names and their persons. We are almost led to believe that those errors were few

and transient, and that the mind, as yet undazzled by its triumphs, questioned nature with that humility and quiet perseverance which could bring no response but truth.

In pursuing the consequences flowing from the prediction of an eclipse, several remarkable results were reached, which we proceed to unfold. It will be recollected that to produce either solar or lunar eclipses, the new or full moon must be in the act of crossing the sun's annual track. This point of crossing, called the moon's node, became therefore an object of the deepest interest. Long and careful scrutiny revealed the fact of its

movement around the ecliptic, in a period of eighteen years and eleven days, during which time there occur 223 new moons, or 223 full moons. If then, a new moon falls on the sun's track to produce a solar eclipse to-day, at the expiration of 223 lunations, again will the new moon fall on the ecliptic, and an eclipse will surely take place. Suppose, then, that all the eclipses which occur within this remarkable period of 223 lunations are carefully observed, and the days on which they fall recorded, on each and every one of these days, during the next period of 223 lunations, eclipses may be expected, and their coming foretold.

This wonderful period of eighteen years and eleven days, or 223 lunations, was known to the Chaldeans, and by its use eclipses were predicted, more than 3000 years ago. It is likewise found among the Hindoos, the Chinese, and the Egyptians, nations widely separated on the earth's surface, and suggesting the idea that it had its origin among a people even anterior to the Chaldeans. It is now known by the name of the Zaros, or Chaldean period.

Let it not be supposed that the application of the Zaros to the prediction of eclipses, can in any way supersede modern methods. While antiquity contented itself with announcing the day on which the dark body of the moon should hide the sun, modern science points to the exact second on the dial, which shall mark the first delicate contact of the moon's edge with the brilliant disc of the sun.

It would be a matter of great interest to fix the epoch of primitive discovery. Though this is impossible, its high antiquity is attested by a few facts, to which we will briefly advert. We find among all the ancient nations, Chaldeans, Persians, Hindoos, Chinese, and Egyptians, that the seven days of the week were in universal use, and what was far more remarkable, each of these nations named the days of the week after the seven planets, numbering the sun and moon among the planets. It is, moreover, found that the order of naming is not that of the distance, velocity, or brilliancy of the planets, and neither does the first day of the week coincide among the different nations; but the order once commenced is invariably preserved by all. If we compute the probability of such a coincidence resulting by accident, we shall find the chances millions to one against it. We are, therefore, forced to the conclusion, that the planets were discovered, and the seven days of the week devised and named,

by some primitive nation, from whom the tradition descended imperfectly, to succeeding generations.

A remarkable discovery, made in the remote ages of the world, throws some farther light on the era of the primitive astronomical researches. The release of the earth from the icy fetters of winter, the return of spring, and the revivification of nature, is a period hailed with uncommon delight, in all ages of the world. To be able to anticipate its coming, from some astronomical phenomenon was an object of earnest investigation by the ancients.

It was found that the sun's entrance into the equinox, reducing to equality the length of the day and night, always heralded the coming of the spring. Hence to mark the equinoctial point among the fixed stars, and to note the place of some brilliant star whose appearance in the early morning dawn would announce the sun's approach to the equator, was early accomplished with all possible accuracy. This star once selected, it was believed that it would remain for ever in its place.

The sun's path among the fixed stars had been watched with success, and it seemed to remain absolutely unchanged; and hence the points in which it crossed the equator, for a long while, were looked upon as fixed and immoveable. And, indeed, centuries must pass away before any change could become sensible to the naked eye and its rude instrumental auxiliaries. But a time arrives at last when the bright star, which for more than five hundred years had with its morning ray announced the season of flowers, is lost. It has failed to give its warning; spring has come, the forests bud, the flowers bloom, but the star which once gave promise, and whose ray had been hailed with so much delight by many generations, is no longer found. The hoary patriarch recals the long experience of a hundred years, and now perceives that each succeeding spring had followed more and more rapidly after the appearance of the sentinel star. Each year the interval from the first appearance of the star in the early dawn, up to the equality of day and night, had grown less and less, and now the equinox came, but the star remained invisible, and did not emerge from the sun's beams until the equinox had passed.

Long and deeply were these facts pondered and weighed. At length truth dawned, and the discovery broke upon the unwilling mind, that the sun's path among the fixed stars was actually

changing, and that his point of crossing the equator was slowly moving backwards towards the west, and leaving the stars behind. The same motion, only greatly more rapid, had been recognised in the shifting of the moon's node, and in the rapid motion of the points at which her track crossed the equator. The retrograde motion of the equinoctial points caused the sun to reach these points earlier than it would have done had they remained fixed, and hence arose the precession of the equinoxes.

This discovery justly ranks among the most important achieved by antiquity. Its explanation was infinitely above the reach of human effort at that early day; but to have detected the fact, and to have marked a motion so slow and shrouded, gives evidence of a closeness of observation worthy of the highest admiration. It will be seen hereafter, that the human mind has reached to a full knowledge of the causes producing the retrograde movement of the equinoxes among the stars. Its rate of motion has been determined, and its vast period of nearly twenty-six thousand years has been fixed. Once revealed, the slow movement of the equinox makes it a fitting hour-hand, on the dial of the heavens, with which to measure the revolutions of ages. As the sun's path has been divided into twelve constellations, each filling the twelfth part of the entire circuit of the heavens, for the equinox to pass the twelfth part of the dial, or from one constellation to the next, will require a period of more than two thousand years. Since the astronomer first noted the position of this hour-hand on the dial of the stars, but one of its mighty hours of two thousand years has rolled away. In case any record could be found, any chiselled block of granite, exhibiting the place of the equinox among the stars at its date, no matter if ten thousand years had elapsed, we can reach back with certainty, and fix the epoch of the record.

No such monument has ever been found; but there are occasional notices of astronomical phenomena, found among the Greek and Roman poets, which at least give colour to conjecture. Virgil informs us that the "White Bull opens with his golden horns this year:"

"Candidus auratis aperit cum cornibus annum,
Taurus."

This statement we know is not true, if applied to the age in which the poet wrote, and seems to be the quotation of an

ancient tradition. If this conjecture be true, this tradition must have been carried down the stream of time for more than two thousand years, to reach the age in which the poet wrote. Although these conjectures are vague and uncertain, the frequent allusions to the constellations of the zodiac in the old Hebrew Scriptures, and in the works of all ancient writers, sufficiently attest the extreme antiquity of these arbitrary groupings of the stars.

In taking leave of the primitive ages of astronomy, and in entering on that portion of the career of research and discovery whose history has been preserved, let us pause for a moment and consider the position occupied by the human mind at this remarkable epoch.

Thus far the eye had done its work faithfully. Through long and patient watching, it had revealed the facts, from which reason had wrought out her great results. The stars grouped into constellations, glittered in the blue concave of a mighty sphere, whose centre was occupied by the earth. Within this hollow sphere, sun, moon, and planets, kept their appointed courses, and performed their ceaseless journeys. Their wanderings had been traced; their pathway in the heavens was known, their periods determined, the inclinations of their orbits fixed. So accurately had the eye followed the sun and moon, that it had learned to anticipate their relative positions, their oppositions and conjunctions, till, reaching forward, it had robbed the dread eclipse of its terrors, and had learned to hail its coming with delight. The pathway of the sun and moon among the stars had been scanned and studied, until their slowest changes had been marked and measured.

Such were the rich fruits of diligence and perseverance which descended from the remote nations of antiquity. With the advantage of these great discoveries, and the experience of preceding ages, it is natural to expect rapid progress, when science found its home among the bold, subtle, and inquisitive Greeks. He who entertains this expectation will meet with disappointment. Not that investigations were less constantly or perseveringly conducted; not that less perfect means were employed, or less powerful talent consecrated to the work; but because a point had been reached of exceeding difficulty. The era of discovery from mere inspection was rapidly drawing to a close. It was an easy matter to count the days from full moon to full