Sect. 12.-Sequel to the Early Stages of Astronomy. EVERY stage of science has its train of practical applications and systematic inferences, arising both from the demands of convenience and curiosity, and from the pleasure, which, as we have already said, ingenious and active-minded men feel in exercising the process of deduction. The earliest condition of astronomy in which it can be looked upon as a science, exhibits several examples of such applications and inferences, of which we may mention a few.

Prediction of Eclipses.-The cycles which served to keep in order the calendar of the early nations of antiquity, in some instances enabled them also, as has just been stated, to predict eclipses; and this application of knowledge necessarily excited great notice. Cleomedes, in the time of Augustus, says, “we never see an eclipse happen which has not been predicted by those who make use of the Tables.” (ὑπό τῶν κανονικών.)

Terrestrial Zones.-The globular form of the earth being assented to, the doctrine of the sphere was appplied to the earth as well as the heavens; and its surface was divided by various imaginary circles; among the rest, the equator, the tropics, and circles at the same distance from the poles as the tropics are from the equator. One of the curious consequences of this division was the assumption, that there must be some marked difference in the

stripes or zones into which the earth's surface was thus divided. In going to the south, Europeans found countries hotter and hotter, in going to the north, colder and colder; and it was supposed that the space between the tropical circles must be uninhabitable from heat, and that within the polar circles, again, uninhabitable from cold. This fancy was, as we now know, entirely unfounded. But the principle of the globular form of the earth, when dealt with by means of spherical geometry, led to many true and important propositions concerning the lengths of days and nights at different places. These propositions still form a part of our Elementary Astronomy.

Gnomonick. Another important result of the doctrine of the sphere was Gnomonick or Dialling. Anaximenes is said by Pliny to have first taught this art in Greece; and both he and Anaximander are reported to have erected the first dial at Lacedemon. Many of the ancient dials remain to us; some of these are of complex forms, and must have required great ingenuity and considerable geometrical knowledge in their construction.

Measure of the Sun's Distance.-The explanation of the phases of the moon led to no result so remarkable as the attempt of Aristarchus of Samos to obtain from this doctrine a measure of the distance of the sun as compared with that of the moon. If the moon was a perfectly smooth sphere, when she was exactly midway between the new

and full in position (that is a quadrant from the sun) she would be somewhat more than a half moon; and the place when she was dichotomized, that is, was an exact semicircle, the bright part being bounded by a straight line, would depend upon the sun's distance from the earth. Aristarchus endeavoured to fix the exact place of this Dichotomy; but the irregularity of the edge which bounds the bright part of the moon, and the difficulty of measuring with accuracy, by means then in use, either the precise time, when the boundary was most nearly a straight line or the exact distance of the moon from the sun at that time, rendered his conclusion false and valueless. He collected that the sun is at 18 times the distance of the moon from us; we now know that he is at 400 times the moon's distance.

It would be easy to dwell longer on subjects of this kind; but we have already perhaps entered too much in detail. We have been tempted to do this by the interest which the mathematical spirit of the Greeks gave to the earliest astronomical discoveries, when these were the subjects of their reasonings: but we must now proceed to contemplate them engaged in a worthier employment, namely, in adding to these discoveries.

CHAPTER II.

PRELUDE TO THE INDUCTIVE EPOCH OF
HIPPARCHUS.

W

ITHOUT pretending that we have exhausted

the consequences of the elementary discoveries which we have enumerated, we now proceed to consider the nature and circumstances of the next great discovery which makes an Epoch in the history of astronomy; and this we shall find to be the Theory of Epicycles and Eccentrics. Before, however, we relate the establishment of this theory, we must, according to the general plan we have marked out, notice some of the conjectures and attempts by which it was preceded, and the growing acquaintance with facts, which made the want of such an explanation felt.

In the steps previously made in astronomical knowledge, no ingenuity had been required, to devise the view which was adopted. The motions of the stars and sun were most naturally and almost irresistibly conceived as the results of motion in a revolving sphere; the indications of position which we obtain from different places on the earth's surface, when clearly combined, obviously imply a globular shape. In these cases, the first conjectures, the supposition of the simplest form,

of the most uniform motion, required no after-correction. But this manifest simplicity, this easy and obvious explanation, did not apply to the movement of all the heavenly bodies. The planets, the "wandering stars," could not be so easily understood; the motion of each, as Cicero says, "undergoing very remarkable changes in its course, going before and behind, quicker and slower, appearing in the evening, but gradually lost there, and emerging again in the morning'." A continued attention to these stars would, however detect a kind of intricate regularity in their motions, which might naturally be described as "a dance." The Chaldeans are stated by Diodorus, to have observed assiduously the risings and settings of the planets, from the top of the temple of Belus. By doing this, they would find the times in which the forwards and backwards movements of Saturn, Jupiter, and Mars recur; and also the time in which they come round to the same part of the heavens3. Venus and Mercury never recede far from the sun, and the intervals which elapse while either of them leaves its greatest distance from

Cic. de Nat. D. lib. ii. p. 450. Ea quæ Saturni stella dicitur, paívwvque a Græcis nominatur, quæ a terra abest plurimum, xxx fere annis cursum suum conficit; in quo cursu multa mirabiliter efficiens, tum antecedendo, tum retardando, tum vespertinis temporibus delitescendo, tum matutinis se rursum aperiendo, nihil immutat sempiternis sæculorum ætatibus, quin eadem iisdem temporibus efficiat." And so of the other planets. * Del. A. A.; i. p. 4. 3 Plin. H. N. ii. p. 204.