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ASTRONOMICAL THEORIES. - 25
C H A P T E R II.
“He sat and read. A book with silver clasps,
THE imperfect historical records of the nations of antiquity preventus from determining with certainty when, and with whom, astronomical science had its origin. It is certain however, that it was cultivated at a very early age by the Egyptians, the Chaldeans, the Bramins of India, and the Chinese. In a fine climate, and fertile country, inhabited by nomadic tribes, we can well imagine the sublime spectacle of the heavens to have arrested early attention. At a later period, when the motion of the sun among the stars began to be noticed, and consequently the helical rising and setting of certain stars, i. e., their rising or setting just before or after the sun, became the signs of approach of certain seasons, the stars were grouped into constellations, and fanciful names given to them. Thus we find Hesiod alludes to the helical rising of Arcturus, and Thales mentions the number of days after the vernal equinox, when the Pleiades set just as the sun arose, by means of which we are now enabled to tell the age in which he lived, as will be explained hereafter.
The constellations being located and named, and the sun’s apparent path determined in the heavens, astronomers began to observe more carefully the motions of the sun, moon, and planets, among the stars, and endeavored to frame a system of the world which would explain all the apparently irregular motions. It was
very early observed that the sun and moon moved around the earth with different velocities from the stars, and that there were certain bodies, five in number, which also appeared to be wandering in the heavens, these were called planets, from a Latin word meaning to wander, and were named in order, according to their supposed distance from the earth, Mercury, Venus, Mars, Jupiter, and Saturn. As soon as these wandering bodies were closely observed, certain irregularities in their motion attracted attention, instead of moving uniformly in a circle in the heavens, like the sun, their paths were often broken, and even turned back, as represented by the lines below, moving from u to b direct, i. e.,
in the order of the signs, from b to c, retrograde, or contrary to their previous motion, at b and c, apparently still, or stationary for a short time, and from c to d moving again direct. In addition to these irregular movements, two of them were observed to always remain in the neighborhood of the sun, viz. Mercury and Venus, while Mars, Jupiter, and Saturn were often seen directly opposite, rising when the sun was setting. Hence, in framing any theory, it was necessary to account for these motions.
All the early astronomers supposed that the earth was the centre of the system, and that all the celestial bodies were revolving around it. The only system of the world which attracted much notice, was that of Ptolemy the great Egyptian king and philosopher, called, from him, the Ptolemaic system. This is the system which we would naturally adopt upon casual thought. Here is the earth occupying the centre, and around it the moon is supposed to be revolving not quite as fast as the sun, next comes Mercury, then Venus, the Sun, Mars, Jupiter, and Saturn, beyondthe whole was supposed to be the grand primum mobile, a sphere
PTOLEMAlic SystEM. 27
to the surface of which the stars were all attached, and revolving
once around the heavens in 24 hours. To account for the irregular motions of the planets before noticed, a modification of this
Mercury around the earth, the planethowever, instead of revolving in this circle, was supposed to be revolving in another smaller circle c a b d, whose centre v was carried forward as the circle A B C revolved around the earth, in the order of the letters, the planet moving in the contrary direction in the small circle c a b d would apparently describe the curve line d e fgh, being stationary at f and h, and apparently moving backward through the arch f g h. Now in order to make Venus and Mercury always accompany the sun, the centre v of the small circle, was supposed to be always in a right line nearly, between the earth and sun. Such was the Ptolemaic system, and as it appeared to explain the irregular motions by really uniform, or true circular motions, it was soon adopted as the true system of the world. In the time of Ptolemy astronomical instruments began to be used; for some time previous however, the eastern nations, in order to ascertain the instant of mid-summer, or mid-winter, had been in the habit of measuring the length of the shadow of a vertical gnomon or style, but Ptolemy introduced the use of graduated spheres. We have already observed that all circles are divided into 360 degrees, and these subdivided into 60 minutes each. Hence it is evident that by means of a graduated circle, angular distances may be measured in the sky. An angle, it must be remembered, is simply the inclination of two lines and has no reference at all to the length of the lines, thus S A B is the angular distance of the star
S from the object B. To observe this angle, or inclination, we may use a small graduated circle thus. Let A C D be a circle graduated into 360°, having a moveable index turning on its centre, which index is furnished at each end with a sight-hole. First look with the index towards the object B, and observe the
MEASUREMENT OF ANGLEs. 29
point where the index marks the circle, say at 10°, then turning
the index towards S, observe where it makes the circle, say 20°, the difference 10°, is the angular distance of S from B. The instruments of Ptolemy were constructed upon this principle though not so perfect, using shadows, and other contrivances, instead of simply observing through two vanes or sight holes. Ptolemy had not intended his system to be received other than an hypothesis, which might account for the observed motions; he did not profess this to be the actual order of the world, but his successors, without their great master's love for truth and careful study, soon gave to these supposed spheres and orbs, a real existence, and the heavens became crowded with crystalline spheres moving in all directions, and with all velocities, and as often as new motions, or irregularities in the old ones were detected, new circles moving at their centres round the old ones, were added, called epicycles, so that at last cycles and epicycles, revolved in all directions, bearing the planets along with them, until amid the crowd of spheres and crystal orbs the brain grew dizzy, and could not comprehend the mysterious revolutions. Amidst all this confusion of “Cycle and epicycle, orb on orb,” a bright luminary arose, and with a master hand dashed aside the crystal spheres of the successors of Ptolemy, substituting instead, the simplicity of truth. This man was Nicholas Copernicus. At the time when the true system was about to be made