310 



ASTRONOMY. 



Egyptians have a tradition that Bel us founded a 

 colony from Egypt on tin- kinks of the Euphrates, 

 in Asia ; and this colony was furnished with priests, 

 according to the custom of the nmiht r country, who 

 wen- free from public taxes, and were allied, by the 

 Babylonians, Chaldees. Hence it may be conjec- 

 tured, that astrology WHS invented by the Egyptians ; 

 among whom the inhabitants of Thebes particularly 

 claimed the honour of the invention. Most of the 

 ancient writers are agreed, that astrology WHS com- 

 municated by the Chaldees to other imtions. rroin 

 this circumstance, astrologers used to becnlled Chat- 

 dees by the ancient writers ; sometimes Genethliuci 

 (see Genethliacon) ; and, in later times, Chaldee lias 

 been synonymous with astrologer. (See Horoscope) 

 The great antiquity of this art may be inferred from 

 the fact, that most astrological observations are 

 founded on the position of the stars in reference to 

 the horizon, which was the first circle recognised in 

 the heavens ; also from its being mentioned in the 

 Mosaic history. As astrology, in later times, fell 

 into disrepute on account of the cupidity and fraud 

 of its practitioners, these assumed the name of mathe- 

 maticians, by which they were generally known at 

 the time of the Roman emperors. They caused so 

 much trouble, thai Tiberius at length banished them 

 from Rome. The law relating to this banishment of 

 astrologers, however, makes a distinction between 

 geometry and the mathematical, i. e. astrological, 

 art. However objectionable astrology may lie in 

 it si -It', it has been of essential use to astronomy. It 

 has excited more interest in, and led to more careful 

 observations of, the heavenly bodies. During the 

 middle ages, astrology and astronomy were culti- 

 vated in connexion by the Arabs, and their works on 

 the subject are still extant. Pico of Mirandola, who 

 manfully combated the errors of astrology towards 

 the close of the 1 5th century, found but little atten- 

 tion paid to his labours. Even in the 16th and 17th 



tion, and then descend, and disappear in the opposite 

 quarter of the heavens. This motion is common to 

 all the heavenly bodies, and is performed in equal 

 times, though they appear to pass through arcs of 

 very different magnitudes. At a certain point, this 

 motion appears to cease : this point is called the pole, 

 which signifies a pivot, on which the heavens appear 

 to turn. The celestial vault being conceived as 

 forming a sphere, there are two of these points: that 

 which is visible in our hemisphere is the north celes- 

 tial pole ; and that which is visible in the opposite 

 hemisphere is the south celestial pole. The circle 

 which bounds our view on nil sides is called the 

 horizon, or boundary: its plane passes through tin- 

 centre of the earth : it is also called the i-Hi : xtinl or 

 rational horizon, to distinguish it from the sensible 

 horizon, which limits the view of objects on the -ur- 

 face of the earth. A circle perpendicular to the 

 horizon, passing through the poles, is called the inrri- 

 dian. Jt divides the celestial hemisphere into two 

 equal parts, so that the heavenly bodies, at the mo- 

 ment they arrive at this circle, are at (he middle of 

 their apparent course : the passage of the sun over 

 this circle determines the instant of noon. The 

 period occupied by the stars in passing from this 

 circle through the celestial sphere, and returning to 

 the same point, is called a sidereal day, and is a little 

 less than twenty -four hours. As we remove from 

 the poles, the arcs described by the stars gradually 

 increase, and at an equal distance between them, we 

 find the largest, which, dividing the celestial sphere 

 into two equal parts, is callea the celestial equator. 

 A line drawn from the centre of the globe, through 

 the place of the observer, ascertains a point in the 

 heavens, perpendicularly over his head, which is 

 called the zenith : the same line produced in the op- 

 posite direction determines a point in the opposite 

 part of the heavens, which is called the nadir. \Ve 

 have thus far spoken of the ascending and descending 



centuries, astrology could boast of literary men, such ; of the heavenly bodies in the heavenly vault. But 

 as Cardano, and even Kepler, among its adherents, does all this train of worlds actually move round the 

 The Copernican system, the correctness of which ex- earth daily ? Or can it be proved that our senses 

 perience has been continually confirming, has shaken deceive us, and that this apparent motion is an illu- 

 the foundations of the ancient science ; but the fa- 



bric is not wholly overthrown. A full account of 

 astrological terminology is given in Lalande's Astro- 

 nomy, vol. i. (and edition), sect. 497. 



ASTRONOMY (from ??, a heavenly body, and <>,;, 

 law) is the science of the heavenly bodies and their 

 motions. It is founded on observation, but it re- 

 ceives its last perfection from calculation. What an 

 interval from the imperfect notions of the Chaldean 

 shepherd and the Phoenician mariner to the Celestial 

 Mechanics of a Laplace! How many centuries of 

 observations were necessary to render the motion of 

 the earth suspected ! How slow the progress to the 

 laws of planetary motion, and from those laws to a 

 universal principle of gravitation ! Founded on geo- 

 metrical considerations, this great principle explains 

 all the celestial phenomena in their minutest details : 

 there is not a single seeming irregularity which does 

 not necessarily result from it. Outrunning the cau- 

 tious advances of observation, it descends from causes 

 to phenomena, and renders astronomy a great me- 



Rcal problem, of which the only data necessary 

 are the motions, figures, and magnitudes of the hea- 

 venly bodies. That part of the science which relates 

 to their motions, magnitudes, and periods of revolu- 

 tion, is called descriptive astronomy ; that part which 

 explains the causes of their motions, and demon- 

 strates the laws by which those causes operate, is 

 called physical astronomy. From a simple view of 

 the heavens, we see stars, with which the ethereal 

 vault is sprinkled, appear regularly in a certain 

 point, to rise with a uniform motion to a certain eleva- 



sion ? The true cause of these appearances is the 

 motion of the earth round its own axis, from W. to 

 E., in the space of nearly twenty- four hours. A 

 moment's reflection will convince us that the horizon 

 of the observer, as it turns along with him during the 

 rotation of the earth, must advance towards the stars 

 successively, so as to give them the appearance of 

 gradually approaching the horizon ; as a vessel leav- 

 ing the shore causes it to appear to recede to a per- 

 son on board. As the meridian turns at the same 

 time, it must arrive successively at the same stars, 

 which will then appear to have ascended to the mid- 

 dle part of the course they describe above the hori- 

 zon. As soon as the star touches the western verge 

 of the horizon, it appears to set, and ceases to be visi- 

 ble until the motion of the earth again brings it back 

 on the eastern ' boundary of the same circle. But 

 has the earth no other motion ? Every one must 

 have observed that the sun, besides its apparent diur- 

 nal motion, which it has in common with all the 

 stars, appears in the course of a year to change its 

 place in a twofold manner. First, it appears to rise 

 and sink alternately towards one or other of the 

 poles ; and, secondly, if we observe its place among 

 the stars, it appears either that the sun recedes daily 

 towards the east, or tliat the stars advance daily to- 

 wards the west ; for the stars, which we see at one 

 time set immediately after the sun, are, on the fol- 

 lowing evening, lost in his rays: some days after, 

 they reappear in the east, and their rising precedes 

 daily more and more that of the sun. At last, afwi 

 a year, or about 365 days, the sun and stars are again 



