ASTROLABE. 



ASTROLOGY. 



G51 



equator, a meridian, and two tropics. It is impossible, from what we 

 know of Hipparchus, that he could have done without something of 

 the sort. At the same time, between Hipparchus and Ptolemy we 

 have no observations to settle this point. 



The description of Ptolemy (' Syntaxis,' book v. ch. i.) is as follows : 

 Fix two perfectly equal circles at right angles to each other, and let 

 one represent the ecliptic, and the other the solstitial colure. In the 

 poles of the ecliptic place cylinders, projecting within and without 

 beyond the rims of the solstitial colure, and fix on these cylinders as 

 pivots outer and inner circles, which shall revolve freely without and 

 within the first-mentioned circles. These are evidently circles of longi- 

 tude. Within the innermost, and in its plane, place a lighter circle, 

 sliding by friction, and having two sights diametrically opposite, by 

 which the latitude of any celestial phenomenon may be observed when 

 the instrument is adjusted ; that is, when the circle representing the 

 ecliptic is in the plane of the true ecliptic. To effect this, cylindrical 

 pivots are inserted in the solstitial colure in the points corresponding to 

 the poles of the equator, and the whole of the apparatus is suspended 

 within a circle which is placed in the meridian of the place of observa- 

 tion. The ecliptic being divided from its interior to its exterior rim, 

 the outer of the circles of longitude is set to the division corresponding 

 to the longitude of the sun, as given in the solar tables, and the whole 



below the cylinder, the orifice of the sight being made just large enough 

 to admit of this. The angular distance of the sight from the equatorial 

 circle is then the declination of the star. To measure the difference of 

 right ascension of two stars, two observers take two sights on the 

 equator, which they adjust till each sees his star just on the axis, both 

 on one side of it and the other. The angular distance of the sights is 



,heu the difference of right ascension of the stars. 

 The plumb-line shows whether the meridian is exactly vertical, and 



,he screws at the feet are employed to raise or lower either end when 

 necessary. 



We give one more step between the ancient and modern instruments, 

 'rom the same work of Tycho Bi-ihd. 



IB then turned round the poles of the equator, until the plane of the 

 ecliptic and the plane of the outer circle of longitude pass through the 

 sun. The instrument is then adjusted, the inner circle of longitude 

 and its sliding limb with sights is turned to the moon, and the angle 

 read off upon this circle is the latitude of the moon, while the angle 

 read off on the interior edge of the ecliptic is the longitude. 



When stars are observed, it is sufficient to make the plane of the 

 outer circle of longitude pass through the moon or any known star after 

 setting that circle to the known longitude of the moon or star. The 

 observation then takes place as before. 



No material improvement upon this construction appears to have 

 been made by the Arabs, who in some instances used very large instru- 

 ments of the kind. A more skilful variety of the astrolabe is here 

 shown, described by Tycho Brahe', from whose ' Astronomise Instauratso 

 Mcchanica ' the preceding cut is taken. 



The outermost circle represents the meridian; the axis passes through 

 the poles, and there is a revolving equator and hour-circle fixed together. 

 The sights on the circles are moveable ; but instead of using opposite 

 sights, the small cylinder which projects from the axis is employed 

 For example, to measure the' declination of a star, the hour-circle is 

 moved till it passes through the star, and a sight is then placed so that 

 the star may be seen through it on the edge of the cylindrical pin 

 which projects from the centre of the axis, both on looking above anc 



The hour-circle is now disengaged from the equator and independent 

 of it. The polar axis is directly supported, and not made to depend 

 upon the position of the meridian. This is perhaps sufficiently near 

 to the modern equatorial to be considered as the first instrument of 

 the kind. 



ASTRO'LOOY. If this word were used in a sense analogous with 

 that of geology or theology, it would mean simply the science of the stars ; 

 while astronomy might mean the science of their order and arrange- 

 ment. But the term, at least when coupled with the epithet judicial, 

 has always signified the discovery of future events by means of the 

 position of the heavenly bodies. The two words astrology (iurrpo\oyta) 

 and astronomy (iurrpovofila) seem to have been used in the same sense 

 by the Greeks, at least till about the Christian era. Cicero (' Offic.' i. 

 6.) uses the word attrologia to express astronomical knowledge. 



It has long been unusual to produce any arguments against this 

 pretended science ; but there are two considerations which make us 

 think it may be useful to show those who are unacquainted with it a 

 few of its details. The first is, that works, seriously professing to 

 inculcate and defend the principles of astrology, have been published 

 within the last twenty years in tlu's country, and are still sold, almost 

 exclusively, by some booksellers : the second, that several of our most 

 popular almanacs do actually give astrological predictions at the present 

 time. This may be a mere matter of amusement with the more 

 enlightened ; but we are afraid there are some who play with edge- 

 tools in reading the fooleries of the works alluded to. The love of the 

 marvellous is not under proper regulation, even in the minds of many 

 who do not go the length of supposing astrology credible ; and we shall 

 therefore perhaps do good service in showing what the system really is, 

 and what consequences its adoption must lead to. 



It must moreover be remembered that our old English writers, par- 

 ticularly the dramatists, cannot be well understood without some 

 information upon the leading terms and principles of this art ; which 

 therefore may be as lawfully studied aa the history of Jupiter and tha 

 Metamorphoses of Ovid. 



