LT.-COL. G. MAC KIN LAY ON BIBLICAL ASTRONOMY. 151 



APPENDIX. 



Explanation of Changes of Amplitude of the Risings 



of Stars. 



If a top (Fig. I) is spun and slightly tilted over, it will perform 

 a number of slow gyrations, and the highest point will slowly 

 describe a horizontal circle (shown by the dotted line) round a centre 

 E, which is vertically above the lowest point. 



Take a precessional globe (Fig. II), which has around it a vertical 

 brass circle : another brass circle revolves inside it on an axis of 

 which N is one of the pivots ; and an ordinary celestial globe is 

 pivoted inside this circle, E being one of its pivots. EN subtends 

 about 23 1° at the centre C of the globe. 



One revolution of the globe inside the inner circle corresponds to 

 the slow precessional gyration of the earth, which is only completed 

 in about 26,000 years'. 



One revolution of the globe and inner circle clamped together corre- 

 sponds to one revolution of the earth on its axis in twenty-four hours. 



Clamp the globe and inner circle together so that N becomes the 

 north pole (N being elevated about 30° to suit the latitude of 

 Memphis in Egypt), it will then represent the heavens at the 

 present time ; the star (Arcturus) rising at P when revolution ensues. 



Unclamp the globe, revolve it through 62^-° and clamp it again to 

 the inner circle, so that N' takes the place of N as the north pole ; 

 then the condition of things about 4500 years ago (B.C. 2596) 



is represented, because ^ ^ = ^500 

 * 360 26000 



Through N' runs the constellation Draco, and a Draconis was then 

 very near the pole, and was the pole star for a long period before 

 and after that time ; it will also be noticed that the constellation of 

 the Great Bear was then nearer to the pole than it is now. 



When revolution ensues the star (Arcturus) will rise at P' 

 instead of P, for as N' has been moved to the right, the star must 

 shift also, since N'P must equal NP'. 



Thus the magnitude of the horizontal angle PGP' represents the 

 change in amplitude in the rising of Arcturus at latitude 30° in that 

 period of 4500 years, and the angle consequently is a measure of 

 time. E and the clotted circles have the same meaning in both 

 Figs. I and II. 



This slow gyratory movement of the axis of the earth also causes 

 the sun at the equinoxes to appear to move through the belt of stars 

 called the signs of the zodiac, of which Aries is one ; as the whole 

 belt is arbitrarily divided into twelve equal parts or signs, the 

 movement through each sign takes a twelfth part of 26,000 years, 

 or about 2,166 years. This apparent movement of the equinoxes was 

 known long ago, and consequently the gyration received the name, 

 now established by long usage, " the precession of the equinoxes." 



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