125 ON THK REVOLUTIONS OK 



time that the equinox or the solstice emploj^s in traversing the con- 

 stellation from one end to the other. 



" Fifty seconds per annum have been taken as the precession, as it 

 is given by a comparison of the catalogue of Hipparchus with modern 

 catalogues. This gave the convenience of round numbers, and an ex- 

 actness that may be depended on. Thus the entire period is 25,920 

 years; the half period 12,960 years; the quarter 6,480 years; the 

 twelfth, or a sign, 2,160 years. 



" We must observe that the constellations leave spaces between, 

 and that sometimes they infringe on each other. Thus, between the 

 last star of Scorpio and the first of Sagittarius there is an interval of 

 six degrees and two-thirds ; on the contrary, the last cf Capricomus is 

 more advanced by fourteen degrees of longitude than the first of 

 Aquarius. 



" Independently of the inequality of the motion of the sun, the con- 

 stellations would give a very unequal and faulty measurement of the 

 year and months. The signs of thirty degrees afford a more convenient 

 and less defective method. But the signs are only a geometric sup- 

 position ; we can neither distinguish nor observe them ; they are 

 continually changing their places by the retrogradation of the equi- 

 noctial point. 



" We have always been able to calculate roughly the equinoxes and 

 solstices ; and we have remarked, that the spectacle of the heavens 

 during the night was not any longer exactly the same as it had been 

 anciently at the times of the equinoxes and solstices. We have never 

 been able to observe accurately the heliacal rising of a star ; we 

 must be a few days out of the calculation, and thus we often speak 

 without having a positive period from which we could reckon. Be- 

 fore Hipparchus we do not find, either from books or traditions, any 

 thing whence we may calculate, and this has caused a multiplicity of 

 systems. We have disputed without having a knowledge of the sub- 

 ject. Those who are not astronomers may form their own ideas of the 

 science of the Chaldeans, the Egyptians, &c. &c. ; no real inconveni- 

 ence will result. We may assign to these people the intelligence and 

 \^'isdom of the moderns, but we can borrow nothing from them, for 

 either thej^ had nothing to leave, or have left nothing. Astronomers 

 will never draw from the ancients anything of the smallest utility. 

 Let us then leave to the learned vain conjectures, and confess our po- 

 sitive ignorance of things useless in themselves, and of which there is 

 not a single existing record. 



" The limits of the constellations vary according to the authors that 

 we consult. We see these limits expand or contract where they im- 

 press, from Hipparchus to Tycho, from Tycho to Hevelius, from Heve- 

 lius to Flamstead, Lacaille, Bradley, or Piazzi. 



" I have said elsewhere, that the constellations were of no use, only 

 that at best they enable us more easily to find out the stars, whilst the 

 stars themselves point out particularly the fixed points whence we may 

 refer the motions of the colures or the planets. A stronomy only begap. 

 at the priod when Hipparchus made the first catalogue of stars, mea- 



