142 



REMARKS on the 



g. These remarks refer equally to all the tables. We are 

 now to take notice of what is peculiar to each, beginning with 

 thofe of Siam. 



In order to calculate for a given time, the place of any of 

 the celeftial bodies, three things are requifite. The firfl is, the 

 poiition of the body in fome pafl inilant of time, afcertained 

 by obfervation ; and this inftant, from which every calculation 

 mufl fet out, is ufually called the epoch of the tables. The fe- 

 cond requifite is, the mean rate of the planet's motion, by 

 •which is computed the arch in the heavens, that it mull have 

 defcribed, in the interval between the epoch and the inftant for 

 which the calculation is made. By the addition of this, to the 

 place at the epoch, we find the mean place of the planet, or 

 the point it would have occupied in the heavens, had its motion 

 been fubjed to no irregularity. The third is, the corredion, 

 on account of fuch irregularity, which mufl: be added to the 

 mean place, or fubtra<5led from it, as circumftances require, in 

 order to have the true place. The corredion thus made is, in 

 the langviage of aftronomy, called an equation ; and, when it 

 arifes from the eccentricity of a planet's orbit, it is called the 

 equation of the centre. 



10. The epoch of the tables of Siam does not go back to any 

 very remote period. M. Cassini, by an ingenious analyfis of 

 their rules, finds that it correfponds to the 2 1 ft of March, in the 

 year 6 }8 of our era, at 3 in the morning, on the meridian of 

 Siam *. This was the inft;ant at which the aftronomlcal year be- 

 gan, and at which both the fun and moon entered the moveable 

 zodiac. Indeed, it is to be obferved, that, in all the tables, the 

 aftronomical year begins when the fun enters the moveable zo- 

 diac, fo that the beginning of this year is continually ad- 

 vancing with refpecfl to the feafons, and makes the complete 

 round of them in 24,000 years. 



From 



• Mem. Acad. Scien. torn. 8. p. 312. Aft. Indienne, p. 11. § 14. 



