Astronomical and Nautical Collections, 197 



ciently well known that this last is the true date, as we may 

 at once infer from the place of Jupiter, which is 198.5^^ at 

 the beginning of this year, and in —805, the nearest alterna- 

 tive, 208'^'^^ in -687, 189««'only. 



If we wish to verify the calculation, from Ptolemy's own 

 tables of Saturn, we have, for the 2560 equinoctial years 

 between - 746 and 1814, 2560 x 365.24222 days, making 

 2561 Egyptian years, and 255.1 days. Then (P. 213) 



352 7 29 

 This is too little by 74°, or -^^ of a revolution, out of 87 

 entire ones, and the agreement would be more perfect if we 

 supposed the time a year longer : but the motion being 

 already slower than that of the modern tables, it is clear 

 that such a supposition is inadmissible. In a similar man- 

 ner we find, for Jupiter, the longitude 

 after 2574 Egyptian years and 258^ days. 

 Hence, (P. 216, 218) 2430^=810x3 

 144 

 240^^ 



m 



355 49 36 

 The error here appears to be only 4° in 257 revolutions, 

 which is little more than ^^j-^jjir » ^^t, in fact, it is a degree 

 or two greater at each end, though still small enough to 

 make the year perfectly certain. 



It is therefore abundantly demonstrated, from the tables 

 of Saturn and Jupiter only, that the Alexandrian noon of the 

 first day of the first year of Nabonassar happened in the 

 equinoctial year preceding the vernal equinox —746; and, 

 according to Ptolemy's reckoning, the sun's mean longitude 

 was 330° 45', whence the date was M. Eq.-746y -29.676^ : 

 or since, according to Ptolemy ""s equation, the true equinox 

 happened when the mean longitude was 359° 23' only, if we 

 call the true equinox ^, the date becomes ^ - 746' - 29.050^ : 



