442 



ASTRONOMICAL AND NAUTICAL COLLECTIONS. 



i. Astronomical Chronology, deduced from Ptolemy, and 

 his Commentators. 



Year 



1 of the Canicular cycle, called by Theon, (MS. " 2390") as 

 cited by Larcher and ChampoUion Figeac, the epoch of 

 Meiiophres, is ascertained by the testimony of Censorinus, 

 chapters 18 and 21 ; he says that the 986th year of Nabo- 

 nassar, in which he wrote, was the 100th of the canicular 

 cycle of 1461 Egyptian years : the first year of that cycle, 

 which may be called the 1462d of the preceding cycle, was 

 consequently the 887 th of Nabonassar, and the 1st of Nabo- 

 nassar the 576th of that cycle, which began 575 Egyptian 

 years before the epoch of Nabonassar, or as many tropical 

 years wanting 139.3 days; and, this epoch having been de- 

 termined to be 0— 746^-30.4'* (Collections for April, 1828), 



in true equinoctial time, the date was nearly 



$-1321y+108.9d. 



It appears from Censorinus, that the canicular period began 

 when the 1 Tlioth was the 20th July. The number of years 

 allotted to it seems to have been very simply deduced from 

 the supposed length of the true year, as consisting of 365j 

 days, without any knowledge of the distinction between the 

 tropical and the sidereal year : and it commenced when the 

 apparent heliacal rising of Sirius was on the first day of the 

 Egyptian year; the sun being supposed to be about ten 

 degrees below the horizon. Professor Ideler has shown 

 (Halma's Ptolemy III., p. 31, 38) that this occurred on the 

 1 Thoth in — 1321 as well as in + 139, exactly at the inter- 

 val of 1460 tropical years; but that in +1599 it must have 

 happened about two days later : and he very truly observes, 

 that there was nothing in this phenomenon that could serve to 

 establish or to correct the supposed length of the year, de- 

 duced, as it must have been, from the regular return of the 

 seasons. 



The nature of the heliacal rising of the stars is illustrated 

 by a passage of Geminus (Halma, p. 57.) " The heliacal 

 risings of the stars are either true or apparent : the true are 

 when the sun and star are at the same instant on the horizon ; 

 but these are not visible, on account of the strength of the 

 sun's light. The sun, however, moving gradually among the 

 stars from west to east, the given star Avill rise every morning 

 afterwards a little more and more before the sun ; when it has 

 become so remote from it as to be visible, the star is said to 

 be at its apparent heliacal rising: and in this manner the 

 risings are predicted and are observed." *' It is a vulgar 

 prejudice," he continues, (p. 67) " to suppose that the rising 

 and setting of the stars have any influence on the atmosphere ; 



