54 THE WORLD. 



ascertained to be about 365 days ; and as the moon, apparently, 

 made near 12 revolutions around the earth in that time the year 

 was subdivided into 12 months, which, in reference to the phases 

 of the moon, were again subdivided into weeks, of seven days 

 each. The time occupied by the sun in the departure from any 

 particular meridian, until its return to that meridian again, is 

 called a Solar day, and a similar revolution, a star being the 

 object, is called a Siderial day. We have already shown that 

 the Solar day was longer than the Siderial day, on account of the 

 apparent backward motion of the sun among the stars ; but it is 

 obvious, that the Siderial day, is the true measure of the time of 

 revolution of the earth on its axis. Now if the earth made an 

 exact number of revolutions on its axis, during the time in which 

 it moves from a particular part of the heavens, back to that par- 

 ticular position again, it is evident we would have an exact 

 number of siderial days to a year. 



It is found, however, that the siderial year does not consist of 

 an exact number of days, but contains, also, a fractional part of a 

 day. When a long interval of time elapses between different 

 observations, so that the earth makes a great number of revolu- 

 tions around the sun, the length of the year maybe very correctly 

 ascertained. Thus On the 1st day of April, 1669, at Oh. 3m. 

 47s., mean solar time, (which we shall explain presently,) 

 Picard observed the distance of the sun from the star Procyon, 

 measured on a parallel of latitude, to be 98 59' 36". In 1745, 

 which was 76 years after, La Caille observed the sun, to deter- 

 mine exactly the time when his difference of longitude should be 

 the same from the star, as in Picard's observation. Now the day 

 of the month in which La Caille observed, had been reckoned on 

 from Picard's time, just as if the year had consisted of exactly 

 365 days, except every leap year, when a day had been added, 

 for a reason that will appear presently. It was not until April 2d, 

 at llh. 10m. 45s., mean solar time, that the difference of 

 longitude was the same as when Picard observed. Now here it 

 was obvious that the earth had in reality, made just exactly 76 

 revolutions. The number of days however, was as follows, viz : 

 58 years, of 365 days each, and 18 leap years, of 366 days each, 



