SCT. XII. ASTRONOMICAL ERAS. 81 



. Since solar and sidereal time are estimated from the 

 passage of the sun and the equinoctial point across the 

 meridian of each place, the hours are different at differ- 

 ent places : while it is one o'clock at one place it is two 

 at another, three at another, &c. ; for it* is obvious that 

 it is noon at one part of the globe, at the same moment 

 that it is midnight at another diametrically opposite to it; 

 consequently an event which happens at one and the 

 same instant of absolute time is recorded at different 

 places, as having happened at different times. There- 

 fore, when observations made at different places are to 

 be compared, they must be reduced by computation to 

 what they would have been had they been made under 

 the same meridian. To obviate this, it was proposed by 

 Sir John Herschel to employ mean equinoctial time, 

 which is the same for all the world, and independent 

 alike of local circumstances and inequalities in the sun's 

 motion. It is the time elapsed from the instant the mean 

 sun enters the mean vernal equinox, and is reckoned in 

 mean solar days and parts of a day. 



Some remarkable astronomical eras are determined by 

 the position of the major axis of the solar ellipse, which 

 depends upon the direct motion of the perigee (N. 102) 

 and the precession of the equinoxes conjointly, the 

 annual motion of the one being ]1"*8, and that of the 

 other 50"-1. Hence the axis, moving at the rate of 

 61"-9 annually, accomplishes a tropical revolution in 

 209-84 years. It coincided with the line of the equinoxes 

 4000 or 4089 years before the Christian era, much about 

 the time chronologists assign for the creation of man. In 

 6483 the major axis will again coincide with the line of 

 the equinoxes ; but then the solar perigee will coincide 

 with the equinox of autumn ; whereas at the creation of 

 man it coincided with the vernal equinox. In the year 

 1246 the major axis was perpendicular to the line of the 

 equinoxes ; then the solar perigee coincided with the 

 solstice of summer, and the apogee with the solstice of 

 winter. According to La Place, who computed these 

 periods from different data, the last coincidence hap- 

 pened in the year 1250 of our era, which induced him to 

 propose that year as a universal epoch, the vernal equi- 

 nox of the year 1250 to be the first day of the first year. 

 6 



