SECT, xii.] ASTRONOMICAL ERAS. 97 



is recorded at different places, as having happened at dif- 

 ferent times. Therefore, when observations made at different 

 places are to be compared, they must be reduced by compu- 

 tation 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 

 o local circumstances and inequalities in the sun's motion. 

 It is the time elapsed from the instant the me*an 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 (K 102) and the pre- 

 cession of the equinoxes conjointly, the annual motion of the 

 one being ll"-8, and that of the other 50"'l. 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 crea- 

 tion 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. Ac- 

 cording to La Place, who computed these periods from dif- 

 ferent data, the last coincidence happened in the year 1250 

 of our era, which induced him to propose that year as a 

 universal epoch, the vernal equinox of the year 1250 to be 

 the first day of the first year. These eras can only be re- 

 garded as approximate, since ancient observations are too 

 inaccurate, and modern observations too recent, to afford 

 data for their precise determination. 



The variation in the position of the solar ellipse occasions 



