86 ASTRONOMICAL ERAS. SECT. XII. 



forty-fifth year is considered as the date of Christ's nativity ; 

 and the forty-sixth year of the Julian Calendar is assumed to be 

 the first of our era. The preceding year is called the first year 

 before Christ by chronologists, but by astronomers it is called 

 the } 7 ear 0. The astronomical year begins on the 31st of De- 

 cember at noon ; and the date of an observation expresses the 

 days and hours which have actually elapsed since that tune. 



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 different 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 oppo- 

 site 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. Therefore, when observa- 

 tions 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 preces- 

 sion of the equinoxes conjointly, the annual motion of the one 

 being 11"'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 4C89 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 



