82 ANCIENT CHRONOLOGY. SECT. XII 



These eras can only be regarded as approximate, since 

 ancient observations are too inaccurate, and modern ob- 

 servations too recent, to afford data for their precise 

 determination. 



The variation in the position of the solar ellipse occa- 

 sions corresponding changes in the length of the seasons. 

 In its present position spring is shorter than summer, 

 and autumn longer than winter ; and while the solar 

 perigee continues as it now is between the solstice of 

 winter and the equinox of spring, the period including 

 spring and summer will be longer than that including 

 autumn and winter. In this century the difference is 

 between seven and eight days. The intervals will be 

 equal toward the year 6483, when the perigee will coin- 

 cide with the equinox of spring ; but when it passes that 

 point, the spring and summer taken together will be 

 shorter than the period including the autumn and winter 

 (N. 147). These changes will be accomplished in a 

 tropical revolution of the major axis of the earth's orbit, 

 which includes an interval of 20,984 years. Were the 

 orbit circular, the seasons would be equal ; their differ- 

 ence arises from the eccentricity of the orbit, small as it 

 is ; but the changes are so trifling as to be imperceptible 

 in the short span of human life. 



No circumstance in the whole science of astronomy 

 excites a deeper interest than its application to chronol- 

 ogy. "Whole nations," says La Place, "have been 

 swept from the earth, with their languages, arts, and 

 sciences, leaving but confused masses of ruins to mark 

 the place where mighty cities stood ; their history with 

 the exception of a few doubtful traditions has perished ; 

 but the perfection of their astronomical observations 

 marks their high antiquity, fixes the periods of their ex- 

 istence, and proves that even at that early time they 

 must have made considerable progress in science." The 

 ancient state of the heavens may now be computed with 

 great accuracy ; and by comparing the results of calcu- 

 lation with ancient observations, the exact period at 

 which they were made may be verified if true, or if 

 false their error may be detected. If the date be accu- 

 rate and the observation good, it will verify the accuracy 

 of modern tables, and will show to how many centuries 



