SECT. XII. ANCIENT CHRONOLOGY. 87 



solstice of winter. According to La Place, who computed these 

 periods from different 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 regarded 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 

 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 

 towards the year 6483, when the perigee will coincide 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. 151). These changes will be accom- 

 plished 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 difference arises 

 from the excentricity 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 chronology. "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 his- 

 tory, 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 existence, 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 calculation 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 accurate and the observa- 



