98 ANCIENT CHRONOLOGY. [SECT. xir. 



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. 147). 

 These changes will be accomplished in a tropical revolution 

 of the major axis of the earth's orbit, which includes an in- 

 terval 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 history, with the exception of a few doubtful traditions, 

 has perished ; but the perfection of their astronomical obser- 

 vations 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 ac- 

 curacy; 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 observation good, 

 it will verify the accuracy of modern tables, and will show 

 to how many centuries they may be extended without the 

 fear of error. A few examples will show the importance of 

 the subject. 



