220 M. Arago on the Thermometrical State 



whicli is called the celestial equator, separates the northern 

 from the southern constellations. The nearer a constellation is 

 to the southern pole the shorter, with us, is the time which 

 elapses between its rising and its setting. The contrary hap- 

 pens in the opposite hemisphere : the constellations included in 

 it, show themselves above our horizon a number of hours greater 

 as they approximate the north. Lastly, the intermediate con- 

 stellations, those which traverse the equator, are visible twelve 

 successive hours, and disappear during the twelve following. 



The sun in its apparent annual course, is found during six 

 months, in the southern constellations, and the other six it is 

 north of the equator. No one can doubt, that at each period of 

 the year, the length of the day is precisely equal to the time 

 which elapses between the rising and the setting of the constel- 

 lation which the sun has then reached,-— of which it seems to 

 form a part, and with which it participates in the diurnal revo- 

 lution of the heaven. The problem whether the winter-days, 

 compared with the summer, are now more or less unequal than 

 they were 2000 years ago, leads us to inquire if, in their excur- 

 sions from the north to the south of the equator, the sun has 

 always stopt at the same constellations, or better still, at the 

 same stars. Mathematically speaking, it has not. Since the 

 most ancient observations, the northern and southern excursions 

 of this luminary have been confined within narrower and nar- 

 rower limits. We add, however, at the same time, that the an- 

 nual change is extremely minute : that the total, at the end of 

 . 2000 years, has scarcely amounted to a quarter of a degree ;•— 

 that, in other terms, the sun in his southern excursion, for ex- 

 ample, now stops previous to his commencing to mount towards 

 the equator, when the inferior edge of his disk is arrived at the 

 star which his centre attained at the commencement of this pe- 

 riod of twenty centuries. A variation so completely trifling, 

 evidently cannot have induced a change worthy of remark, either 

 in comparative lengths of summer and winter days, or in the 

 phenomena of agriculture.* 



• Geometricians have discovered that the variation which is observable in 

 the enlargements of the annual oscillations of the sun, to the north and south 

 of the equator, is Periodical ; — that after having diminished for a certain 

 number of centuries, these oscillations begin to increase again, and thus in 

 indefinite succession, without ever surpassing the narrowest limits. 



