of the Terrestrial Globe. ;. 44 21S 



go on to shew, that for the determination of what has been the 

 duration of the sidereal day for 2000 years, we have proofs 

 infinitely superior to antique machines, whose correspondence 

 and identity might be contested ; and besides, that unless this 

 duration were the same, the years must inevitably have become 

 shorter. 



The moon is not immoveable in space. She moves from west 

 to east. It is from west to east that we see her successively 

 traverse all the constellations of the zodiac. 



The moon's movements have, in all times, attracted the espe- 

 cial attention of mankind ; and they have been peculiarly soli- 

 citous to measure its velocity. But the measure of velocity im- 

 plies the choice of an unit of time ; and it will be readily ad- 

 mitted that this unit of time has been the sidereal day. 



That the choice of the sidereal day as the unit of time, should 

 furnish no objection in the problem concerning the velocity of 

 the moon, it is necessary that the duration of this day, or, what 

 is the same thing, it is necessary that the duration of the rota- 

 tion of the earth, should be independent of the velocity of our 

 satellite. But it is manifest that this independence exists. Even 

 should the earth, on a sudden, cease to rotate upon its axis, the 

 moon would nevertheless continue to traverse the constellations 

 of the zodiac, as she at present does. 



The Alexandrian school has left us observations, from which 

 we can deduce, with the greatest accuracy, what, 2000 years 

 ago, was the mean course which the moon ran during a sidereal 

 day. The astronomy of the Arabians supplies us with elements 

 for the determination of the same point, for the time of the 

 Caliphs. And there is not a single catalogue of modern obser- 

 vations, where we do not find, for the present epoch, the value 

 of the mean course of the moon during the duration of the side- 

 real day. 



Well then, it happens that the arc described in a sidereal day 

 by our satellite, proves to be exactly the same, whether we cal- 

 culate by the Grecian observations, or by the Arabian, or by 

 those of our own day *. 



• It is true that if we take the rough observations, the arcs described by 

 the moon at the three distinct epochs, the Grecian, Arabian, and Modem, 

 will not be equal Since the time of the Chaldeans, the velocity of the moon 



