l68 PHILOSOPHICAL TRANSACTIONS. [aNNO 1764. 



right ascension. The question therefore comes to this, How many minutes and 

 seconds of mean solar time does the mean sun take to move this distance up to 

 or from the meridian ? Astronomers hitherto have allowed 1 minute of time to 

 every 15 minutes of right ascension, and so in proportion; and I apprehend 

 justly too ; for does not the mean sun, in returning to the meridian, describe 

 360° about the pole in 24 hours of mean solar time ; whence it is plain that his 

 departure from the meridian is at the rate of 15° to 1 hour, and 15' to 1 minute 

 of mean solar time. Therefore astronomers have not converted the equation of 

 right ascension into time according to the motion of the primuin mobile ; for 

 the equation of time being mean solar time, and the motion of the primum 

 mobile being compleated in SS** 56™ 4^ of mean solar time, therefore 15° motion 

 of the primum mobile do not answer to 1 hour of mean solar time (though it 

 does to 1 hour of sidereal time) but to the 24th part of 23'' 56"" 4' or 59"* 50-f'. 

 And it appears that the equation of time in the Connoissance des Mouvements 

 Celestes has been computed in this manner, and the table in the 7gth page 

 of the Connoissance for 1761 has been made use of, entitled, " A table to con- 

 vert into degrees the time of a clock regulated according to the mean motion of 

 the sun." The degrees of this table are evidently degrees of the primum 

 mobile, 1 hour of mean solar time giving 15' 2' 27.8", which answers to the 

 motion of the stars from the meridian, but not to the mean motion of the sun 

 from it, which is 15° to 1 hour of mean solar time: whence it appears, that this 

 writer has evidently fallen into the mistake of taking motion or space of the 

 primum mobile, instead of the mean motion of the sun from the meridian ; a 

 mistake equal to that Of which he erroneously supposes former mathematicians 

 to have been guilty, in computing the equation of time. So that the equation 

 of time in this ephemeris, besides the mistake arising from taking the equation 

 of the equinoctial points into the account, is constantly too small in the propor- 

 tion of 24 hours to 23'' 56'" 4% or of 366 to 365, or too small by 1 second on 

 every 6 minutes of the equation of time: and the mistake of 24- seconds, which 

 was supposed to be found in the old manner of reducing the equation of right 

 ascension into time, really takes place in this new method ; which, added to 1 

 second of time, arising from the mistake in taking the precession of the equi- 

 noxes into the account, produces 3^- seconds, an error which I apprehend, the 

 astronomical equation tables used since Mr. Flamsteed's time have but rarely 

 exceeded. 



To some, who are not well acquainted with the present improved state of 

 astronomy, the difference in question may seem a matter of indifference, and too 

 trifling for notice. But if truth be the object of all our inquiries, why should 

 we wilfully go beside it in the smallest matters.? And is it not a justice due to 

 past astronomers, to whom we owe the foundations of all our knowledge, to vin- 



