DETERMINING THE LONGITUDE. Q55 



Rules for deducing the Right Ascension of the Mooiis Limb froiu the 

 Observed Transit of the Sun and Moon^ and thence the Longitude. 



If the sun's motion in the Ecliptic were uniform, that is, if liis daily mo- 

 tion was equal to the acceleration of the fixed stars (o"* 55' 9), the inter- 

 val, in*mean solar time, between the transits of the sun and moon, would be 

 equal to the difference of their right ascensions at the instant of the moon's 

 transit; but, as this is not the case, as the sun's daily motion in ARn. is some- 

 times greater and sometimes less, than the mean acceleration of sidereal on 

 solar time, it follows, that the interval in solar time must be corrected by the 

 daily difference of the equation of time given in the Ephemeris. When the 

 daily difference of the sun's right ascension exceeds 3"* 5^^^ the proportional 

 part of the daily difference of the equation of time is subtractive, otherwise 

 additive to the interval. 



Thus, after the 2d November 1825, the sun's daily motion is greater than 

 3'" 55^ 9. — For instance, between the 18th and 19th, it is 4" 10', being about 

 14' in excess, which is the daily difference of the equation of time nearly. 

 Hence it appears, that an apparent day, or twenty-four hours of apparent time, 

 on the 18th and 19th, was in excess of a mean solar day, by 14% and so on, 

 till about the middle of February, when an apparent day becomes less than a 

 mean solar day. Hence, — 



1. — With a solar chronometer, correct the observed interval for the rate, 

 and then we have the interval in mean solar hours. 



2. — If an apparent day is in retardation of mean solar time, that is, if the 

 interval between two transits of the sun, exceeds twenty-four hours mean 



