14- Mr. Galbraith on the Obliquity of the Ecliptic. 



that obtained in winter, as may be seen in a table (20) pub- 

 lished by Dr. Pearson in the first volume of his Treatise on 

 Practical Astronomy, page 175; and that the winter obliquity 

 is in general less than the summer, which seems to have been 

 increasing in amount since the year 1754, in the time of Dr. 

 Bradley. In the first volume of Dr. Maskelyne's Observa- 

 tions, page v. of his explanation of the tables, he expressly 

 states that Bradley adopted 51° 28' 39^" N. as the latitude 

 of Greenwich, which he was induced to increase by half a 

 second, making it finally 51° 28' 40"; and it is believed that 

 this value of it continued to be used till his death, and per- 

 haps to a still later period. From some calculations made 

 from the Greenwich Observations by the French Refractions, 

 I was induced to think the latitude about 51° 28' 38^", or ex- 

 actly a second less than Bradley, and a second and a half less 

 than Maskelyne. 



Bessel has also investigated this matter in Schumacher's 

 Astro?wmische Nachrichten, No. 73, apparently with great care, 

 and obtains 51° 28' 38"*34, affected with a very small error, 

 which cannot by the Greenwich Observations be removed, 

 since the mural circle is incapable of reversion. 



Again, in the Greenwich Observations for 1826, the lati- 

 tude is deduced from numerous observations connected by 

 various tables of refraction. But if a star near the zenith, such 

 as y Draconis, be chosen as least affected by any small error 

 in the refractions, and a mean of the refractions in highest 

 esteem be employed, as those of the French, Bessel, Brinkley, 

 Young and Ivory, the latitude will be 51° 28' 38"-5, from 

 which all those derived from tables of most authority deviate 

 very slightly. This result may therefore, I presume, be con- 

 fidently trusted as the true latitude ; from which future obser- 

 vations will, in all probability, make no sensible deviation. 

 The difference between the most common estimation of it, or 

 51° 28' 40", and 51° 28' 3S"\5, is l"-5 = E., and therefore 

 2e = 3" will be the difference between the summer and win- 

 ter obliquity arising from this cause alone ; that is, the winter 

 obliquity will be 3" less than the summer. 



It will next be necessary to inquire what effect the refrac- 

 tions will have on the zenith distance in winter as being the 

 greatest, and consequently most affecting, when taken from 

 different tables. The meridian zenith distance of the sun at 

 Greenwich near the winter solstice will be about 75°, as ex- 

 treme nicety is not necessary to form an estimate in this case, 

 at which few tables, especially Bradley's, are without the sus- 

 picion of error. For this purpose, suppose the atmosphere 



is 



