OPPOSITION OF' MARS: 

 1849-50. 



INTRODUCTORY REMARKS TO THE CAMBRIDGE OBSERVATIONS. 



Observations upon Mars and comparison stars, made at the Observatory of Harvard Col- 

 lege, 1849-50. 



The following observations upon Mars and comparison stars in its neighborhood were made 

 at Cambridge, with the 23-feet Equatorial, in the autumn and winter of 1849-50. The imme- 

 diate object proposed in undertaking them was the determination of the parallax of the planet 

 by differential observations. Those in declination were intended for comparison with corre- 

 sponding ones made by the Expedition sent by the American government to Chile, under 

 charge of Lieut. J. M. Gilliss, U. S. N. The Right Ascension observations afford data also 

 for deducing the parallax, by comparing the place of the planet at the evening and morning 

 observations, when occupying a large hour angle west and east of the meridian. 



They are comprised in three divisions, viz : 



Comparisons of the planet with neighboring stars 



let. By times of transits, marked with a chronometer. 



2d. By differences of A. R., measured with the micrometer. 



3d, By differences of declination, measured with the micrometer. 



In the reduction of the latter, the value of a revolution of the micrometer head used was 



r = 9".800 + 0".00026 (t 50,) 



t being the temperature in degrees of Fahrenheit's scale. 



For the Right Ascension observations, those stars of sufficient brightness nearest to the decli- 

 nation of the planet were commonly taken, preference being given to the stars of the catalogue 

 in the circular of the Chile Expedition, when they were near enough to admit of frequent repe- 

 tition. But in all cases the same comparison stars were used for the successive evening and 

 morning observations. 



The stars compared in declination were taken from the catalogue of- the Expedition. 



All the observations are reduced to the means for each wire separately, and the times by the 

 chronometer are reduced to the sidereal times at the Observatory. The small corrections ap- 

 plied to the means of the micrometer differences are for the effect of temperature upon the 

 value of the screw. 



For the name of the observer, the initials W. C. B. for W. C. Bond, and G. P. B. for G. P. 

 Bond, are employed. 



W. C. BOND. 



CAMBRIDGE OBSERVATORY, July 21, 1853. 



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