The Great Comet o/^1861. 281 



The nucleus admitted of very precise observations ; indeed it 

 is a curious fact tliat it would be quite possible by means of pro- 

 per comparisons with neighbouring stars, to obtain the differences 

 of terrestrial longitudes of the principal points at which it was 

 observed, with a degree of precision only surpassed by the more 

 refined methods known in astronomy. 



The near approach of the present comet to the earth and the 

 sharply defined point of its nucleus, illustrates the practicability 

 of a method of determining the solar parallax with perhaps greater 

 exactness than can be attained by any other means. Many comets 

 have stellar points for their nuclei, visible in the larger telescopes, 

 which admit of as accurate comparisons with neighbouring stars 

 as is practicable in measurements among the stars themselves. 

 Many such have appeared within the last fifteen years. Suppose 

 such a comet to be suitably placed so as to be observed simulta- 

 neously in different quarters of the globe, when at a distance 

 from the earth of less than one-twentieth of the sun's distance. 

 Under favorable circumstances it would not be hazarding too 

 much to say, that in the course of its apparition the probable 

 error of the solar parallax could be reduced within smaller limits 

 than is possible by means of transits of Venus or of any othet 

 method. Such an opportunity might possibly afford an improved 

 value of the mass of the earth. 



From the above elements, the diameter of the nucleus may be 

 variously estimated at from one hundred and fifty to three or four 

 hundred miles. On July 2d the breadth of the head at the nu- 

 cleus was 156,000 miles, the height of the inner envelope 11,500 

 miles, and the length of the tail about 15,000,000 miles. 



The comet was seen between one and two o'clock on Sunday 

 morning, June 30th, by Dr. Brunnow, at the Observatory of Ana 



