144 ON THE COMET OF SEPTEMBER 1862. No. 1. 



in the tables to whose elements those of the present one bear a 

 complete resemblance. There is a rough coincidence between 

 its perihelion distance, position of orbit-plane, and direction of 

 heliocentric motion, and the corresponding elements of the great 

 comet of 1811 ; the greater axes of the two orbits are, however, 

 at right angles to each other. The most remarkable feature of 

 the orbit of the present comet is the fact that it nearly intersects 

 that of the earth at the descending node, or point where the 

 comet crosses from the north to the south side of the plane of 

 the ecliptic. The value of the comet's radius vector, at the node, 

 expressed in parts of the earth's mean distance from the sun, is 

 1-0191352. The radius vector of the earth, corresponding to the 

 same point, is 1'0132944. If we multiply the difference 

 0*0058408 by ninety-five millions of miles, the assumed mean 

 distance of the earth from the sun, we have very approximately 

 the distance between the two orbits, as measured in the plane of 

 the ecliptic namely, 554,876 miles. Now, it will be remembered 

 that when the great comet of last year crossed the plane of the 

 earth's orbit on the 29th June, the earth and comet had nearly 

 the same longitude as seen from the sun. The comet's head, 

 however, was thirteen millions of miles within the line of the 

 earth's orbit ; consequently the earth merely performed a journey 

 through the more diffused part of the comet's tail. Had the 

 earth and comet in the present instance been in heliocentric 

 conjunction at the time of the nodal passage, we should have 

 witnessed a phenomenon surpassing, it may be, the magnificent 

 apparitions of ancient times. The condition necessary to have 

 brought the two bodies into such close neighbourhood would be 

 that the perihelion passage should occur about thirty-two days 

 earlier than it actually did. If we assume six minutes of arc as 

 the apparent diameter of the comet's head on the evening of the 

 1st September, we have 58,000 miles as the real diameter of 

 the nebulosity surrounding the nucleus, and this would subtend 

 an angle of six degrees, supposing the earth and comet to be 

 both in the line of nodes at the same time. An object 

 like this would strike even the enlightened people of the 

 nineteenth century with amazement. The earth, however, 

 had passed the point on the 10th of August, or thirty- 



