84 H. A. Nt wtoiv—The Story of BielcCs Comet. 



ing toward you, and the part APD retreating from you. The 

 parts near D must rise about the half-diameter of the earth's 

 orbit to give the true position of the two planes. Notice that 

 the comet's and the earth's orbits cut each other at the node on 

 the line AB. The importance of this fact will by and by ap- 

 pear. The two orbits seem to cut each other at another point 

 (below P), it is true, but because of the angle of the planes the 

 cutting is only apparent. 



Like all other comets, this one was visible only when near 

 the earth and near the sun. Through the outer part of its path 

 it was never seen, even with a telescope. The comet was seen 

 in 1826 for the third time. 



Positions in 1772 and 1805. — In March, 1772, it was first seen 

 from A in the direction Aa (fig. 2). It was last seen four 

 weeks later from B in the direction BZ>. In November, 1805, 

 Pons found it when the earth was at A' and the comet at a r 

 (fig. 2). Both the earth and the comet were going to the node, 

 the comet going faster than the earth. The earth passed 

 the node just ahead of the comet. I have told you that the 

 comet was then visible to the naked eye even in moonlight, 

 and well it might be. On the 8th of December, with the scale 

 of the figures before you, it was only •§ th of an inch from the 

 earth at the node. On the same scale the moon is -^ of an 

 inch from the earth. The comet passed T x -g- of an inch outside 

 the earth's orbit, but the earth was already past that point. 



Dr. Schroter describes the comet: To the naked eye it was 

 (Dec. 8) a large round cloud of light nearly as large as the 

 moon. In a 13-foot telescope it had the same appearance, 

 though it was much smaller, and it had a bright, star-like 

 nucleus. This nucleus had not sharp edges, not even a defi- 

 nitelv round form, but was like a light shining through a fog. 

 Its diameter was about 112 miles, or, if we take only the cen- 

 tral light, 70 miles ; speaking roughl} 7 , as large as the State of 

 Connecticut. The whole cloud, as seen in the telescope, was 

 some 6,000 miles in diameter ; to the naked eye perhaps 30,000 

 miles. How much smaller than 70 miles was the hard part of 

 the nucleus, we cannot say. 



Position in 1826. — In 1826 it was first seen from A in the 

 line Aa (fig. 3). Astronomers followed it with care, as they 

 had come to know that it was a comet of short period, and not 

 manv such were then known. Its path then crossed just inside 

 the earth's orbit at the node, but only -g-L of an inch in the 

 diagram, or 20,000 miles, in fact, from it. 



Position in 1832. — Six and two-thirds years brings us to 1832, 

 and you can readily imagine with what interest this first pre- 

 dicted return was watched for. Some of you also remember 

 the wide-spread, though groundless, fears at that time of a col- 



