Feb. 25, 1886J 



NATURE 



393 



Thirty-three years later, November 1805, another 

 Frenchman, named Pons, saw the comet. It passed 

 rapidly from the northern heavens, and in a month went 

 below our horizon. It came this time very cloie to the 

 earth, and I shall in a moment tell you how it appeared. 

 It was visible to the naked eye, even in strong moonlight. 

 Twenty years later, February 1S26, an Austrian officer, 

 Von Biela, again found the comet. So soon as an orbit 

 could be computed, it was seen that the three comets of 

 1772, 1805, and 1826 were the same body. This has 

 since been known as Biela's comet. Its e.xact path around 

 the sun could now be told. Let me show it to you. 



Let us look down upon the solar system from a point 

 several hundreds of millions of miles north of it. Look- 

 ing southward we should see the sun in the centre. The 

 earth, with its moon, would travel around the sun in a 

 path or orbit denoted by the circle in the figure (Fig. l). 



It goes about the sun once a year, being, on the loth 

 days of January, April, July, and October, at the points 

 so marked on the diagram. The motion is opposite to 

 that of the hands of a watch. Outside, five times as far 

 from the sun as is the earth, will be the huge planet 

 Jupiter, a part of whose path you see. It goes about the 

 sun once in twelve years. The paths of the other planets 

 are not in the figure, as I have nothing to say about them 

 to-night. In the figures which I show you the earth's 

 orbit is tuentv inches in diameter, or one inch to nine 



million miles. An express railway train travelling all the 

 time for a fortnight would pass over about the thousandth 

 of an inch in this figure. The comet's path is the ellipse. 

 Around this ellipse it travelled three times in twenty years, 

 or once each 65 years. When nearest to the sun, or at peri- 

 helion, it went within the earth's orbit, and when most 

 distant it passed beyond Jupiter. 



The comet's motion is very unequal. At D it moves 

 very slowly. As it falls towards the slin the sun's attrac- 

 tion makes it move faster and faster, so that it whisks 

 rapidly by P. As it then rises from the sun on the other 

 side of the orbit, the sun not only turns it ever out of the 

 straight path it would move in, but it stops its upward 

 niomentum, so that when it reaches D again it has only 

 its old velocity with which to repeat its circuit. At P its 

 velocity is twenty-eight miles, at D four miles, a second. 

 In fact, to pass over the part lying apparently outside of 

 Jupiter's orbit, just half of the whole 5| years is required. 

 I said apparently outside, for another fact must be noticed : 

 while Jupiter and the earth may be said to move in the 

 same plane, that of the figure, the comet's orbit, lies at an 

 angle. Suppose the ellipse to be a metal ring, and let it 

 turn about the line A l! as a hinge, the part A D B rising 

 toward you, and the part A P D 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 A B. The importance of this fact 

 will by and by appear. 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 A a (Fig. 2). It was last 

 seen four weeks later from B in the direction b b. In 

 November 1805 Pons found it when the earth was at a' 

 and the comet at a' (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 December 8, with the scale of the figures before you, it 

 was only fth of an inch from the earth at the node. On 

 the same scale the moon is j'jjtli of "I'l inch from the earth. 

 The comet passed j'jth 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 ap- 

 pearance, though it was much smaller, and it had a bright, 

 star-like nucleus. The nucleus had not sharp edges, not 



Fig. 2. Fig. 3. 



even a definitely round form, but was like a light shining 

 through a fog. Its diameter was about 1 1 2 miles, or, if 

 we take its central light, 70 miles ; speaking roughly, as 

 large as the State of Connecticut. The whole cloud, as 

 seen in the telescope, was some 6000 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 can- 

 not say. 



Position in 1826. — In 1826 it was first seen from a in 

 the line a^ (Fig. 3). Astronomers followed it with care, 

 as they had come to know that it was a comet of short 

 period, and not many such were then known. Its path 

 then crossed just inside the earth's orbit at the node, but 

 only ,-, Jijth 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-predicted return was watched for. Some of you also 

 remember the widespread, though groundless, fears at that 

 time of a collision of the earth and the comet. The comet 

 was first seen by Sir John Herschel in September. In his 

 20-foot reflecting telescope he saw it pass centrally over a 

 group of small stars of the i6th or 17th magnitude. The 

 slightest bit of fog would have at once blotted out the 

 stars. Through the comet, however, they looked like a 

 nebula, resolvable, or partly resolvable, into stars. How 

 thick the cometic matter was we do not know. Its extent 



