82 II. A. Newton — The Story of Bieltfs Comet 



Thirty-three years later, November, 1805, another French- 

 man 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 close to the earth, and I shall in a mo- 

 ment tell you how it appeared. It was visible to the naked 

 eye, even in strong moonlight. Twenty years later, Februaiy 

 1826, 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 exact 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. Looking southward 

 we should see the sun in the center. The earth, with its moon, 

 would travel around the sun in a path or orbit denoted by the 

 circle in the figure (fig. 1). 



It goes about the sun once a 3 r ear, being, on the 10th 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 noth- 

 ing to say about them to-night. In the figures which I show 

 you the earth's orbit is twenty inches in diameter, or one inch 

 to nine million miles. An express railway train traveling 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 traveled three times in twenty years, or 

 once each 6§ years. When nearest to the sun, or at perihelion, 

 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 toward the sun the sun's attraction 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 momentum, so that when it 

 reaches D again it has onty its old velocity with which to re- 

 peat its circuit. At P its velocity is twenty-eight miles, at D 

 four miles, a second. In fact, to pass over the part lying ap- 

 parently outside of Jupiter's orbit, just half of the whole 6§ 

 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 AB as a hinge, the part ADB ris- 



