Sky Study 907 



sun and its planets and its relation to the stars in space. He explains that 

 if here in the Atlantic States we should make a model of our solar system by 

 putting an apple down in a field to represent the sun ; then our earth could 

 be represented by a mustard seed forty feet away revolving around the 

 apple; and Neptune, our outermost planet, could be represented as a small 

 pea circling around the apple at the distance of a quarter of a mile. Thus, 

 our whole solar system could be modeled in a field one-half mile square, 

 except for comets which might extend out in their long orbits for several 

 miles. But to find the star nearest to our earth, the star that is only four 

 and one-half light-years away from us, we should have to travel from this 

 field across the whole of North America to California, and then take steamer 

 and go out into the Pacific ^ "an before we should reach our nearest star 

 neighbor, which would be another sun like our own and be represented by 

 another apple. 



COMETS 



Besides planets and stars there are in space other bodies spinning around 

 our great sun, and following paths shaped quite differently than those 

 followed by our earth and its sister planets. We move around the sun 

 nearly in a circle with the sun at the center, but these other heavenly bodies 

 swing around in great ellipses, the sun being near one end of the ellipse and 

 the other end being out in space beyond our farthest planet. These bodies 

 do not revolve around the sun in the same plane as our world and the other 

 planets, and indeed they often move in quite the opposite direction. The 

 most noticeable of these bodies whose race-track around the sun is long 

 instead of circular are the comets, and we know that some of these almost 

 brush the sun when turning at the end of their course. The astronomers 

 have been able to measure the length of the race-tracks of some of the 

 comets and thus tell when they will come back. Encke's comet, named 

 after the German astronomer, makes its course in three and one-half years 

 and this is the shortest period of any we know. There are about thirty 

 comets whose courses have been thus measured ; the longest period belongs 

 to Halley's comet, which makes such a long trip that it comes back only 

 once in seventy-six years ; but there are other comets which astronomers are 

 sure travel such long routes that thev come back onlv once in hundreds or 



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even thousands of years. About nine hundred comets have been dis- 

 covered, many of them so small that they can only be seen through the aid 

 of the telescope; and it has been found that in one instance, at least, 

 three comets are racing around the sun on the same track. 



A comet is a beautiful object, usually having a head which is a point of 

 brilliant light and a long, flaring tail of fainter light, which always extends 

 out from it on the side opposite the sun. The head of a comet must be 

 nearly twice as thick through as the earth in order to be large enough for our 

 telescopes to discover it. Some of the comet heads have been measured, 

 and one was thirty-one times, and another one hundred and fifty times, as 

 wide as our earth. If the heads are this large, imagine how long the tails 

 must be ! Some of them are far longer than the distance from our earth to 

 the sun. 



The head of a comet is supposed to be a mass of gas which is made to 

 glow by the sun's heat, and is so volatile and thin that the heat evaporates 

 it. In fact, this gas has so little weight that light can push it; one would 

 never believe that light could push anything because we cannot feel it 



