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SIR I'EANK W. DYSON, F.R.S., ON 



it is, that its density is, in the mean, something like that of 

 water — we know how hot it is, say 7000° C. near the surface 

 and increasing greatly as we penetrate inwards — we know 

 that it consists, at any rate near the surface, of many chemical 

 elements with which we are familiar on the earth, all in a 

 gaseous condition owing to the high temperature — we know 

 that it rotates on its axis in twenty-five days, that a number of 

 planets, including the earth, revolve around it, and that it is 

 moving through space at the rate of twelve miles a second. 



Now when we look at the stars they are simply points of 

 hght in the sky : we have no notion whatever of their distances. 

 They are all so small that they have no perceptible disc, such 

 as the sun has. When we look at them with a telescope, how- 

 ever large, they still remain the merest points. If you will 

 admit that they are bodies Hke the sun and comparable with it 

 in size, you will see that they must be at a very much greater 

 distance. I suppose that our largest telescopes would show the 

 sun with a disc of sensible size if it were twenty or thirty 

 thousand times as far away. But it is begging the question to 

 begin by assuming that the stars are like the sun, and we will 

 show how their distances are found with no assumptions except 

 those of elementary geometry. 



I dare say you are familiar with the method used by surveyors 

 in finding the distances of inaccessible objects. They take two 

 points, A and B, and measure carefully the distance from A to 

 B, and then measure, by an instrument called a theodolite, the 

 two angles, 0 A B and 0 B A. When this is done it is easy 

 to calculate the distances 0 A and 0 B by a branch of elementary 

 mathematics called trigonometry. There is nothing at all 

 mysterious or difficult about it ; suppose that A B is 1 mile, 

 and on a sheet of paper we put down ah = 1 inch and draw the 

 angles at a and h equal to those at A and B, then o a, o b will 

 give us the distances we require in the scale of an inch to a 

 mile. 



This same method can be easily applied to determine the 

 distance of the moon. If the moon is observed simultaneously 

 from two places on the earth, let us say the observatories at 

 Greenwich and the Cape, one angle corresponding to that at A 

 is measured at Greenwich, another corresponding to B is 

 measured at the Cape, and the distance A B represents the 

 length of the straight line joining Greenwich to the Cape. In 

 practice, if one wishes to obtain an accurate result there are 

 a number of minutice to be attended to, but the general principle 

 is simplicity itself. 



