4 i 6 THE POPULAR SCIENCE MONTHLY. 



inanac." Taking the distance as 92,250,000 miles, this makes the 

 sun's diameter 860,500 ; and the probable error of this quantity, de- 

 pending as it does both on the error of the measured diameter and of 

 the distance, is some 4,000 or 5,000 miles; in other words, the chances 

 are strong that the actual diameter is between 855,000 and 865,000 

 miles. 



Measurements made by the same person, however, and with the 

 same instrument, but at different times, sometimes differ enough to 

 raise a suspicion that the diameter is slightly variable, which would be 

 nothing surprising considering the nature of the solar surface. There 

 is no sensible difference between the equatorial and polar diameters, 

 the rotation of the sun on its axis not being sufficiently rapid to make 

 the polar compression (which must, of course, necessarily result from 

 the rotation) marked enough to be perceived by our present means 

 of observation. 



It is not easy to obtain any real conception of the vastness of this 

 enormous sphere. Its diameter is 108.7 times that of the earth, and its 

 circumference proportional, so that the traveler who could make the 

 circuit of the world in 80 days would need nearly 24 years for his jour- 

 ney around the sun. Since the surfaces of spheres vary as the squares, 

 and bulks as the cubes, of their diameters, it follows that the sun's 

 surface is nearly 12,000 times, and its volume, or bulk, more than 

 1,280,000 times, greater than that of the earth. If the earth be rep- 

 resented by one of the little three-inch globes common in school ap- 

 paratus, the sun on the same scale will be more than 27 feet in diam- 

 eter, and its distance nearly 3,000 feet. Imagine the sun to be hollowed 

 out and the earth placed in the centre of the shell thus formed, it 

 would be like a sky to us, and the moon would have scope for all her 

 motions far within the inclosing surface; indeed, since she is only 

 240,000 miles away, while the sun's radius is more than 430,000, there 

 would be room for a second satellite 190,000 miles beyond her. 



The mass of the sun, or quantity of matter contained in it, can also 

 I>e computed when we know its distance, and comes out 325,600 times 

 as great as the earth. The calculation may be made either by means 

 of the proportion given in the note to page 413, or by comparing the 

 attracting force of the sun upon the earth, as indicated by the curva- 

 ture of her orbit (about 0.119 inch per second), with the distance a 

 body at the surface of the earth falls in the same time under the action 

 of gravity, a quantity which has been determined with great accuracy 

 by experiments with the pendulum. Of course, the fact that the sun 

 produces its effect upon the earth at a distance of 92,250,000 miles, 

 while a falling body at the level of the sea is only about 4,000 miles 

 from the centre of the attraction which produces its motion, must also 

 enter into the reckoning. 1 



1 The calculation of the sun's mass, from the data given, proceeds as follows : Let M = 

 the sun's mass, and m that of the earth ; R = the distance from the earth to the sun, 



