DISTANCE AND DIMENSIONS OF THE SUN. 415 



already been said, to a distance of 92,250,000 miles, with a probable 

 error of about one-half per cent., or 450,000 miles. 



But, though the distance can thus easily be stated in figures, it is 

 not so easy to give any real idea of a space so enormous ; it is quite 

 beyond our power of conception. If one were to try to walk such a, 

 distance, supposing even that he could walk 4 miles an hour, and 

 keep it up for 10 hours every day, it would take 68^ years to make a 

 single million of miles, and more than* 6,300 years to traverse the 

 whole. 



If some celestial railway could be imagined, the journey to the sun, 

 even if our trains ran 60 miles an hour, day and night and without a 

 stop, would require over 175 years. Sensation, even, would not travel 

 so far in a human lifetime. To borrow the curious illustration of 

 Prof. Mendenhall, if we could imagine an infant with an arm long 

 enough to enable him to touch the sun and burn himself, he would die 

 of old age before the pain could reach him, since, according to the 

 experiments of Helmholtz and others, a nervous shock is communi- 

 cated only at the rate of about 100 feet per second, or 1,637 miles a 

 day, and would need more than 150 years to make the journey. Sound 

 would do it in about 14 years if it could be transmitted through ce- 

 lestial space, and a cannon-ball in about 9, if it were to move uni- 

 formly with the same speed as when it left the muzzle of the gun. If 

 the earth could be suddenly stopped in her orbit, and allowed to fall 

 unobstructed toward the sun under the accelerating influence of his 

 attraction, she would reach the central fire in about four months. I 

 have said if she could be stopped, but such is the compass of her 

 orbit that, to make its circuit in a year, she has to move nearly 

 19 miles a second, or more than fifty times faster than the swiftest 

 rifie-ball ; and in moving 20 miles her path deviates from perfect 

 straightness by less than one-eighth of an inch. And yet, over all 

 the circumference of this tremendous orbit, the sun exercises his do- 

 minion, and every pulsation of his surface receives its response from 

 the subject earth. 



By observing the slight changes in the sun's apparent diameter, 

 we find that its distance varies somewhat at different times of the 

 year, about 3,000,000 miles in all ; and minute investigation shows 

 that the earth's orbit is almost an exact ellipse, whose nearest point 

 to the sun, or perihelion, is passed by the earth about the 1st of Jan- 

 uary, at which time she is 90,750,000 miles distant. 



The distance of the sun being once known, its dimensions are easi- 

 ly ascertained at least, within certain narrow limits of accuracy. 

 The angular semi-diameter of the sun when at the mean distance is 

 almost exactly 962", the uncertainty not exceeding 2o 1 00 of the whole. 

 The result of twelve years' observations at Greenwich (1836 to 1847) 

 gives 961.82", and other determinations oscillate around the value 

 first mentioned, which is that adopted in the " Amei'ican Nautical Al- 



