KN()\VLi:i)GE. 



Sei'TEMber, 1912. 



high nhcne the ecUptic ? <-W)(ur tlic trliiitic is. of 

 coursi'. ;» fiii,-(>n tic Piirlcr, for tluTc is no up or down, 

 as \\f use tile terms, in celestial matters; what is 

 meant is, on that sitie of the ecliptic plane in which 

 the north pole of the earth is situated. Let this 

 point he somewhere about twice the diameter of 

 Jupiter's orbit "above" the Sun, so as to open out 

 his orbit and those of the planets within. Imagining 

 our friend gifted with the necessary keenness of 

 vision, at a casual glance the whole of the planets 

 would seem to him to be at rest. There they would 

 be, hung in space, apparently fi.xed points of light. 

 It would be the same with the .several families of 

 satellites, supposing the observer could visit each of 

 the motlier planets, and note the outlook thence, as 

 he studied the children of each. With the e.xcep- 

 tion of a few of the satellites he would have to watch 

 some little time before the motions of each system 

 could be detected. Even a few stray comets would 

 appear quite motionless in the inter-planetary spaces. 

 All this is on the proviso that the observer has no 

 telescope, but is watching things with ordinarv 

 unaided vision. 



The same remarks apply w ith more force to the 

 stellar svstem. Here, there is not so much need to 

 move from the earthly point of view, as the Earth is 

 surrounded by the stellar universe in all directions. 

 And what is observed ? The configurations of the 

 stars remain practically the same from age to age. 

 For thousands of years the old historic constellations 

 have presented much the same shaped groupings : 

 yet it is certain that every star is in motion — all 

 have their proper motions — moreover, there are 

 streams and classes of motions which individually, 

 and taken absolutely, are of immense magnitude. 

 The real motions of individual stars are known to 

 range from ten miles up to forty or fifty miles per 

 second, and in one case two hundred miles per 

 second. All the while their apparent angular 

 motions are extremely small. The principle of 

 relativity must be borne in mind, and then it is seen 

 that if the movements of the stars are compared 

 with their enormous distances apart, the former are 

 really quite slow and gradual. 



Absolute terrestrial movements may, of course, be 

 compared arithmetically with celestial ones, but to 

 reall}' grasp the enormous disparity- between them is 

 another matter, seeing we are dealing with speeds 

 be\ond our own experience. However, an effort 

 may be made to discuss the case of the Earth's 

 orbital motion, compared with that of an express 

 train, as best representative of each class. Such 

 motions as those of a rifle bullet or projectiles dis- 

 charged by heavy ordnance, as well as molecular 

 motions connected with light, electricity, and so on, 

 are purposely left out of account. E\-ervone knows 

 the terrific rush and momentum of a train travelling 

 at sixty miles an hour, as seen from the platform of 

 some country railway station, about twentv feet from 

 the rails. A cloud of steam is first noticed and a 

 distant roar heard, which rapidly increases in inten- 

 sity. In the course of a few seconds, with a sense 



of momentum which thrills the spectator, the train, 

 weighing himdreds of tons, dashes by, a w ind follows, 

 tilling up the \acuum thus caused, and all is over. 



Suppose, now, a position is taken up. on a clear 

 day, on some elevated ground, such as the southern 

 spurs of Dartmoor, or the South Downs in Sussex, 

 some miles distant from the railway. From such 

 a point the same express ma\' be watched with a 

 field glass, wending its way along the valleys and 

 over the viaducts of the lower district. How the 

 apparent motion is now reduced ! There is the long 

 trail of steam, but by comparison the train seems to 

 crawl along in the distance. It is brought home to 

 one that the sense of motion of a body depends on 

 proximity to it. 



This idea is well expressed in the following lines 

 from a poem on the South Downs by Mr. R. Bridges, 

 the reference being to steamships in motion, seen a 

 long distance away. 



I climb your crown, and lo ! a sight surprising 

 Of sea in front uprising, steep and wide : 



.■\nd scattered ships ascending 



To heaven, lost in the blending 

 Of distant blues, where water and sky divide. 

 Urging their engines against wind and tide. 



And all so small and slow, 

 They seem to be wearily pointing the way they would go. 



So much for an exam[)le of terrestrial speeds. 



Is it now possible to place the ether-man so that 

 he can satisfactorily observe the Earth which moves 

 one thousand times as fast as the express ? To 

 begin with, the moving body in this case has a 

 diameter of some eight thousand odd miles, and he 

 must be situated, at the very least, four thousand 

 miles from the centre line of the Earth's track, if onlv 

 to avoid collision, if such can be iinagined between 

 an atom of a being and such a vast globe. To get 

 anything approaching a reasonable view of the 

 motion, proportionate, in fact, to the conditions of 

 position at a railway station, he must be placed 

 about twelve thousand miles from the centre of the 

 track. But seen from this distance the Earth's 

 motion would seem comparatively slow : for, as Sir 

 K. Ball says, it would take eight minutes to pass 

 over its own diameter, which from the selected point 

 of view would subtend an angle of about thirty-nine 

 degrees. This corresponds to an angular rate of 

 motion, when near the spectator, of about 5' a 

 second. This rate, which is certainly above our 

 limit, is yet not fast, corresponding to a body at one 

 hundred vards distance moving across the field of 

 view at a speed of about one-third of a mile per hour. 

 .At the first glance the Earth would seem to be 

 moving very slowly, in fact more or less " poised in 

 space." Morally speaking, it is pretty certain that 

 no human lieing could see such a sight — and live. 



So far, motions of revolution only have been 

 considered, but motions of rotation of the planets, 

 although much more rapid, are still below our limit. 

 .\ person on the Earth's equator, we know, is whirled 

 round, in space, at an absolute speed of over one 

 thousand miles per hour. Yet on account of the 

 smoothness of the motion, and every surrounding 



