SCIENCE-GOSSIP. 



SOME LITTLE WORLDS. 

 By Sik Robert Stawell Ball, C.B., LL.D., F.R.S. 



T ET us visit the observatory of an astronomer 

 who is about to make a discovery of one of 

 the little worlds, of which we know there are so 

 many. You will find the watcher of the stars 

 seated at a telescope in a building covered by a 

 dome. There is a shutter which can be drawn aside 

 to render the skies visible, and the dome can be 

 made to revolve so as to keep the opening .directed 

 towards the part of the heavens which is under 

 observation. Of course, you know that the stars 

 are constantly moving, or rather seeming to move, 

 from east to west, just as the sun goes through 

 a daily journey from sunrise to sunset. The 

 telescope must, therefore, be carried round so 

 as to insure that it shall remain directed to 

 the same spot. This must be done regularly and 

 smoothly if reliable observations are to be made. 

 He cannot trust to the hand for performing the 

 task with the necessary delicacy ; a clockwork 

 mechanism driven by a heavy weight is therefore 

 applied to every such instrument. It steadily urges 

 the telescope forward to follow the stars, and by 

 careful adaptation this can be done so perfectly 

 that the stars seem fixed in the field of view. The 

 astronomer is thus almost able to forget their 

 movements, and is only reminded that the instru- 

 ment is in motion by the fact that he has occasion- 

 ally to adjust the position of the chair on which he 

 is sitting and to give the dome a pull round. 



Thus the astronomer gazes through his tube, 

 and what he generally sees is a number of stars of 

 varying degrees of brightness. Beside him is a 

 map or chart of the heavens on which the positions 

 of stars have been drawn. This chart has been pre- 

 pared either by himself or by some other astronomer 

 as the result of a study of this portion of the sky on 

 some previous occasion. All the stars that a good 

 telescope can show, even down to the minutest 

 twinkling points, are marked on the map in their 

 true positions. The preparation of such a chart 

 with the requisite accuracy and fullness is a very 

 laborious piece of work, but it is an indispensable 

 preliminary to the discovery of the little worlds of 

 which we are to write in this paper. I may mention 

 the case of one distinguished Professor of Astronomy 

 who had drawn some maps of a particular part of 

 the sky with exquisite care and used them with 

 such skill in discovering the little worlds, that 

 other astronomers gracefully withdrew their tele- 



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scopes from the search in this particular part and 

 relinquished the region, which became known as 

 the " Professor's Preserve." 



Let us suppose that the evening on which we 

 have made our visit to the Observatory happens to 

 be one of those red-letter days in the Astronomer's 

 Calendar when he is to be so fortunate as to add 

 another world to those already known. He is 

 deligently comparing the stars that he sees in the 

 heavens with the stars that are marked on the 

 chart ; point by point he verifies the correctness of 

 the former work upon the map. As a general rule, 

 each star in the heavens appears duly in its place 

 on the map. Each star depicted on the map is 

 observed to have its counterpart in the sky. For 

 hours, for the whole night, for many nights, the 

 patient observer has been carrying on this 

 work. He has, in fact, been searching to find 

 some discrepancy between the sky stars as 

 they appear in the sky and the stars as they 

 appear on his map. Perhaps he will make the 

 comparison scores of times without any noteworthy 

 result. At last the supreme moment has arrived ; 

 among the stars on the heavens he finds one which 

 is not depicted on his map. Instantly his hopes 

 are aroused. It seems highly probable that the 

 star, or rather the object which looks like a star, 

 cannot have been in that part of the heavens at 

 the time the map was drawn. It would indeed 

 have been possible, but extremely unlikely, that it 

 was a star which had been overlooked by the con- 

 structor of the map. We do not suppose that a 

 new star has been called into being. No doubt 

 it has sometimes happened that the astronomer has 

 chanced upon a variable star ; it was faint and 

 invisible when the map was made and has waxed 

 bright in the interval between the drawing of the 

 map and the comparison with the heavens which 

 we are now supposing to be made. There is, how- 

 ever, another supposition which is what has actually 

 happened in the present instance ; it is that the 

 celestial body has wandered into its present position 

 since the map was made. In fact, what has been 

 found is a moving object and is thus of a funda- 

 mentally different character from the stars properly 

 so-called. We thus see that this body, though so 

 starlike in its aspect, is in truth not a star at all : it 

 is a veritable planet, that is to say, one of those 

 little worlds which revolve around the sun. The 

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