HOW ro MAKE STEREOSCOPIC STAR CHARTS. 



By A. H. STUART. B.Sr., F.R.A.S. 



A 1 KW years ago, an American firm placed upon 

 tile market a series of star charts arranged as stereo- 

 scopic slides, so that in the stereoscope the\' present 

 the very beautiful appearance of shewing the stars 

 in relief. Very few of these slides seem to have 

 found their way into England. Much instructive 

 pleasure may be derived by preparing these slides 

 for oneself. For this [purpose we must be able to 

 determine the position of the stars as the\' appear to 



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much tile same relative positions as they appear in 

 the lieavens. Now right ascension is, of course, 

 given in hours, w hile declination is given in degrees, 

 and as twenty-four hours are equivalent to three 

 hundred and si.xty degrees, we have one hour of 

 right ascension equivalent to fifteen degrees of 

 declination. This, however, is only true at the 

 equator, the great (hour) circles converging as we 

 approach the poles. It is easy to see that the 

 distance between a pair of these circles at any place 

 compared with the distance measured along the 

 equator is proportional to the cosine of the declina- 

 tion. If the constellation we are plotting is not too 

 large, we may take the cosine of the mean declination 



Ursa Major. 



FlGLkii 245. 



US. and we must also have some know ledge of their 

 relative distance from us. The first of these require- 

 ments is supplied by a knowledge of the star's right 

 ascension and declination, which may be taken from 

 the Xciittical Almanac or similar publication. The 

 relative distance of the stars would be best given by 

 their parallax, but as the vast majority of stars are 

 too remote to shew any parallax, their relative dis- 

 tances have to be estimated by a consideration of 

 their type, magnitude, and 

 so on. 



The method of preparing 

 a slide will be best demon- 

 strated by taking a concrete 

 example. Suppose we take 

 the constellation Ursa Major, 

 using the magnitude only as 

 an indication of distance. 

 Table 20 is prepared by 

 consulting The Nautical 

 Almanac. 



It will be wise, at any rate 

 for the beginner, to make a 

 preliminary diagram similar 

 to that shewn in Figure 245. 

 Trouble is saved if this opera- 

 tion is performed on squared 

 paper. The vertical scale 



is one of declination and the horizontal one shews 

 the right ascension. These two scales must be so 



for our adjustment without introducing any sensible 

 error. Now the mean declination of the stars in 

 Ursa Major is about 50°, and cos. 50"^= -643. 

 Suppose we decide to represent one degree of 

 declination by a distance of two millimetres, then at 

 the equator one hour of R.A. would have to be 

 represented by 2x15 = 30 millimetres. But at the 

 declination of Ursa Major this distance becomes 

 30X-643=:19-3 millimetres. Thus Figure 245 is 



57-31- 



FlGURE 246. 



proportioned that the diagram shews the stars in 



drawn, using a scale of two centimetres to 10° of 



declination, and 1 • 93 centimetres to one hour of R.A. 



Having settled this matter, the ne.xt thing is to 



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