328 



CATALOGUE. — ASTROXOiAIY, SYSTEMS OF STARS. 



When a spherical surface has been projected on a plane, 

 it has been usual to consider it as viewed from a particular 

 point, either infinitely remote, as in the orthographical 

 projection, or situated in the opposite surface of the sphere, 

 as in the sterecgraphical. The latter method produces 

 the least distortion, and is the most commonly used, but 

 even here, at the extremities of the hemisphere, the scale is 

 twice as great as in the middle. Sometimes, another prin- 

 ciple is employed, and the hemisphere is divided into seg- 

 ments, by omitting portions in the directions of their radii' 

 as if the paper were intended to be fixed on a globe ; and in 

 the same form as if a spherical surface were cut in the di- 

 rection of its meridians, and spread on a plane. If the 

 number of these divisions be increased without limit, the 

 result will be the projection, which is employed in the cir- 

 cular part of this diagram, and in the same manner the zone 

 on each side the equinoctial, being cut open by innumera- 

 ble divisions, so as to be spread on a plane, will coincide 

 with the two remaining portions. By these means the 

 distortion becomes inconsiderable. In the common stereo- 

 graphical projection indeed, the distortion would be of no 

 consequence, if it represented always those stars only, 

 which are at once above the horizon of a given place, for 

 we actually imagine the stars in the zenith to be much nearer 

 together, than when they are nearer the horizon, and the 

 picture would appear to agree very well with the original : 

 but their positions being continually changing, -the inconve- 

 nience remains. 



It is not. however necessary, in projections of the stars, to 

 refer them in any instance to a spherical surface. Among 

 Doppelmayer's charts, published at Nuremberg, there are 

 six, which represent the sides of a cube, on which the va- 

 rious parts of the constellations are represented : the eye 

 being probably supposed to be situated in the centre. 

 Funck and others have represented the stars as projected 

 on the inside of two flat cones. But the most convenient 

 representation of this kind, and which would approach very 

 near to the projection here employed, would be to con- 

 sider the eye as placed 4n the centre of a hollow cylinder, 

 so proportioned that all the circumpolar stars should be re- 

 presented on one of its flat ends, and all those which rise 

 and set on its concave surface ; or if it were desired to have 

 a division without referring to any particular latitude, the 

 circular part might extend to the limits of the zodiac, and 

 the parallelogram, into which the cylinder unfolds, might 

 comprehend all the stars to which the planets approach. 

 The horizon, and other great circles, would form lines of 

 :<;'arious and contrary curvatures. 



Systems of Stars, Nebulae, and 

 Double Stars. 



BuUialdi monita duo. 



Ace. Ph. tr. 1665—6. 1. 387. 



On the nebulosa Andromedae. 



Account of nebulae lately observed. Ph. tr, 



1716. XXIX. 390. 

 Halley. Ph. tr. 1720. XXXI. '22. 



Says, that it would be hard to place 13 points on a sphere 

 at the distance of the radius. 



Kaestner shows, that it would be impossible. Dissertat. 

 Math. 



Derham on nebulous stars. Ph. tr. 1733. 



XXXVIII. 70. 

 Wright's theory of the universe. 

 Kants Allgemeine naturgeschichte. 



Lambert Photometria. §. 1139. 1140. 



Thinks the milky way as it were the ecliptic of the fixed 

 stars. That the greater stars belong to the solar nebula, 

 the other nebulae being confused together in the milky way. 



Figure of the nebula in Orion, by Messier. 



A. P. 1771. 458. 

 A figure of the nebula in Orion, supposed to 



be changed. Roz. XXII.34. 

 I'igotton a nebula, and on double stars. Ph. 



tr. 1781. 82, 84. 

 Herschel's catalogue of double stars. Ph. tr. 



1782. 112. 

 Herschel on the construction of the heavens. 



Ph. tr. 1784. 437. 1785. 213. 1802. 477. 



Nich. 8. V. 75. Magnified figures of ne- 

 bulae. 1784. 1785. 



Conjectures, that the milky way is the projection of our 

 nebula, and that the sun has a motion towards its node, 

 near Cepheijs, and Cassiopeia, 1784. In acircle of is' dia- 

 meter 588 stars were counted ; if these wen; at equal dis- 

 tances in a cone, the length of the cone must have beea 

 497 times their distance. From calculations of this kind a 

 figure of the nebula is drawn, showing a section passing 

 through its poles at right angles to the line of the nodes. 

 The right ascension of the pole is 166°, its polar distance 

 i8°; 1785. 



