DESCRIPTION AND USE OF THE SIDEKEAL MAPS, 



THESE Maps have been constructed to show the sidereal hemisphere visible on the parallel of Greenwich, 

 and being also adapted to the meridian of Greenwich, they are drawn on the plane of that horizon. To 

 insure the greatest amount of accuracy, the stereographic projection has been made use of, because of all 

 projections that occasions the least possible disarrangement of the relative position of the stars and of the 

 angles they form one with another. 



There is a difficulty in reducing a concave surface to a plane without distortion taking place somewhere, 

 and in the projection here adopted a little compression will be found, gradually increasing from the horizon 

 to the centre of the map. The constellations when at or near the zenith, will be found to be somewhat 

 smaller than when at the extremities of the projection or near the horizon. Three, four, or five stars may 

 appear in the heavens so as to form a group, and present to the observer the appearance of a triangle, a 

 rhomboid, trapezium, or parallelogram, which figures are more correctly preserved by this projection than 

 by any other which might have been made use of. 



The difference between celestial and terrestrial maps should not be lost sight of. When a comparative 

 observation is made between one of these maps and the heavens at any of the times given on the next page, 

 the map should be held up in a vertical position, placing that part of the map downwards towards which 

 the observer is directing his attention. For instance, if the stars in the south are to be examined, the 

 person's face must be turned that way, with the south or bottom of the map downwards ; if for the north, 

 the map must be reversed, with the north or top of the map downwards, when a complete view of the 

 heavens in either of those directions will be obtained. If for the east and west, the sides of the map are to 

 be similarly held, corresponding with the aspect required. 



The centre of each map represents the zenith, that part of the heavens which is exactly over the obser 

 ver's head, and will answer equally well for any other place upon the same parallel of latitude, making 

 the allowance of four minutes for each degree, east or west, sooner or later ; which shows that all persons 

 living on the same parallel of latitude have in succession the same view of the starry concave. Another 

 appearance would be presented if the observer were at either of the poles. Supposing there were inhabitants 

 at the North Pole, to them one half of the firmament would never set, and the other half would never rise. 

 The polar star would be their zenith, and appear quite stationary, with all the other stars in view revolving 

 round it in circles. To such inhabitants the equator would be the horizon, and at whatever elevation a star 

 was first seen in their winter, there it would remain, and appear to complete a circle at that elevation once 

 in every twenty-four hours. 



If there were inhabitants at the South Pole, they would be similarly situated with regard to stars in the 

 southern hemisphere ; they would never see the stars on the north side of the equator or northern hemi 

 sphere, nor would those in the southern hemisphere ever set to them. 



To the inhabitants of the equator, the whole of the stars from pole to pole, rise and set perpendicularly 

 to their horizon once in every twenty-four hours. As the equator has no parallel of latitude, so has its 

 zenith no declination, because the celestial equator passes immediately over it in a line from east to west. 

 If an observer moves towards either pole from the equator; for every degree of his progress his zenith will 

 have just so many degrees of declination, and as many degrees can he see beyond the pole towards which 

 he is advancing, and he will lose sight of the pole from which he is receding in the same proportion. For 

 example, as the inhabitants of London are situated 5U from the equator northwards, their zenith is 51^ 

 elevated above the celestial equator. As 51 i is the distance from the zenith to the equator, it follows that 

 38^ must be seen by an inhabitant of London below the equator to make up the complement of the 

 quadrant, or 90. Between the zenith and the pole will be found 38^, requiring 51^ beyond the pole to 

 complete the other quadrant of 90, thus together completing the hemisphere of 180. 



With these preliminary explanations a few words will explain the use of the maps. 



Each map may be supposed to represent the heavens at the hours named. The dotted circle crossing 



