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 observer'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 hemisphere, 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 510 from the equator northwards, their zenith is 5110 elevated above the celestial equator. As 510 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 3810, requiring 510 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

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DESCRIPTION AND USE OF THE SIDEREAL MAPS.

the graduated meridian line at 41°, is the circle of perpetual apparition, the stars within that circle being visible at all times from the meridian of Greenwich.

The maps may, by a little calculation, be made to represent the aspect of the heavens at other times than is named; for instance, reckoning backwards, and allowing a difference of about twenty minutes for every five days, or four minutes for every twenty-four hours, we find that on the 21st of January at 40 minutes past 1 o'clock in the morning, the stars will appear as they are represented in the map for March, and so on for every other month, following the same reckoning for every day in the year.

TABLE SHOWING THE HOUR AND DAY WHEN THE STARS OCCUPY THE POSITION INDICATED IN THE MAP.