Page:The Moon (Pickering).djvu/137

 CHAPTER XIII

of the maps of the Moon, prepared by hand, that have hitherto been published, contain much detail, but all of them show large errors in position, shape and size, even of the important formations, while the minor features are often very inadequately and incorrectly represented. This is only what would naturally be expected in maps based so largely on drawings, and can only be avoided with certainty by having recourse to photography. Even the trigonometrical positions given at the end of Nelson's work on the Moon are found to be largely discordant when compared with the more recent photographic measurements. The positions lately published by Franz (Stemwarte Breslau, L), based on five negatives of the full moon taken at the Lick Observatory, have been, adopted as standards in the location of the paralels and meridians on the present map.

Of the 479 craters common to Nelson's map and the one given at the end of this volume, more than forty per cent, of the positions were found to differ from one another by more than one degree. Many of these deviations occur near the limb, especially in high latitudes, but still a very large number of them were found near the centre of the disk. In none of these cases did the deviation exceed two degrees, although in high latitudes deviations of ten and even twenty degrees sometimes occurred. Owing to the method of construction here employed, based on photography, it is believed that these large discordances are due to errors in Nelson's map, rather than in the present one.

The first step in the construction of this map consisted in the selection of a suitable negative of the full moon. The one finally chosen was taken at Jamaica on the night of August 29, 1901, at 16$h$ 50$m$ G. M. T. The image measured 15.7 inches or 399 millimetres in diameter. This was enlarged on bromide paper to a diameter of 705 millimetres. The craters, parallels and meridians were then inserted. When completed the map was reduced by photography to an approximate diameter of 13.7 inches, or 348 millimetres, which is just one ten-millionth of the diameter of the Moon. This 95