Page:The Moon (Pickering).djvu/133

Rh eight equal parts, and erect perpendiculars at the dividing points. This, as shown in the figure, divides the visible surface into sixteen regions, eight north of the equator and eight south of it. According to the original plan, three views of each region were to be taken, one at lunar sunrise, one at lunar noon and one at lunar sunset. It was soon found, however, for certain regions, notably those in the southern half of the disk, that the change in appearance produced by the difference in lighting rendered it absolutely impossible to identify the same formation upon the plates taken at sunrise and sunset and those taken at noon. To overcome this difficulty, two more plates, designated as morning and evening plates, were added, one taken about two days after sunrise and the other about two days before sunset. We thus obtained eighty plates, covering the entire surface of the Moon five times. Certain small regions where the images overlapped might appear on as many as twenty plates, representing the region under ten different phases of illumination.