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Rh must have lasted till eleven o’clock in the day. Furthermore, it must have been mist of a singularly mathematical turn of mind, for it stretched from one pole to the other, quite oblivious of the fact that every hour from sunrise to sunset lay represented along the limb, including high noon. What is more, as the disk passed, in course of time, from the gibbous form to the full, and then to the gibbous form on the other side, the limb-light obligingly clung to the limb, regardless of everything except its geometric curve. But as it did so, the eleven o’clock meridian swung across it from one side of the disk to the other. As it passed the centre the regions there showed perfectly clear; not a trace of obscuration visible as it lay beneath the observer’s eye.

From the first observation it is evident that Martian sunrise and sunset had nothing to do with the phenomenon, since it was not either Martian sunrise or sunset at the spot where it was seen; and, from both observations taken together, it is evident that the phenomenon did have to do with the position of the observer. For nothing on Mars had changed in the mean time) but only the point of view of the observer on Earth. It is clear, therefore, that it was not a case of Martian diurnal meteorological change, but a case of foreshortening of some sort.

To what, then, was the limb-light due? At