Page:Mars - Lowell.djvu/48

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discrepancy, but for the further fact that the discrepancies should usually be on the side of an increase of the apparent polar flattening. This factor is the recognition of a perceptible twilight upon the planet, not only of enough account to be visible, but to have been actually measured, quite unconsciously, by Mr. Douglass, and disclosed only when the measures came to be compared with each other. Of this I shall speak more at length when we reach the subject of atmosphere. Here it is only necessary to say that the presence of a twilight fringing the surface of the planet would have the effect of increasing the apparent size of the equatorial diameter at all times, but to a different degree at different times, and almost always more than it would the polar one. In consequence, the polar flattening, which is the ratio borne by the difference of the equatorial and polar diameters to the equatorial diameter, would be seemingly increased.

The value of Mr. Douglass’ measures is heightened by a certain happy event of an unprecedented nature,—the first observed disappearance of the polar cap, and that at the very time the most important measures were made. The presence of the polar cap enters as a disturbing element into measures of the planet’s disk, on account of the increased irradiation it causes at the extremity of the polar diameter,