Page:Mars - Lowell.djvu/49

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which makes the polar diameter measure more than it otherwise would. For the polar cap is the most brilliant part of the disk; and for the same reason that any bright body seems larger than a dark one of the same size, it dilates the planet unduly in that direction. The resulting effect is further complicated by the fact that the polar cap is eccentrically situated with regard to the pole of rotation, as we shall see later; and as the pole is tilted, the cap is sometimes on the edge of the disk and the irradiation in consequence large, and sometimes well on the disk ite1f where its irradiation is little or nothing. As the amount of its magnifying effect is not accurately known, there enters with it an unknown error. Now, last autumn Nature herself kindly eliminated this source of error.

The measures made by Mr. Douglass are thus entitled to special regard, not only because of their number (a great many of them were taken), but chiefly because Nature made the disturbing influence of the polar cap nil. When, in addition, the twilight arc is allowed for, the measures show a most satisfactory accordance and give for the value of the polar flattening 1/190 of the equatorial diameter.

Now, it is interesting that this value should receive corroborative support from two quite different directions. The first of these is that 1/190 is just about the flattening which would re-