Page:Mars - Lowell.djvu/63

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41
ATMOSPHERE

would come out too big in proportion to the amount added by the twilight arc.

Pursuant, therefore, to the supposition that such was the cause of the increase, I took the means of the polar and of the equatorial diameters with regard to the time from opposition, at which the measures were made, to find myself confronted by a series of values counterparting what we have just seen would be given by the presence of a visible twilight arc. The resulting values are:—

Polar Diameters:
October 15 to 23 inc. 9”.35
October 12 and 24 to 30 inc. 9”.35
November 2 to 21 inc. 9”.36
Equatorial Diameters:
October 15 to 23 inc. 9”.40
October 12 and 24 to 30 inc. 9”.43
November 2 to 21 inc. 9”.53

The measure of the 12th of October and those of the 24th to 30th are taken together, because equidistant from opposition on October 20.

The agreement of this table with that deducible by theory from the effect of an atmosphere is striking. But the agreement is even more exact than appears. For, as the polar axis was not in the same line as the axis of phase, the twilight arc to some extent affected the polar diameter at all times, but specially