Page:Mars - Lowell.djvu/59

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37
ATMOSPHERE

a visible twilight unconsciously measured for a part of the planet's surface. Like the Downeaster who shingled fifty feet on to the fog, Mr. Douglass had measured several miles into the Martian air.

A word or two will explain this. The planet came to opposition on October 20. The mid-measures of the series, therefore, were taken within a few days of opposition, just before and just after that event. The subsequent ones, on the other hand, were made at a gradually increasing distance from this position, as the planet passed toward quadrature. Now, at opposition, the disk of the planet is full, like the full Moon; while, as it passes to quadrature, it loses something of itself, becoming gibbous, as the Moon does two or three days after the full. This loss from phase chiefly affects the equatorial diameter, the polar one remaining substantially unchanged by it. It would remain absolutely unchanged if the planet moved in the plane of the ecliptic. It does not so move, but the quantity resulting from lack of accordance is so small that for the present explanation it may be neglected. Now, this question of phase was the only point, practically, in which the equatorial and polar diameters differed during the interval under consideration. This, then, was the clue to the discrepancy.

It was not, however, the loss of phase that