Page:Mars - Lowell.djvu/68

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46
MARS

part of the central portions of the planet, from any Martian cause, has been detected by any one of three observers. Certain peculiar brightish patches have from time to time been noted, but, with a courtesy uncommon in clouds, they have carefully refrained from obscuring in the slightest degree any feature the observer might be engaged in looking at.

The only certain dimming of detail upon the Martian disk has been along its bright semicircular edge or edges, as the case may be,—what is technically called its limb. Fringing this is a permanent lime of light that swamps all except the very darkest markings in its glare. This limb-light has commonly been taken as evidence of sunrise or sunset mists on Mars. But observations at Flagstaff during last June show that such cannot be the case. In June Mars was gibbous,—that is, he showed a face like the Moon between the quarter and the full,—and along his limb, then upon his own western side, lay the bright limb-light, stretching inward about thirty degrees. Since the face turned toward us was only in part illumined by the Sun, the centre of it did not stand at noon, but some hours later, and the middle of the limb consequently not at sunrise, but at about nine o’clock of a Martian morning. As the limb-light extended in from this thirty degrees, or two hours in time, the mist, if mist it was,