Page:Is Mars habitable - Wallace 1907.djvu/83

64 temperature of the moon by Mr. Very in his Prize Essay on the Distribution of the Moon's Heat (published by the Utrecht Society of Arts and Sciences in 1891). He shows, by a diagram of the 'Phase-curve,' that at the commencement of the Lunar day the surface just within the illuminated limb has acquired about 1/7 of its maximum temperature, or about 70°F. abs. As the surface exposed to the Bolometer at each observation is about 1/30 of the moon's surface, and in order to ensure accuracy the instrument has to be directed to a spot lying wholly within the edge of the moon, it is evident that the surface measured has already been for several hours exposed to oblique sunshine. The curve of temperature then rises gradually and afterwards more rapidly, till it attains its maximum (of about +30 to 40°F.) a few hours before noon. This, Mr. Very thinks, is due to the fact that the half of the moon's face first illuminated for us has, on the average, a darker surface than that of the afternoon, or second quarter, during which the curve descends not quite so rapidly, the temperature near sunset being only a little higher than that near sunrise. This rapid fall while exposed to oblique sunshine is quite in harmony with the rapid loss of heat during the few hours of darkness during an eclipse, both showing the prepotency of radiation over insolation on the moon.

Two other diagrams show the distribution of heat at the time of full-moon, one half of the curve