Since the surface of the moon is unscreened by air and water vapor to absorb the incoming rays from the sun, and the outgoing radiations from the surface, the extremes of temperature between day and night are very great, and are augmented by the fact that the lunar day equals the lunar month in length, so that fourteen days of untempered heat are followed by fourteen days of frigid darkness. Observations of the rate of radiation from the moon's surface during total eclipses of the moon indicate that the moon's radiation is very rapid, and that its temperature during the height of the lunar day probably approaches 200° F., while at the lunar midnight it may have fallen to 100° below zero F., or even lower.