The Bright Earth

How bright is the earth as seen from space? In the Journal of the British Interplanetary Society, Meteorologist Alan E. Slater decides that it is quite a bright planet, although rather blue.

The albedo (brightness) of the earth depends on the amount of sunlight that it reflects, and this figure cannot be determined accurately by looking up from the surface. Even rockets shot above the atmosphere get too limited a view, and the dry southwestern part of the U.S., where most rockets have been fired, is certainly not typical.

The best way to estimate the earth's overall albedo is to measure the brightness of "earthlight" on sunless parts of the moon, allowing for the earth's size and distance, and how much of its disk is in sunlight. Figured this way, the earth's albedo is about .40, which means that it reflects four-tenths of the sunlight that falls on it. It is dimmer than cloud-covered Venus (.64) but much brighter than the naked, rocky moon (.073). The brightness varies a good deal with the season, probably because of changes in the amount of cloud-cover. The oceans and surface reflect less light than clouds.

The color of the earth as seen from space can be judged in the same way. Earthlight is definitely blue, apparently because of blue light reflected by fine particles in the atmosphere. So the earth looks blue from above for the same reason that its sky looks blue from below.

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