L 4nRf oTq

If the Bond albedo of the body is A, the fraction of the radiation absorbed by the planet is (1 — A). This is later emitted as heat. If the body is at a distance r from the Sun, the absorbed flux is

There are good reasons to assume that the body is in thermal equilibrium, i. e. the emitted and the absorbed fluxes are equal. If not, the body will warm up or cool down until equilibrium is reached.

Let us first assume that the body is rotating slowly. The dark side has had time to cool down, and the thermal radiation is emitted mainly from one hemisphere. The flux emitted is

where T is the temperature of the body and 2nR2 is the area of one hemisphere. In thermal equilibrium, (7.50) and (7.51) are equal:

Rf T4

whence

A body rotating quickly emits an approximately equal flux from all parts of its surface. The emitted flux is then

and the temperature

The theoretical temperatures obtained above are not valid for most of the major planets. The main "culprits" responsible here are the atmosphere and the internal heat. Measured and theoretical temperatures of some major planets are compared in Table 7.3. Venus is an extreme example of the disagreement between theoretical and actual figures. The reason is the greenhouse effect: radiation is allowed to enter, but not to exit. The same effect is at work in the Earth's atmosphere. Without the greenhouse effect, the mean temperature could be well below the freezing point and the whole Earth would be ice-covered.

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