Question

Why the earth is not in thermal equilibrium with the sun?

Answer 1

The Earth is not in thermal equilibrium with the sun as the temperature of the sun is higher than that of the Earth, and thus the Earth will gain thermal energy from the sun. Since there is a net movement of heat energy, we say the Earth and the Sun is not in thermal equilibrium.

As to why that is the case is not so simple. First, the sun is far away, and radiation is the only way of heat transfer, and since the rays are given out equally in all direction, the energy decays with the square of distance. That's a very rapid decrease in energy. You might imagine that after so many years of Earth existence the Earth should heat up due to the sun. Sadly, the answer is not so simple. The Earth and ice and clouds. These have high albedo, which reflects a good portion of the rays from the sun elsewhere. Plus the Earth radiates heat out as the space around Earth is colder, so this dynamic process keeps the Earth away from Earth reaching the same temperature as the sun.

Answer 2

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