Question 1: Why is the earth not in thermal equilibrium with the sun?
When two bodies are in thermal equilibrium, they are at the same temperature. There must not be a net heat flow between them. As the temperature of the sun is very high (6000 K) than earth for example 300 K, therefore, they are not in thermal equilibrium. So, heat flows from the sun to the earth. Principally, it should increase the temperature of the earth so that they are at the same temperature. However, it does not happen due to the following reasons.
- Earth is spherical in shape. Therefore, at any instant of time, only half of the earth is receiving heat radiations from the sun.
- Part of the heat radiations are absorbed in the atmosphere.
- Earth also continuously radiate heat energy back to the atmosphere which significantly decreases temperature of the earth.
Therefore, earth does not attain temperature as high as sun.