Thermal Equilibrium
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.