The Earth is not in thermal equilibrium with the Sun due to several reasons:
Temperature difference: The Sun is much hotter than the Earth. The surface temperature of the Sun is around 5,500 degrees Celsius, while the average surface temperature of the Earth is around 15 degrees Celsius. This significant temperature difference means that energy flows from the Sun to the Earth, rather than the two bodies being in thermal equilibrium.
Energy transfer: The primary mode of energy transfer between the Sun and the Earth is radiation. The Sun emits a vast amount of electromagnetic radiation, including visible light, ultraviolet light, and infrared radiation. This energy is radiated outwards in all directions. Some of this radiation reaches the Earth and warms its surface, but not all of it. The Earth also reflects and scatters some of the incoming radiation back to space. Therefore, there is a net energy transfer from the Sun to the Earth, and thermal equilibrium is not achieved.
Radiative balance: While the Earth absorbs a portion of the Sun's incoming radiation, it also radiates energy back into space. The Earth's surface and atmosphere emit infrared radiation as a result of their temperature. This outgoing thermal radiation is crucial for maintaining a balance. If the Earth were in thermal equilibrium with the Sun, the energy received from the Sun would be exactly equal to the energy radiated back into space. However, due to various factors like the greenhouse effect and atmospheric composition, the Earth's radiative balance is not perfectly equal, leading to a non-equilibrium state.
Overall, the Earth and the Sun are not in thermal equilibrium primarily because of the temperature difference, the net energy transfer from the Sun to the Earth, and the radiative imbalances that occur due to factors like atmospheric composition and greenhouse effect. These factors contribute to the dynamic climate system we observe on Earth.