thermal equilibrium

• Heat is the flow of energy from a high temperature to a low temperature.
• When these temperatures balance out, heat stops flowing, then the system (or set of systems) is said to be in thermal equilibrium.
• Thermal equilibrium also implies that there's no matter flowing into or out of the system.
• For example, when molten rock comes up from a volcano, it will give off heat to the atmosphere until the rock and the atmosphere are at the same temperature. Even though the two systems (rock and air) are very different, thermal equilibrium allows a definition of temperature for both.
• All systems tend towards thermal equilibrium over time
• It is very important for the Earth to remain in thermal equilibrium in order for its temperature to remain constant. 
• the Earth must radiate just as much as heat as it receives
•  The greenhouse effect slows down the heat transfer from Earth to space, making the planet hotter. The temperature of the Earth relies on the natural greenhouse effect in order to be habitable by humans and other animals and without it, the Earth would be far too cold.