Equilibrium, in general, is a state in which opposing forces are balanced, and hence a system does not change in time.
[2][3] Prevost in 1791 offered the following definitions (translated): Absolute equilibrium of free heat is the state of this fluid in a portion of space which receives as much of it as it lets escape.
Prevost went on to comment that "The heat of several portions of space at the same temperature, and next to one another, is at the same time in the two species of equilibrium."
D. Mihalas and B. Weibel-Mihalas (1984)[5] emphasise that this definition applies to a static medium, in which the matter is not moving.
Subrahmanyan Chandrasekhar (1950, page 290)[9] writes of a model of a stellar atmosphere in which "there are no mechanisms, other than radiation, for transporting heat within the atmosphere ... [and] there are no sources of heat in the surrounding" This is hardly different from Schwarzschild's 1906 approximate concept, but is more precisely stated.
[12][13][14][15] Global radiative equilibrium can be defined for an entire passive celestial system that does not supply its own energy, such as a planet.
Prevost[1] would say then that the Earth's surface and its atmosphere regarded as a whole were in absolute radiative equilibrium.
[21] A radiative equilibrium temperature is calculated for the case that the supply of energy from within the planet (for example, from chemical or nuclear sources) is negligibly small; this assumption is reasonable for Earth, but fails, for example, for calculating the temperature of Jupiter, for which internal energy sources are larger than the incident solar radiation,[22] and hence the actual temperature is higher than the theoretical radiative equilibrium.
[23] When there is enough matter in a region to allow molecular collisions to occur very much more often than absorption or emission of photons, for radiation one speaks of local thermodynamic equilibrium (LTE).
In this case, Kirchhoff's law of equality of radiative absorptivity and emissivity holds.