  A principle on the macroscopic motion of systems in thermodynamic
equilibrium, rarely discussed in texts, is reviewed: Very small but still
macroscopic parts of a fully isolated system in thermal equilibrium move as if
points of a rigid body, macroscopic energy being dissipated to increase
internal energy, and increase entropy along. It appears particularly important
in Space physics, when dissipation involves long-range fields at
Electromagnetism and Gravitation, rather than short-range contact forces. It is
shown how new physics, Special Relativity as regards Electromagnetism, first
Newtonian theory then General Relativity as regards Gravitation, determine
different dissipative processes involved in the approach to that equilibrium.
