Secular cooling কী?

The secular cooling of the Earth is its slow loss of the heat stored inside it since it formed, the primordial heat: the part of the heat flow that radioactivity does not replace, by which the mantle and the core grow colder over the ages.

Secular cooling is the part of the Earth’s heat loss supplied by the slow decrease of the temperature of its interior, mantle and core: the release of heat stored since the planet’s formation, at a rate $-\sum_j M_j c_j\,\mathrm dT_j/\mathrm dt$ for reservoirs of mass $M_j$, specific heat $c_j$ and mean temperature $T_j$.