If Bohr’s quantisation postulate (angular momentum = `nh/2pi)` is a basic law of nature, it should be equally valid for the case of planetary motion also. Why then do we never speak of quantisation of orbits of planets around the sun?

Explanation

Angular momenta associated with planetary motion are incomparably large relative to h. For example, angular momentum of the earth in its orbital motion is of the order of `10^(70)`h. In terms of the Bohr’s quantisation postulate, this corresponds to a very large value of n (of the order of `10^(70)`). For such large values of n, the differences in the successive energies and angular momenta of the quantised levels of the Bohr model are so small compared to the energies and angular momenta respectively for the levels that one can, for all practical purposes, consider the levels continuous.