Question

If Bohr’s quantisation postulate (angular momentum = nh/2π) 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?

Answer 1

according to Bohr's postulate, “ angular momentum of an electron in nth orbit is integral multiple of (h/2π).”

i.e., $mvr=\frac{nh}{2\pi}$ , it 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 $10^{70}\frac{h}{2\pi}$.

for such large value of n, the difference in successive energies and angular momentum of the quantised levels of the Bohr's model so small that one can predict energy level continuous.