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

According to Bohr.s quantisation,
`mvr=(nh)/(2pi)`
`rArr n=(mvrxx2pi)/h`
For angular momentum associated with planetary motion , say the orbital motion of the Earth, we have
Mass of the Earth , `m=6xx10^24` kg
Orbital velocity , `v=3xx10^4` m/s
Radius of orbit, `r=1.5xx10^11` m
`h=6.63xx10^(-34)` Js
`therefore n=(6xx10^24xx3xx10^4xx1.5xx10^11xx2xx3.14)/(6.63xx10^(-34))`
`=2.55xx10^74`
The differences in the successive energies and angular momenta large values of n will be insignificant compared to the energies and angular momenta of the different levels. Therefore, the energy levels can be considered continuous, for all practical purposes.