question_answer 16)

If Bohr's quantisation postulate (angular momentum ) 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:

Applying the Bohr's quantization postulate, we get  For the motion of a planet (say earth), we may consider the data : and   i.e., n is very large. Since n is very large, the difference between the two successive energy or angular momentum levels is very small and the levels may be considered continuous.