That he was at best a crackpot with no knowledge of basic physics, and at worst a deliberate fraud.
Ding! Ding!
You win the prize, a free URL full of pointers to information about just how cracked Velikovsky's pot really was:
http://www.astrosociety.org/educatio...eudobib07.html
Probably the best reference in this list is to the work of a former 'follower' of Velikovsky who finally figured out how ridiculous his ideas were:
http://abob.libs.uga.edu/bobk/velidelu.html