Perhaps the most extreme version of the persistent astronomy crackpot theory is Velikovsky's whole "Worlds in Collision" business. The basic idea behind this is that in Earth's recent past— that is, within the last several thousand years or so— there have been global catastrophes caused by objects in the Solar System moving around. Among other things, Velikovsky's ideas suggest that Venus was originally a comet (...yes...), that was ejected from Jupiter (...yes...), and migrated around the Solar System, having close passes with Earth and Mars, thereby causing the catastrophes that Earth supposedly had in the past. His most famous (infamous?) book is Worlds in Collision.
The various movements around the Solar System that Velikovsky needs, on the timescales he needs them, require massive violation of things like conservation of angular momentum, if the orbits are purely gravitational. His response to that was, well, electric forces must be responsible for planetary orbits! I've already pointed out how utterly unphysical and batty the whole "electric universe" thing is in my previous post. The Velikovsky business makes no sense, because the physical model of the solar system it needs to work makes no sense.
There's another basic reason why we can be pretty sure that Velikovsky is wrong. That is, there's absolutely no reason whatsoever to think that he might be right! His methodology for coming up with his model for the Solar System had absolutely nothing to do with science or scientific considerations. Rather, what he did was look at the myths of ancient cultures, and assumed that they were true— that is, when the myth said that there was a global catastrophe, there in fact was a global catastrophe. I would point out that this methodology, looking at ancient myths to determine natural history, is exactly the mythology used to come up with Young-Earth Creationism. As such, from a scientific point of view, there's absolutely no reason to pay any more attention to Velikovsky than there is to young-earth creationists. Trying to make a scientific refutation of it is like trying to explain the color blue in terms of musical theory.
So: Velikovsky's whole idea was based on non-scientific considerations, and as such isn't even worthy of debate on a scientific forum. Trying to shoehorn the physics to make it work requires resorting to the "electric universe" stuff that is crazy. And, as if that weren't enough, there's no evidence in the geologic record of the global catastrophes that Velikovsky was trying to "explain".
(So where do these ancient myths come from? If you've read the news for the last ten years, it's not very hard to imagine. Think of the people who lived through the east Asian tsunami in 2004— it's not very difficult to imagine somebody, especially somebody without the benefit of a world-wide media, believing that a global catastrophe had occurred after living through that! Flood myths are almost certainly so common in human cultures not because there ever was a global flood, but because there are floods, all the time, and sometimes they're really bad.)
Sadly, despite the fact that Velikovsky's ideas were ill-founded and have little or no connection to actual physics and astronomy, there remains a small fringe that think that the astronomy community is doing him a disservice by not taking him seriously. It's just like the electric universe business. Those of us in the "mainstream" are either deliberately hiding the truth, or are blinded by the dogma, and don't want to allow "outside" ideas to undermine the ideas that are the basis of our careers. Or, so they say. And, so, you can find books and websites out there saying that Velikovsky never got a fair hearing.
The truth is that Velikovsky has gotten way more attention and hearing than he ever deserved, at least as far as natural history is concerned. The fact that he wrote his stuff more than half a century ago, and I still feel some motivation to mention how wrong it is on this blog, indicates that his ideas have somehow garnered far more staying power than the ideas themselves would warrant.