User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-2253300-20130130083731

You are assuming that the attacking ships can hit their target at such advanced range. Yes objects move in a straight line in space, and the PDFs are by their nature stationary targets. However at the distance you are talking about the PDF is no longer stationary. The planet's revolution and rotation enter the equation. Barring all of this we are talking about what is essentially an extremely precise strike from said distance. We are talking about vast amounts of space. Mathematically you can calculate where a projectile will hit, but understanding the exact position of the target which is relatively small in relation to the position of the attacker is very near impossible. Besides all of this I don't believe for a second that the civilizations in Mass Effect would not have come up with countermeasures for bombardment from space. Anyway this is Sci-fi, don't think to hard about it.