What's the difference between 'ignorant' and 'silly'?

Ignorance is lack of knowledge.

lack of knowledge or information : he acted in ignorance of basic procedures.

Silly is behaving in a foolish manner, or showing poor judgment.

having or showing a lack of common sense or judgment; absurd and foolish : another of his silly jokes | “Don't be silly!” she said

[Both definitions from NOAD]

"Ignorant" is (at least on the surface) factual, while "silly" is purely an expression of opinion. For example, my manager makes decisions I consider silly (and sometimes downright stupid), while he thinks them wise (if not brilliant). But we agree they aren't ignorant decisions: he has all the necessary information before deciding, including my sage advice.