Monadic second order logic over countable linear orderings

We will look at words over countable linear orderings (for example, omega words, words over rationals etc). We will then look at languages over such words definable by MSO[<]. Is satisfiability of MSO decidable? Is WMSO (weak MSO) as powerful as MSO? All these questions can be answered by algebra. Finally we will take a look at circle algebra, some known results, and many unknown problems.