The Circle Line is no longer a true circle anymore!
Trains start at the Hammersmith terminal (well off the circular route), run Edgware Road and then continue round the circle in the CW direction back Edgware Road. They then terminate there, reverse and go back round the circle in the CCW direction to Hammersmith via Edgware Road. [This is shown on the current London tube map.]
The reason for this is that it hard to regulate the train service (maintain an even interval) on a continuous circular route when that route is shared with other services.
So circle lines are not a good idea!