Based on the shared root, the common subject for all the words is CIRCLES.
-CIRCULAR: Something that has a circle form
-CIRCUS: A circus is an artistic show, usually itinerant, that is presented in a large tent that has tracks and seating galleries for the public. Circus tracks are usually circular areas where functions are presented.
-CIRCUMNAVIGATE: Circumnavigate is to travel around a place, be it an island, a continent, or the Earth. The term is usually referred to the navigation around the world.
-CIRCUMFERENCE: A circumference is a flat, circular and closed curve, so that all its points are equal distance from the center.
-CIRCUMSTANCE: A non-essential condition or characteristic that surrounds a person or thing and that influences them or facts related to them.