$a$ is the point on the line such that $Oa$ is perpendicular to the line, and A is its pair. For any point p on the line, it's pair $P$ forms a trianle $OAP$ which is similar to triangle $Oap$, since $$\frac{|OP|}{|OA|} = \frac{1/|Op|}{1/|Oa|} = \frac{|Oa|}{|Op|}$$. Therefore $\angle OPA$ is a right angle, and as p moves along the line, P traces out a cirlce with diameter $OA$.