Constructing a hyperbolic circle in the Poincare disk

Back to Geometry homepage


Recall that a hyperbolic circle with hyperbolic centre P is the set of all points in the Poincare disk which have the same hyperbolic distance from P. This looks like a regular Euclidean circle, but the Euclidean centre is not the same as the hyperbolic centre.
A useful property of the circle centred at P is that it is orthogonal to every hyperbolic geodesic passing through P.
The picture below contains an example of a hyperbolic circle with centre P and a point Q on the circle. The centre of the Poincare disk is labelled O. Can you construct a hyperbolic circle with centre A and passing through B?

To help you get a feel for what the solution should be, there is already a tool for constructing hyperbolic circles. Try and construct them yourself, using only the other given tools.
Hint. You can construct two hyperbolic lines (geodesics) through A. How to construct a circle which is orthogonal to both of these hyperbolic lines?