The competing program Cabri géometrie provides similar capabilities.
2.
The illustrations adopt O. Pedersen's notation (A survey of the Almagest (Odense, 1974)) rather than O. Neugebauer's (A history of ancient mathematical astronomy (3 vols, New York, 1975)). They can be changed easily, but the free demonstration version of The geometer's sketchpad does not permit the user to save changes made to a file.
