Parallel courses

Episode 17 (usually known by the Homeric name Ithaca) takes the form of a catechism, the kind of Q & A dialogue by which Catholic schoolchildren are indoctrinated in the faith. (Joyce had to memorize two of them before his 10th birthday.) But rather than using theology to explain the ways of God, this catechism uses science to explain the ways of the material world. Joyce once called it "a mathematical catechism." Scientific inquiry is certainly not evoked by all of its hyper-rationalistic questions and answers, but the first pair sets a distinctive tone by using geometry to describe the "parallel courses" which Stephen and Bloom take to Bloom's house.

John Hunt 2014

Question #21 from the Westminster Shorter Catechism. Source:

Two parallel courses. Source:

A green chord "subtending" a red arc. Source:

Diagonal crossings allow pedestrians to navigate Oxford Circus "diametrically." Source: