It always sounded strangely in my ears, like the word gnomon in the Euclid and the word simony in the Catechism - James Joyce, Dubliners.
Pronounced no mon, this word comes from the same word in Ancient Greek (γνωμων) meaning 'interpreter', 'the one who knows' or 'the one that shows the way'.
A gnomon is a part of a sundial. It is the central spike which casts the shadow, the bit that looks like an upside-down letter y in the picture. In early sundials, the gnomon was vertical, that is, perpendicular to the ground. Hence the word 'gnomonwise' means at right angles. In more recent sundials, the shadow-casting edge is normally parallel to the Earth's axis and points to the celestial pole, the fixed point in the sky around which all the heavens revolve. In a sense, the gnomon always points the same direction while the world turns around it.