The tilt between the ecliptic and the equator. It causes the seasons, and the signs' unequal rising times.

This page is a seed. The full write-up is coming; for now it exists so the links that point here resolve, and so the shape of the codex is visible while it grows.

what belongs here

  • the plain-language definition
  • how it is actually calculated or read
  • what it is good for, and its known limits
  • traditional, modern, or both