The Celestial Poles and Equator
Projecting the Earth's axis and equator outward onto the sky, giving the celestial sphere an axis it turns about and a middle to measure from.
Projecting the Earth's axis and equator outward onto the sky, giving the celestial sphere an axis it turns about and a middle to measure from.
The celestial equator and the ecliptic set side by side — one built on the Earth's rotation, one on its orbit, and both already met.