In astronomy the "mean sun" is a fictitious sun which moves uniformly in the celestial equator and has its right ascension always equal to the sun's mean longitude.
If a mirror is mounted on a truly adjusted polar axis, the plane of the mirror being parallel to that axis, the normal to that mirror will always be directed to some point on the celestial equator through whatever angle the axis is turned.
Since the sun moves in the ecliptic, it is in the last-named sense the point of intersection of the ecliptic and the celestial equator.
Equinox The two points at which the Sun crosses the celestial equator in its yearly path in the sky.
The equinoxes are the two points on the celestial sphere where the ecliptic intersects the celestial equator.
