Where does the clock start for right ascension?

Answer 1

The "zero" or reference for right ascension in the sky is the meridian that joins

the north and south poles of the sky and passes through the vernal equinox ...

that's the point where the sun crosses the celestial equator moving north, on

March 21.

Answer 2

The clock for right ascension starts at the vernal equinox, which is the point in space where the celestial equator intersects the ecliptic. This point is used as the zero point from which right ascension is measured in the sky.