Final answer:
The position in the sky where both right ascension and declination are zero is called the vernal equinox. This occurs around March 21 when the Sun crosses the celestial equator.
Step-by-step explanation:
The position in the sky where the right ascension and declination are zero is known as the vernal equinox. This point marks the intersection of the celestial equator and the ecliptic, and the Sun passes through this position around March 21 each year, signaling the start of spring in the Northern Hemisphere. The vernal equinox is historically referred to as the first point of Aries, although due to precession, it is no longer situated in the constellation Aries.