The vernal equinox marks the zero point for which coordinate?

• A. Right Ascension
• B. Declination
• C. Ecliptic Longitude
• D. Azimuth

Answer: (A)

Positions on the sky are referenced from the vernal equinox. In the common equatorial coordinate system, the zero point for right ascension is exactly at the vernal equinox, and right ascension is measured eastward along the celestial equator in hours, minutes, and seconds. This is like longitude on Earth, but using time as the unit. Declination, on the other hand, is the angular distance north or south of the celestial equator, so its zero is the celestial equator itself, not the vernal equinox. Ecliptic longitude does use the vernal equinox as its zero point as well, but that coordinate sits in the ecliptic (not the equatorial) system. So the vernal equinox specifically defines zero for right ascension in the equatorial framework.