What is the Celestial Prime Meridian?

• A. The projection of the terrestrial prime meridian onto the celestial sphere.
• B. The line perpendicular to the celestial equator at its intersection with the ecliptic.
• C. Earth's equator extended to the celestial sphere.
• D. The great circle that passes through the celestial poles and is 90° from the prime meridian.

Answer: (C)

In celestial coordinates, the zero longitude on the sky—the celestial prime meridian—is the great circle that passes through both celestial poles and the First Point of Aries (the vernal equinox). This line defines zero for celestial longitude (right ascension). It is distinct from the celestial equator, which is the projection of Earth’s equator onto the celestial sphere and serves as the zero line for declination. So describing the celestial prime meridian as Earth’s equator extended onto the sky mixes up two different reference circles. The other options describe different sky lines—one is a line perpendicular to the celestial equator at the ecliptic intersection (not the prime meridian), another is a meridian 90° away from zero, and another ties to the terrestrial prime meridian in a way that doesn’t define the celestial prime meridian.