a half-circle (semicircle) that runs across the surface between
the geographicpoles of a planetary body. Meridians intersect the
equator at right angles. Observers on the same meridian have the same
solar time.
the half-circle that runs from the equatorial north pole (near the
Pole Star) via the zenith to the equatorial south pole. When the Sun
goes through the meridian, then it is highest in the sky for the day,
and then it is noon. Also called the celestial meridian.