What is the relationship between daylight and nighttime during an equinox?

At the equator, the sun is directly overhead at noon on these two equinoxes. the nearly equal hours of day and night is due to the refraction of sunlight or bending of lights Ray's that causes the sun to appear above the horizon when the actual position of sun is below the horizon.

On equinox, the sun moves across the celestial equator, which lies directly above the Earth's equator. When the sun crosses the plane, it rises exactly in the east and sets exactly in the west.

Bottom line: There's slightly more day than night on the day of an equinox. That's because the sun is a disk, not a point of light, and because Earth's atmosphere refracts (bends) sunlight.