On the day of the vernal equinox, the day length for all places on Earth is actually slightly longer than 12 hours. Explain why.

Equinox is referred as the time when Earth's equator passes the Sun's center.

At equinox, day and night length is same which is of 12 hours but it does not happen like this. Day time is slightly longer than 12 hours because of the refraction of Sun's light on Earth's surface.

Earth's atmosphere refracts sunlight that causes the Sun's upper brim to be visible from Earth's horizon even though when the Sun does not reaches the horizon. This time from sun rise and sun sets adds extra minutes to the 12 hours of the day thus making day longer even at vernal equinox.