The Fall Equinox arrives today at 5:54 a.m., marking the start of the northern autumn. It's the moment the sun crosses the celestial equator - the plane of Earth's equator extended out into space - and heads south. That makes this the first day of spring for the Southern Hemisphere. On paper, night and day are equal lengths today. But it's the center of the sun's disk that rises and sets 12 hours apart. We see Sol's top half for an extra seven minutes.