Astronomically speaking, today is the day that the Sun crosses the celestial equator (an imaginary line directly above the equator) heading north.
This diagram to the right
is the analemma. It traces the path of the Sun in its annual migration to the northern and southern hemispheres. In other words, for the next 6 months, the Sun is in the northern hemisphere! Of course, any fifth grader can tell you that the cause of this is the tilt of the Earth on its axis as it orbits the Sun.
Some people believe that the equinox is a day when sun rise and sun set are exactly 12 hours apart – resulting in equal day and nights. That day – when sunset and sunrise are 12 hours apart – is called the equilux. The equilux is different for different locations on Earth.
The image below shows the illumination of the Earth by the Sun today.