The autumnal equinox, like its vernal counterpart, is not fixed on any certain date. You might think it ought to be, but it may fall on any one of several days around this time. Almost by definition, it comes around at exactly yearly intervals, but the fact that our year is not an integral number of days, combined with our odd habit of inserting a leap day into the annual cycle every now and then to rectify the matter, means that the autumnal equinox oscillates backwards and forwards on our calendar . Some years it might occur today; it often falls on September 22nd; and this year, it will occur in the early hours of Thursday next, September 23rd.
If you ask an astronomer to tell you about the autumnal equinox, he or she will probably mumble something about it being one of the two points where the ecliptic crosses the celestial equator. It may be easier, although less accurate, however, to think of it as the instant when the sun, moving southwards and abandoning our hemisphere to the rigours of a northern winter, has reached that point on its apparent journey when it is directly over the equator.
It is this biannual migration of the sun that gives us the four seasons of our year. The Earth moves around the sun in an orbit which is very nearly circular, and at the same time it spins like a top on its own axis. If this axis were exactly at right angles to the plane defined by the Earth's orbit, there would be no such thing as seasons, but this is not the case: the northern hemisphere is inclined towards the sun for part of the year, and away from the sun at other times.
This means that during our summer, the sun's rays shine vertically down on the lower latitudes of the northern hemisphere, maximising the absorption of solar radiation. During our winter, on the other hand, the sun hits the northern hemisphere at a very oblique angle; as a result, much of the radiation is reflected back out to space, and that which does reach the surface of the Earth is diffused over a comparatively large area, diminishing its heating effect.
But just twice each year, at the autumnal and the vernal equinoxes, both hemispheres get an equal share of the sun's warmth and other forms of energy. And as the name of the event implies, around this time of year, day and night are of equal length, at 12 hours each, in any region of the world.