Autumnal Equinox - September 22nd
The Autumnal Equinox signals the end of the summer months and the beginning of winter. At this time of year, days have been shortening since the Summer Solstice some three months earlier, and the Equinox is the point where nights reach the same length as days.

After this point, the Sun will shine lower and lower on the horizon until the Winter Solstice in about three months' time.

Equinoxes occur because the Earth's axis of rotation isn't aligned with the plane of its orbit around the Sun: it tilts over by about 23½°.

The direction of this tilt is effectively constant, relative to the stars, so that the Earth's north pole always points towards Polaris, the Pole Star, and the south pole always points at the constellation of Octans. (In fact, this direction is not completely constant, and the poles move against the stars by about a fifth of a degree every century).

Each year, the Earth completes a circuit of the Sun, and for its poles to remain fixed against the stars, their direction must rotate relative to the Sun. This effect gives us the seasons. When a pole is angled towards the Sun, its hemisphere receives more hours of sunlight, and when a pole is turned away from the Sun, its hemisphere experiences long cold nights.

The equinoxes represent the points where the direction of poles are at a right angle to the Sun. They represent the point of transition from summer to winter, or from winter to summer. The Autumnal Equinox occurs in late September, and is named for the fact that it marks the end of summer and the entrance into winter of the northern hemisphere. South of the equator, its name is less appropriate, since it corresponds to the beginning of summer.

What's so special about it? There are a few special things you can talk about.
As summer wears on, the nights have been growing longer and the days shorter. On this date, the night becomes longer than the day! That's just for us in the Northern Hemisphere. For our friends in Australia, it's reversed. Spring has just begun for them.

A minor point ... if you look up the rising and setting time of the Sun in the newspaper, it'll look as if the day is still a bit longer than the night. That's mostly because "sunset" and "sunrise" are defined by the top edge of the Sun, not the middle of it, and the middle of the sun sets (rises) a few minutes before (after) the edge does.

On this day,the Sun will begin to rise at the South Pole after six months of darkness. It's going to be daytime there for the next six months! And yes, there are scientists who are living and working down there now! They have a party to celebrate. Wouldn't you if it were eighty below zero outside and the Sun was just coming up? What would it be like living in a place where day and night each last six months?

On this day, the Sun rises directly in the East, and sets directly in the West.. At sunrise and sunset, the shadow of the Sun Tower will point exactly in those directions. So this is a special time to talk about where east and west are (and if you don't know, you can look at the compass rose on the Sun Tower!)

On this day, the Sun passes straight overhead, at the "zenith" for people on the equator, like in Kenya or Ecuador. When the Sun passes straight overhead, there aren't any shadows! What kind of a shadow would the Sun Tower (or any other vertical pole or building) show when the Sun is at the zenith?

Since the nights are getting longer, and the days are getting shorter, the sun is shining on us less and less every day. Also, since it's lower, its slanting rays spread out over more area. What does all this have to do with winter being colder than summer? One of the great occurances of autumn is the fall, and all the leaves on the plants starting to turn radiant colours before they begin sheding their leaves.