Sunset on 29 August 2005 from Sydney Observatory. At the equinox the Sun sets much further to the left or south. Photo Nick Lomb

There are four turning points in the year: the summer and winter solstices and the spring and autumn equinoxes. Tuesday 20 March 2012 is the day of the autumn equinox in the southern hemisphere. At 4:14 pm AEDT on that day the Sun crosses from the southern to the northern part of the sky.

At the equinox day and night are equal at 12 hours each or, more precisely, almost equal. In Sydney the Sun rises at 6:58 am AEDT and sets at 7:06 pm so that the day is 12 hours and eight minutes in length. Why not exactly 12 hours? There are two factors at work. One is that sunrise and sunset are calculated as the times when the top edge of the Sun touches the horizon while the expected equality of day and night refers to the centre of the Sun. In addition, the atmosphere bends the light from the Sun in such a way that we see it at sunrise and sunset even though geometrically it is below the horizon. These two factors come together so that daytime is always slightly longer than the night at the equinox.

Diagram of the seasons. In this diagram we are looking the Earth’s circular path around the Sun from side on and hence the path appears oval. The seasons take place because of the tilt of the Earth’s axis. If there was no tilt life would be boring with no seasons. Note that “NCP” on the diagram stands for north celestial pole, which is the point directly above you if you were standing at the north pole. Drawing Nick Lomb

As indicated on the above diagram the seasons occur due to the path of the tilted Earth around the Sun. At the equinoxes the relationship of the tilt to the Sun is that neither hemisphere is tilted towards or away from it. Hence on the day of the equinox day and night are approximately equal everywhere on Earth.

Another feature of the equinox is that the Sun rises due east and sets due west. Again this statement is a slight approximation and its accuracy depends on how close to sunrise or sunset the exact occurrence of the equinox takes place. On the autumn equinox on 20 March 2012 there is less than three hours between the time of the equinox and the time of sunset, hence sunset takes place on the day almost exactly due west.

‘The descent of the snake’ at Chichen Itza in Mexico during the March 2009 equinox. Courtesy Wikimedia Commons

The most famous equinox happening is ‘the descent of the snake’ at Chichen Itza in Mexico at sunset at the spring and autumn equinoxes. Chichen Itza is a pre-Columbian city built by the Maya civilization over a thousand years ago. One of its buildings is a tall stone pyramid, on part of which at the equinoxes the setting Sun creates a pattern of light and shadow that gives the impression of a descending snake.

If clouds permit have a look at the setting Sun on the day. You may not see a snake, but you will establish the exact position of due east and due west.