The seasons are governed by the tilt of the Earth’s axis in space as it journeys around the Sun in a year. When the South Pole of the Earth is tilted towards the Sun, this is our Summer. Six months later, when the South Pole is tilted away from the Sun, it's our Winter. In between these we have Autumn and Spring.
Temperatures on our planet are not determined by the distance of the Earth from the Sun. Rather it is the angle of the Sun’s rays striking the Earth. In Summer, the Sun is high in the Sky and the rays hit the Earth at a steep angle. In winter, the Sun is low in the Sky and the rays strike the Earth at a shallow angle.
The seasons don’t begin on one day and finish on another. That's because our orbit around the Sun is continuous. It actually takes quite some time for the Earth to heat up or cool down, and that’s why the seasons change gradually.
So when do we actually start the seasons?
In some parts of the world, such as Australia, seasons begin on the first day of a particular calendar month - in March for Autumn, June for Winter, September for Spring and December for Summer. In other countries such as Britain, it’s accepted that the seasons begin on the dates that the Earth passes four special points in its orbit about the Sun.
Spring Equinox (AEST)
2008 September 23, 1:44am
2009 September 23, 7:18am
2010 September 23, 1:09pm
The Sun in spring
Artist: Frey Micklethwait. Source: Museum Victoria.
On the day of the Spring Equinox, the Earth’s poles are the same distance from the Sun. In Melbourne, the Sun rises due east, sets due west and gets to 52° above the horizon at noon. On this day there are roughly 12 hrs of day and 12 hrs of night.
Summer Solstice (AEDT)
2008 December 21, 11:04pm
2009 December 22, 4:47am
2010 December 22, 10:38am
The Sun in summer
Artist: Frey Micklethwait. Source: Museum Victoria.
On the day of Summer Solstice, the Earth’s south pole is tilted towards the Sun. The Sun rises south of east, sets south of west and reaches 751/2° above the horizon at noon. This is, usually, the longest day of the year.
Autumn Equinox (AEST)
2008 March 20, 3:48pm
2009 March 20, 9:44pm
2010 March 21, 3:32am
The Sun in autumn
Artist: Frey Micklethwait. Source: Museum Victoria
On the day of the Autumn Equinox, the Earth’s poles are the same distance from the Sun. The Sun rises due east, sets due west and reaches 52° above the horizon at noon. There are roughly 12 hrs of day and 12 hrs of night.
Winter Solstice (AEST)
2008 June 21, 9:59am
2009 June 21, 3:45pm
2010 June 21, 9:28pm
The Sun in Winter
Artist: Frey Micklethwait. Source: Museum Victoria.
On the day of Winter Solstice, Earth’s south pole is tilted away from the Sun. The Sun rises north of east, sets north of west and reaches 281/2° above the horizon at noon. This is, usually, the shortest day of the year.