Wednesday, May 30, 2012

Defining the Seasons

As we move between the solstices and equinoxes it is sometimes asked how we measure the start and end of the seasons in Australia and why this is different to other parts of the world.

In Australia we use the beginning and end of calendar months to describe our seasons – winter is June 1st to August 31st, spring from September 1st to November 30th, summer from December 1st to February 28th (29th in a leap year) and autumn from March 1st to May 31st.

However in other parts of the world the seasons are defined through the solstices and equinoxes. In the Southern Hemisphere the summer equinox, or the longest day, falls around December 21, and the winter solstice, or shortest day, around June 21. (There is some minor variation of these dates from year to year). The spring or vernal equinox (equal hours of night and day) falls around September 21st and the autumnal equinox around March 21st.

Darwin thunderstorm - common during the southern hemisphere summer. (Image form Wikipedia Commons - click to enlarge)

If we use these dates to define our seasons we find that summer occurs between December 21st and March 21st, autumn from March 22nd to June 21st, winter from June 22nd to September 21st, and spring from September 22nd to December 20th. This system displaces our present seasons forward by about three weeks.

So why did Australia not adopt this solstice – equinox system rather than the calendar month option that has considerably less physical significance? The answer appears to be lost in the mists of time but an interesting theory has been advanced that may or may not be true.

In 1789 the New South Wales Corps was formed in England as a permanent regiment to oversee the infant settlement of Sydney. Apparently they changed from summer dress to winter dress on March 1st of each year and then back to summer dress on September 1st. This may have been the reason behind defining our seasons using calendar months. 

In any case it’s also become rather obvious that the four season European model has little relevance in many parts of Australia. In Darwin, for example there’s a strong wet season dry season cycle with spring and autumn hard to find.

Indigenous Australians have a different way of describing the seasons that varies from place to place and some further details about this can be found at