Yesterday the maximum temperature in Brisbane reached almost 31 degrees. Today it was just over 32 degrees. Quite warm for this summer, which has seen lower temperatures than average thanks to a La Niña event that has increased cloud cover and rainfall amounts. Perhaps I should refer to 'last' summer? Since yesterday was the first day of autumn in Australia.

This is where the sun set tonight.

At the height of summer, the sun was setting behind the middle of the big fig tree on the left of of the picture below.

Now, anyone who knows anything about the passage of the seasons, no matter which hemisphere they live in, is aware that seasonal variation is determined by the tilt of the earth on its axis as it journeys around the sun.

Four seasons are characteristic of temperate regions of the earth: one season gradually leads into another as far as observable weather patterns are concerned. For convenience, the duration of each season is defined – in a variety of ways, it turns out. In many countries, the start of autumn is determined by the earth passing a certain point in its orbit, at which both poles are at the same distance from the sun. (In the southern hemisphere's summer, the south pole is tilted towards the sun.) This is how astronomers define the start of autumn, at what they call the autumnal equinox, when day and night are roughly equal in length. This happens on or around 21 March*.

Not everyone follows the astronomers. In the States, there's a tradition that autumn starts on Labor Day (the Tuesday after the first Monday in September), which is before the northern hemisphere's autumnal equinox around 21 September, and finishes on Thanksgiving (the fourth Thursday in November).

Meteorologists tend to define seasons by months: so, autumn is September, October and November in the northern hemisphere; March, April and May in the southern hemisphere. Australia follows this definition. 

I've read that in the early days of the colonies, the New South Wales Corps and the navy changed from summer to winter uniforms on 1 March, and back to summer uniforms on 1 September.

There are other factors to muddy the waters, however. I have always been inclined to follow the astronomers. And my friend and I, who prefer to celebrate pagan dates (based on the cycles of the sun) rather than those determined by religion, make a point of dining out on equinoxes and solstices. I have always been a bit bothered, however, by the fact that winter in northwest Europe clearly doesn't start on 21 December. It's often been winter-cold for weeks by then because winter lasts for more like five months than three at those latitudes.

Southeast Queensland isn't even in a temperate zone; it's sub-tropical. It is often said to have two seasons: the Wet and the Dry. Especially since the state doesn't implement Daylight Saving Time, the difference between sunrise in summer and sunrise in winter (and sunset in summer and sunset in winter) is little more than an hour an a half. No light evenings for us; ever! Inevitably, if autumn starts at the beginning of March, there'll be autumnal days that are hotter than many days in summer. Midwinter days can reach a very pleasant mid-20s.

The power of suggestion is sometimes very strong: and the media here make more fuss of the start of a new season than I've been used to. A number of people have commented to me how hot it's been over the last couple of days. I'm willing to bet that, a couple of weeks ago, no one would have batted an eyelid at these temperatures. But it's autumn, init?

*The time varies slightly from year to year because the earth's orbit is an ellipse, not a circle, and the speed of the earth around the sun is therefore not constant (it moves faster when closer to the sun), which produces variations in the length of the solar day

This post was written on 2 March 2012