For much of Australia, temperatures are on their way down, frost and fog are getting more widespread and snow is settling more frequently on the alps. Most would agree that these are signs of autumn nearing its end and that winter is almost here.

However, despite these clear signs that the seasons are changing, when this change actually takes place is a muddier issue.

In Australia, seasons are defined by grouping three calendar months together:

• Summer = December to February
• Autumn = March to May
• Winter = June to August
• Spring  = September to November

Some other countries use the dates of the equinoxes and solstices to identify when each new season starts. These dates relate to the changing 'tilt' of our planet relative to the sun. The summer and winter solstices occur when Earth's poles face more towards the sun in December and June, while the spring and autumn equinoxes occur when the sun passes directly over our planet's equator, in March and September.

While Australia's definition of seasons starts at the beginning of each calendar month, the solstices and equinoxes occur between 20 and 23 days later.

So, which one is right?

Just like Australia uses metres and Celsius while America uses miles and Fahrenheit, the answer depends on where you live.

In Australia, the accepted definition that is used by the Bureau of Meteorology identifies seasons based on calendar months. 

As many locations in Australia are cooler during the first few weeks of June than the same period in September, it makes sense for early June to be part of winter. For example, Sydney's average temperature (combined minimums and maximums) is 13.4 degrees during the first 21 days of June and 15.3 degrees during the first 21 days of September.

In some other countries, defining seasons using the solstice and equinox is widely accepted and should be used to ensure consistency between agencies, both nationally and internationally.

In reality, the three weeks difference between each definition doesn't make too much of a difference in terms of weather.