According to a report published by the Bureau of Meteorology last week, Australia just had its second-warmest summer on record. However, some may argue that summer isn't even over yet. So, why is there a discrepancy about our seasons and when do they actually start and end? Seasons are caused by Earth's tilt. An object roughly the size of Mars slammed into Earth about 4.5 billion years ago, causing our planet to lean over a bit. Earth now orbits the sun with a slant of about 23.5 degrees. Earth's tilted axis causes each pole to point towards the sun during summer, away from the sun in winter, and transitions between these two orientations during spring and autumn. In Australia, seasons are defined by grouping calendar months together. Spring is September to November, Summer is December to February, Autumn is March to May and Winter is June to August. These seasonal groups combine the hottest and coldest months together for summer and winter and the transition months in spring and autumn. They also make it easier to compile and compare climate statistics about seasons. Some countries refer to seasons based on the astronomical definition, using the dates of the solstices and equinoxes. Solstices are the moments in time that Earth's poles reach their furthest orientation towards (summer) or away (winter) from the sun. The equinoxes (spring and autumn) mark the moment halfway between each solstice. In reality, the change of seasons for any one point on Earth doesn't happen on a single day and is a transition that happens gradually. For example, the shift in weather between summer and autumn spans enough time to encompass both the end of February and the equinox in late-March. In a matter of weeks it will be a bit cooler for much of the country, frosts will be developing more in southern Australia and some farmers will be sowing their winter crops.