Autumn leaves.

Many trees lose their leaves in autumn.

Image: Willem van Aken/CSIRO

Wherever you are in the world, you may have noticed a change in the weather. Over the course of a year, there are periodic changes in things like temperature, rainfall and the amount of daylight. These changes allow us to divide the year into seasons.

How many seasons are there? Many places around the world count four seasons: summer, autumn, winter and spring. The summer season has longer days and warmer temperatures, while winter is the opposite: shorter days and colder temperatures. Autumn and spring are in between. Seasons in the northern and southern hemispheres are opposite – for example, when it is summer in the southern hemisphere, it’s winter in the northern hemisphere.

The main reason for these different seasons is to do with Earth’s orbit around the Sun. The Earth rotates on its axis – one full rotation of the Earth takes a day. Relative to Earth’s orbit around the Sun, this axis is tilted. Sometimes, the northern hemisphere is tilted towards the Sun, meaning the southern hemisphere is tilted away. During this time, it’s summer up north and winter down south. As Earth continues its orbit around the Sun, the situation reverses – the southern hemisphere tilts towards the Sun and experiences summer.

Not everywhere has four seasons. There are many different ways to divide the year into seasons. For example, places close to the equator don’t have weather patterns that fit into four seasons. The year might be divided into only two: the wet season, and the dry season. As their names suggest, the main feature of these seasons is the amount of rainfall.

Plants and animals adapt to suit their environments, including the seasons. Changes in temperature, rainfall and daylight can trigger changes in plant growth and animal behaviour. Such ecological patterns are used by Aboriginal and Torres Strait Islander cultures to define different seasons with their own calendars. Some groups have four seasons, but many have more or less – the Ngan’gi calendar has 13!

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