Why do we have seasons?

A myth that makes sense, but is wrong

Lots of people think summer happens when Earth is close to the Sun, and winter happens when Earth is far away. It sounds sensible. A campfire is hotter when you stand close to it. But it is not true.

Earth's path around the Sun is not a perfect circle. It is a little bit squashed. Earth is closest to the Sun in January, at about 147 million kilometres. It is farthest away in July, at about 152 million kilometres. That is a difference of about 5 million kilometres, but it is only about 3 per cent of the whole distance. It is too small to make summer and winter.

Here is the best proof. In January, when Earth is closest to the Sun, it is summer in Australia. But it is winter in countries in the northern hemisphere, like Canada and Japan. If the distance made the seasons, everyone on Earth would have summer at the same time. So something else must be going on.

The real reason: Earth leans

Earth spins around an invisible line called its axis. The axis runs from the North Pole to the South Pole, through the middle of Earth. It is not straight up and down compared with Earth's path around the Sun. It is tilted by about 23.5 degrees.

As Earth travels around the Sun, which takes a year, the axis keeps pointing the same way. So during part of the year the southern half of Earth leans towards the Sun, and during another part it leans away. Around December, the southern hemisphere leans towards the Sun. Around June, it leans away. In March and September, neither half leans towards the Sun.

The half that leans towards the Sun has summer. The Sun climbs higher in the sky, and its light shines more directly on the ground. The same amount of light is squeezed into a smaller patch, so the ground heats up more. The days are also longer, so there is more time to warm up. The half that leans away has winter. The Sun stays low, its light is spread over a bigger patch, and the days are short. Because the two halves lean opposite ways, they have opposite seasons.

Long days, short days

The days really do change length. In Hobart, there are only about 9 hours of daylight on the shortest day of the year, but about 15 hours on the longest. Darwin is much closer to the equator, so its days hardly change. They go from about 11.5 hours of daylight to nearly 13 hours.

The longest day in Australia is at the summer solstice, around 21 or 22 December. The shortest is at the winter solstice, around 21 June. In late March and late September, there are two equinoxes, when day and night are almost the same length.

Australia's weather calendar follows the temperatures. The Bureau of Meteorology counts summer as December, January and February, autumn as March to May, winter as June to August, and spring as September to November. The warmest and coldest times of year come a few weeks after the solstices. This is because the oceans around us take time to warm up and cool down. In much of southern Australia the coldest week is usually in July, and the warmest is in late January or early February.

Only two seasons up north

The tilt matters less near the equator, because the heat that places there get from the Sun hardly changes during the year. So the four seasons do not fit the far north of Australia. Instead, the year is split by rain. A hot, sticky "build-up" starts around October. The wet season then runs from about November to April, and the monsoon rain usually arrives around late December. The dry season is generally from May to September.

People in the south often envy the dry season. The days are sunny, and the top temperatures are usually in the low 30s. The wet season is when the heaviest rain falls. The exact date when the rains start changes from year to year and from place to place.

Reading the year from the land

The four seasons we learn about at school came from Europe. For thousands of years, Aboriginal and Torres Strait Islander peoples have kept their own seasonal calendars. These divide the year using what is happening in nature, such as which plants are flowering, which animals are breeding, and what the wind and rain are doing. The knowledge is local. Each language group has its own calendar for its own Country, and the number and the length of the seasons are different from place to place.

The Noongar people of south-west Western Australia (the Bureau of Meteorology spells it Nyoongar) describe six seasons: Birak, Bunuru, Djeran, Makuru, Djilba and Kambarang. Months are only a rough guide. The seasons can be long or short, and people know that one is changing into the next by looking at nature. For instance, they look for flowering plants, reptiles waking up, and swans changing their feathers. Makuru, in about June and July, is the coldest and wettest time. The Noongar calendar has two summers and two springs.

The Gulumoerrgin (Larrakia) people of the Darwin region describe seven seasons, and one of them, Dalay, is the monsoon season. These calendars belong to the people who hold them. If you want to learn the seasons of the place where you live, ask the local Aboriginal community, land council or language centre.

📚 Words to know

Axis

The invisible line through Earth from the North Pole to the South Pole. Earth spins around it once a day.

Hemisphere

One half of Earth. The equator splits Earth into the northern hemisphere and the southern hemisphere.

Solstice

The day of the year when a hemisphere leans most towards or away from the Sun, giving the longest or the shortest day.

Equinox

One of the two days of the year when day and night are nearly equal, in late March and late September.

Tilt

The lean of Earth's axis. It is about 23.5 degrees, and it causes the seasons.

🧪 Try this: model the seasons with a torch

1. Ask an adult to help. In a dim room, hold a torch about 20 centimetres above a sheet of paper and shine it straight down. Trace around the bright circle with a pencil.
2. Without moving the torch away, tip it so the light hits the paper at a slant. Trace the stretched oval. The same light now covers a bigger patch, so it looks dimmer. A low winter Sun does the same job on the ground, so it warms the ground less.
3. Now model Earth's orbit. Put a lamp with an LED bulb on a table, or use a torch, as the Sun. A desk globe already leans at about 23.5 degrees. Walk the globe slowly around the lamp, and keep its North Pole pointing towards the same corner of the room all the way round. Do not turn the lean towards the lamp. Stop four times, and look at whether Australia is leaning towards the lamp or away from it.
4. Challenge: measure the seasons with a shadow. Stand a stick upright in a pot of soil in a sunny spot. Never look straight at the Sun. Around the middle of the day, measure the shadow length with a ruler. Measure again in a month or two. In summer, the Sun is higher and the shadow should be shorter. When is it shortest?