Why we have seasons

Earth spins like a top. It spins around a line called its axis.

The axis is not straight up and down. Earth leans a little to one side. That lean gives us summer, autumn, winter and spring.

Imagine a rod pushed through Earth from the North Pole to the South Pole. That imaginary rod is Earth's axis, and Earth spins around it once a day, giving us day and night. Earth also travels all the way around the Sun once a year.

Here is the key: the axis does not stand up straight. NASA measures it tilted 23.4 degrees, and many weather services round it to about 23.5. As Earth goes around the Sun, the axis keeps pointing in the same direction, so for part of the year the northern half leans toward the Sun and for part of the year the southern half does.

Earth rotates once every 23.9 hours and completes an orbit in 365.25 days (NASA). Its rotation axis is tilted 23.4 degrees with respect to the plane of the orbit, and because the axis keeps pointing in the same direction in space, the hemisphere tilted toward the Sun changes as Earth moves around its orbit. NASA states that this tilt causes the yearly cycle of seasons.

The angle itself is not fixed forever. The U.S. National Weather Service gives an average of 23.5 degrees and notes that the tilt varies between about 22 and 24.5 degrees. NASA's Earth Observatory lists Milankovitch cycles, small variations in the shape of Earth's orbit and in its axis that happen over thousands of years, among the natural causes of climate change.

Some people think summer comes when Earth is close to the Sun.

That is not right! Earth is closest to the Sun in January. That is winter in the north.

Many people believe summer happens because Earth is closer to the Sun. It makes sense, but it is wrong. Earth's path around the Sun is a little lopsided, so the distance does change during the year.

NASA Space Place gives the numbers. At its closest, in January, Earth is about 91.4 million miles (about 147 million kilometers) from the Sun. At its farthest, in July, it is about 94.5 million miles (about 152 million kilometers) away. If distance made summer, the north would be hot in January. Instead, January is winter there.

Earth's orbit is slightly elliptical. Perihelion, the closest point, is about 91.4 million miles (about 147 million km) from the Sun and falls in January; aphelion, the farthest, is about 94.5 million miles (about 152 million km) and falls in July. The difference is over 3 million miles, but NASA notes that compared with the whole distance it does not make much difference to our weather.

The timing is the clearest evidence. The Northern Hemisphere has winter at perihelion and summer at aphelion, which is the opposite of what a distance explanation predicts. The National Weather Service makes the same point: if distance were the cause, the north would be hotter in January than in July.

When the Sun is high in the sky, its light feels strong and warm.

When the Sun is low, its light spreads out. Spread-out light warms the ground less.

Shine a flashlight straight down and you get a small, bright spot. Tilt it and the same light spreads into a bigger, dimmer oval. Sunlight works the same way. When your half of Earth leans toward the Sun, the Sun climbs high in the sky and its rays hit the ground more directly, so each patch of ground gets more energy.

The lean also changes how long the Sun stays up. The National Weather Service explains that in summer the high Sun brings long days, while in winter the low Sun brings short days. More hours of stronger sunlight add up to warmer weather.

NASA's Earth Observatory explains that from the equator toward the poles, the Sun's rays meet Earth at smaller and smaller angles, so the same light is spread over larger surface areas. At the equinoxes, it reports, the tropics receive about 90 percent of the energy that reaches the equator, the mid-latitudes at 45 degrees roughly 70 percent, and the Arctic and Antarctic Circles about 40 percent.

The tilt shifts this pattern north and south through the year. The National Weather Service adds two more effects for the summer hemisphere: sunlight travels through less atmosphere, so less of it is scattered before reaching the ground, and the high Sun angle comes with long days. The winter hemisphere gets the opposite: low, slanted rays, more scattering and short days.

Two days a year are special. One is the longest day. One is the shortest day.

Two more days are in between. On those days, day and night are about the same length.

A solstice is when one half of Earth leans most toward the Sun and the other leans most away. Around June 21, the Sun is straight overhead at the Tropic of Cancer, north of the equator. Around December 21, it is over the Tropic of Capricorn, south of the equator. Each solstice brings one hemisphere its longest day and the other its shortest.

Halfway between come the equinoxes, in late March and late September. Then the Sun is right over the equator, neither half leans toward it, and day and night are close to equal everywhere.

The solstices mark the two moments when Earth's tilt angles the hemispheres furthest toward or away from the Sun. The National Weather Service places the Sun directly over the Tropic of Cancer, at 23.5 degrees north, near June 21, and over the Tropic of Capricorn, at 23.5 degrees south, near December 21. The tropics sit at the same latitude as the tilt.

At the equinoxes, in late March and late September, the axis is side-on to the Sun and the Sun is directly over the equator. Australia's Bureau of Meteorology notes that 'equinox' comes from the Latin for 'equal night' and 'solstice' from the Latin for 'sun-stopping', because at a solstice the Sun stops appearing to move further north or south each day. Day and night are not exactly equal at an equinox: the Bureau explains that the atmosphere bends sunlight, so most of Australia sees about 12 hours 8 minutes of daylight at the spring equinox.

The north and the south take turns.

When it is summer in the north, it is winter in the south. In Australia, summer comes in December!

Because the two halves of Earth take turns leaning toward the Sun, their seasons are opposite. In the United Kingdom, the Met Office counts summer as June, July and August and winter as December, January and February. In Australia, the Bureau of Meteorology counts summer as December, January and February, exactly six months later.

The difference in day length is easy to measure. In Hobart, in southern Australia, the Bureau says there are only about 9 hours of daylight at the winter solstice in June, but about 15 hours at the summer solstice in December.

Seasons in the two hemispheres are always reversed: six months after the north tilts toward the Sun, the south does, and NASA notes the heating pattern simply swaps. The Met Office uses meteorological seasons for the UK (spring March to May, summer June to August, autumn September to November, winter December to February), and Australia's Bureau of Meteorology uses the same four-season model shifted by six months.

The size of the swing depends on latitude. The Bureau reports that Hobart, at about 43 degrees south, has around 9 hours of daylight at the winter solstice and 15 at the summer solstice, while tropical Darwin varies only between about 11.5 and 12.5 hours. Where the tilt barely changes the Sun's strength, temperature is not what defines the year, as the next section shows.

Not every place has four seasons.

Near the equator, it is warm all year. Some places have a rainy time and a dry time instead.

Spring, summer, autumn and winter fit places far from the equator, such as the United Kingdom or southern Australia. Near the equator, the Sun is high all year, so temperatures change much less. The Met Office explains that some places have only two seasons, a winter and a summer or a wet and a dry season, and some have just one.

In northern Australia, the Bureau of Meteorology divides the year by rain: a wet season from October to April and a dry season from May to September. Over thousands of years, Indigenous Australians have kept local calendars with seasons based on weather and on what plants and animals are doing.

The four-season calendar is a model built for the mid-latitudes. Australia's Bureau of Meteorology says that model is not relevant in the country's northern tropics, where average temperatures in 'winter' and 'summer' differ little; there the year is divided by rainfall into a wet season (October to April, with the heaviest rain) and a dry season (May to September), and the arrival of heavy rain varies by year and place.

The Bureau also highlights Indigenous Australian calendars, maintained over thousands of years, which divide the year into seasons by local weather patterns and plant and animal life-cycle events. The Met Office notes that cycles such as El Niño and La Niña can change the weather a season would normally bring. The tilt sets the astronomy; local geography decides what each season feels like.

You can see this with a flashlight!

Shine it straight at a paper. Draw around the bright circle. Now tip the paper back. The light spreads out and looks dimmer.

Scientific American, with Science Buddies, describes a simple test. Tape a flashlight on its side on a box, and tape a sheet of paper to a book. Hold the book upright in front of the light and draw around the bright circle. Then, without moving it closer or farther, tilt the book back about 45 degrees and draw around the light again.

The second shape is a bigger, dimmer oval, almost twice the size of the first. The same light is spread over more paper, just as slanted winter sunlight is spread over more ground.

The flashlight activity from Scientific American (with Science Buddies) turns the geometry into a measurement. Keep the distance fixed, trace the lit area with the paper square to the beam, then tilt it back 45 degrees and trace again; the outline becomes an oval almost twice the size. Repeat on graph paper and count the lit squares, or use a protractor to test 20, 30 and 40 degrees.

The result matches what NASA's Earth Observatory describes for the real planet: the lower the angle of the rays, the larger the area each beam covers and the less energy each square meter receives. Try predicting the shape at 60 degrees before you trace it.