Why does the Earth have 4 seasons

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How Are Seasons Different in the Northern and Southern Hemispheres?
But what caused Earth to tilt?
Earth's lopsided orbit
Related Resources for Educators

Tree through the four seasons (Studio Light and Shade, iStockphoto)

Two things cause the seasons to change. First, the Earth moves around the Sun. Second, the Earth has a tilted axis of rotation.

The Earth spins around an axis. This imaginary line extends from the South Pole to the North Pole. But the Earth’s axis is not vertical. It’s actually tilted at an angle of 23.5°. The planet is always tilted in the same direction as it orbits the Sun.

Did you know?

The tilt of Earth’s axis hasn’t always been 23.5°. Every 40 000 years, it cycles between 22° and 24.5°.

The Earth spins around its axis. The planet’s rotational axis is tilted at 23.5 degrees from perpendicular (Let’s Talk Science using an image by shoo_arts via iStockphoto).

What does the Earth’s tilted axis have to do with seasons? It means that different parts of the planet are tilted toward the Sun at different times of the year. It’s also why the seasons are different in different parts of the world.

Not all parts of the Earth have four distinct seasons. But they all experience seasonal variation. Closer to the North Pole and the South Pole, daylight and temperatures change with the seasons. Days are longer and temperatures are colder in summer than in winter. Near the Equator, days are always about 12 hours long. But these areas usually have a wet season and a dry season.

Misconception Alert

People often think that the Earth is closer to the Sun during the summer. And it’s farther away during the winter. This is not correct. In fact, the Earth is closest to the Sun in January! The distance between the Earth and the Sun does not affect the seasons. Seasons change because of the tilt of the Earth and the planet’s movement around the Sun.

Did you know?

It takes about 365.25 days for the Earth to orbit the Sun. We have leap years to make up for the extra ¼ day!

How Are Seasons Different in the Northern and Southern Hemispheres?

Seasons happen at different times in different parts of the world. The tilt of the Earth doesn’t change as it rotates around the Sun. But the part of the planet that gets the most direct sunlight does change.

The Northern Hemisphere is tilted away from the Sun from September to March. That means the northern half of the planet doesn’t get as much light and heat from the Sun. This causes autumn and winter. During the same months, the Southern Hemisphere is tilted towards the Sun. That means the southern half of the planet gets spring and summer.

The movement of the Earth around the Sun, showing which part of the Earth is tilted toward the Sun in different seasons (Source: Let’s Talk Science using an image by shoo_arts via iStockphoto).

From March to September, the Northern Hemisphere is tilted towards the Sun. So that’s when the northern half of the Earth experiences spring and summer. During the same months, the Southern Hemisphere experiences autumn and winter.
Other planets also have seasons. But the length and intensity of each season varies from planet to planet.

On Earth, seasons last between 90 and 93 days.
On Venus, seasons last between 55 and 58 days.
On Mars, seasons change about once every six months. Summer lasts 199 days and winter lasts 146 days.
On Saturn, seasons last about seven years.
And if you lived on Neptune, you would have to wait more than 40 years for the seasons to change!

Many people believe that Earth is closer to the Sun in the summer and that is why it is hotter. And, likewise, they think Earth is farthest from the Sun in the winter.

Although this idea makes sense, it is incorrect.

It is true that Earth's orbit is not a perfect circle. It is a bit lop-sided. During part of the year, Earth is closer to the Sun than at other times. However, in the Northern Hemisphere, we are having winter when Earth is closest to the Sun and summer when it is farthest away! Compared with how far away the Sun is, this change in Earth's distance throughout the year does not make much difference to our weather.

There is a different reason for Earth's seasons.

Earth's axis is an imaginary pole going right through the center of Earth from "top" to "bottom." Earth spins around this pole, making one complete turn each day. That is why we have day and night, and why every part of Earth's surface gets some of each.

Earth has seasons because its axis doesn't stand up straight.

But what caused Earth to tilt?

Long, long ago, when Earth was young, it is thought that something big hit Earth and knocked it off-kilter. So instead of rotating with its axis straight up and down, it leans over a bit.

By the way, that big thing that hit Earth is called Theia. It also blasted a big hole in the surface. That big hit sent a huge amount of dust and rubble into orbit. Most scientists think that that rubble, in time, became our Moon.

As Earth orbits the Sun, its tilted axis always points in the same direction. So, throughout the year, different parts of Earth get the Sun’s direct rays.

Sometimes it is the North Pole tilting toward the Sun (around June) and sometimes it is the South Pole tilting toward the Sun (around December).

It is summer in June in the Northern Hemisphere because the Sun's rays hit that part of Earth more directly than at any other time of the year. It is winter in December in the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted toward the Sun.

Earth's lopsided orbit

Earth's perihelion (point closest to Sun) = 91,400,000 miles from Sun

Earth's aphelion (point farthest from Sun) = 94,500,000 miles from Sun

While that is a difference of over 3 million miles, relative to the entire distance, it isn’t much.

And, believe it or not, aphelion (when Earth is farthest from the Sun) occurs in July, and perihelion (when we are closest) occurs in January. For those of us who live in the Northern Hemisphere where it's summer in July and winter in January, that seems backwards, doesn't it? That just goes to prove that Earth's distance from the Sun is not the cause of the seasons.

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