Why Do We Have Seasons?

📚 Study helper: This page helps you learn and practice for school. Read it, listen to it, and try to answer this: why does tilted Earth give us summer and winter?

Throughout the year, different parts of Earth receive the Sun's most direct rays.

When the North Pole tilts toward the Sun, it is summer in the Northern Hemisphere.

And when the South Pole tilts toward the Sun, it is winter in the Northern Hemisphere.

Elementary: A Tilted Earth Makes the Seasons

Earth does not stand up straight as it spins. It leans a bit. That lean is why we have seasons.

Earth's axis — an imaginary pole

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, NASA Space Place says.

And that is why every part of Earth's surface gets some of each.

The tilt

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

It leans over a bit.

NASA Science says you can picture Earth like a spinning top tilted to one side.

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.

That big thing that hit Earth is called Theia.

The big hit sent a huge amount of dust and rubble into orbit.

Most scientists think that that rubble, in time, became our Moon, NASA Space Place says.

The axis points the same way all year

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.

June and December

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.

Opposite ends, opposite seasons

The poles are on opposite ends of Earth, NASA Space Place says.

So they experience opposite timing for the seasons.

When a part of Earth tilts toward the Sun, it has summer. When it tilts away, it has winter.

So when the north has summer, the south has winter.

Is it because Earth gets closer to the Sun?

Many people believe that Earth is closer to the Sun in the summer and that is why it is hotter.

Although this idea makes sense, it is incorrect, NASA Space Place says.

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.

Earth's distance from the Sun is not the cause of the seasons.

Middle School: Seasons, Solstices, and Equinoxes in More Detail

NASA Science says Earth's axis of rotation is the line going through the North and South geographic poles around which Earth rotates.

The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane, NASA Science says.

The substantial tilt, about 23.5 degrees, is thought to be the result of an ancient planetary body, Theia, smashing into Earth about 4.5 billion years ago, in the same collision that formed the Moon, NASA Science says.

As Earth orbits the Sun, its tilted axis always points in the same direction. So the northern and southern hemispheres are tilted either toward or away from the Sun at different times of year.

Toward the Sun means summer

When a hemisphere is tilted toward the Sun, it experiences summer; when it's tilted away, it experiences winter, NASA Science says.

Spring and autumn arrive when the hemispheres are roughly equal in their tilt toward the Sun, with spring following winter as a hemisphere starts tilting toward the Sun and autumn following summer as it starts to tilt away.

Earth's tilt means that places farther away from the equator receive more or less sunlight during the summer and winter months.

For example, the High Arctic can have almost 24 hours of daylight in the middle of summer, or almost 24 hours of darkness in the middle of winter, NASA Science says.

The solstices: June and December

Around the June solstice, the Northern Hemisphere leans toward the Sun, bringing more direct sunlight and longer days.

Around the December solstice, the Southern Hemisphere does the same.

That's why June ushers in summer and warm weather in the Northern Hemisphere, while December does so in the Southern Hemisphere, NASA Science says.

The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere.

It is not the Sun that is moving north or south through the seasons, NASA Science says. It is a change in the orientation and angles between the Earth and its nearest star.

The equinoxes: March and September

The March and September equinoxes serve as the midpoints between these two seasonal extremes, NASA Science says.

On those days, the terminator — the boundary between the sunlit and dark sides of Earth — runs directly through both poles.

As a result, the Northern and Southern Hemispheres receive almost the same amount of sunlight, and day and night are nearly equal in length.

At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.

Equinox means "equal night" in Latin, NASA Science says. It captures the idea that daytime and nighttime are equal lengths everywhere on the planet.

The shape of the terminator line between night and day varies with the seasons, which means different lengths of days and differing amounts of warming sunshine.

How the Sun's path changes in North America

If you live in North America, during the winter the sun's path across the sky is more toward the south, rising in the southeast and setting in the southwest, NOAA says.

During the summer, the sun tracks more directly overhead, rising in the east, overhead at noon, and setting in the west.

The distance proof, with NASA Space Place's numbers

Earth's orbit is not a perfect circle. It is a bit lop-sided, so during part of the year, Earth is closer to the Sun than at other times.

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, NASA Space Place says.

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.

Words to Know

Axis: Earth's axis is an imaginary pole going right through the center of Earth from "top" to "bottom." Earth spins around this pole. NASA Science also calls it the line going through the North and South geographic poles around which Earth rotates.

Tilt: Tilt means leaning over. Earth's axis doesn't stand up straight. It leans over a bit, and that lean causes the seasons.

Hemisphere: A hemisphere is one half of Earth. The Northern Hemisphere is the northern half of the planet. The Southern Hemisphere is the southern half.

Orbit: To orbit is to travel around something. Earth orbits the Sun, and as it travels through the year, different parts of Earth get the Sun's direct rays.

Direct rays: The Sun's rays that hit a part of Earth most straight-on. In summer, the sun's rays hit that region more directly than at any other time of the year.

Solstice: The June solstice is when the Northern Hemisphere leans toward the Sun. The December solstice is when the Southern Hemisphere does the same.

Equinox: The March and September equinoxes are the midpoints between the solstice extremes. Day and night are nearly equal in length. Equinox means "equal night" in Latin.

Terminator: The terminator is the boundary between the sunlit and dark sides of Earth. NASA Science also calls it the edge between the shadows of nightfall and the sunlight of dusk and dawn.

Perihelion: Perihelion is Earth's point closest to the Sun.

Aphelion: Aphelion is Earth's point farthest from the Sun.

Theia: Theia is the big thing that hit Earth long ago. Scientists think that hit tilted Earth, and that rubble from the hit became our Moon.

Ecliptic plane: NASA Science calls it the flat "plane" going through the Sun's equator, along which most planets' orbits can be roughly traced. Earth's tilt is measured compared to this plane.

📚 Study helper: Great job! Now try this. Finish the sentence: Earth has seasons because its axis is ______. (Tilted!) Then answer this: when the North Pole tilts toward the Sun, what season is it in the Northern Hemisphere? Summer!

Sources

NASA Space Place — What Causes the Seasons?: https://spaceplace.nasa.gov/seasons/en/

NOAA NESDIS — Why does Earth have Seasons?: https://www.nesdis.noaa.gov/about/k-12-education/understanding-our-planet/why-does-earth-have-seasons

NASA Science — Helio and You: Seasons on Earth, Mars, and Beyond: https://science.nasa.gov/helio-and-you-seasons-on-earth-mars-and-beyond/

NASA Science — An Epic View of the Seasons: https://science.nasa.gov/earth/earth-observatory/an-epic-view-of-the-seasons/

NASA Science — Seeing Equinoxes and Solstices from Space: https://science.nasa.gov/earth/earth-observatory/seeing-equinoxes-and-solstices-from-space-52248/

Why Do We Have Seasons? — For Kids