Earth has seasons because its axis is tilted.
The planet does not spin straight up and down relative to the plane of its orbit around the Sun. Its axis is tilted about 23.4 degrees.
As Earth travels around the Sun during the year, that tilt changes how directly sunlight strikes each hemisphere and how many hours of daylight it receives.
Earth’s distance from the Sun is not the main reason
This is one of the most common misconceptions about seasons.
It feels logical that summer should happen when Earth is closer to the Sun and winter when it is farther away.
But the timing does not work.
Earth is actually closest to the Sun in early January, during winter in the Northern Hemisphere. It is farther away in early July, during northern summer.
The small change in orbital distance is not what creates the familiar seasonal cycle.
Tilt changes the angle of sunlight
Imagine shining a flashlight straight down onto a table. The light is concentrated in a relatively small area.
Now tilt the flashlight. The same amount of light spreads over a larger area.
Sunlight works similarly.
When a hemisphere is tilted toward the Sun, sunlight strikes it more directly. Solar energy is more concentrated, which contributes to warmer temperatures.
When the hemisphere tilts away, sunlight arrives at a lower angle and spreads over a larger area.
Tilt also changes day length
Summer is not just about the angle of the Sun.
When your hemisphere is tilted toward the Sun, the Sun stays above the horizon longer each day. Longer daylight gives the ground and atmosphere more time to absorb solar energy.
In winter, days are shorter and the Sun stays lower in the sky.
Less direct sunlight plus fewer daylight hours creates a powerful seasonal difference.
Why are the seasons opposite in the two hemispheres?
When the Northern Hemisphere tilts toward the Sun, the Southern Hemisphere tilts away.
That is why June can mean summer in the United States and winter in Australia.
Six months later, the geometry reverses.
If distance from the Sun caused the seasons, both hemispheres would have summer at the same time.
They do not.
That is another easy way to test the idea.
What happens during spring and fall?
Around the equinoxes, neither hemisphere is tilted strongly toward or away from the Sun.
Day and night are close to equal in length across much of Earth, and the angle of sunlight falls between the summer and winter extremes.
The seasons then continue shifting as Earth moves along its orbit.
Why are seasons weaker near the equator?
Near the equator, day length and the angle of sunlight change less dramatically through the year.
Higher latitudes experience much larger changes, including very long summer days and very short winter days.
That is why the difference between seasons is generally stronger as you move farther from the equator.
Does Earth’s tilt ever change?
Yes, slowly.
Earth’s axial tilt varies over very long time periods. Those changes can influence climate patterns over tens of thousands of years.
But for the everyday yearly seasons you experience, the explanation is straightforward.
Earth is tilted.
It goes around the Sun.
The tilt keeps pointing in nearly the same direction as the planet travels.
That is enough to turn one orbit into spring, summer, fall, and winter.
A related part of the natural world appears in Why Does the Moon Have Phases?, where the same kind of physical or biological reasoning answers another familiar question.
And that’s the short explanation.
Related explanations: Why Does the Moon Have Phases? and Why Do We Have Day and Night?