Big question
Why do we have seasons?
Seasons happen because the Earth's axis is tilted about 23.5 degrees, so different parts of the planet lean toward or away from the Sun as Earth orbits it. This is not caused by distance from the Sun — Earth is actually closest to the Sun in early January.
A Tilted, Not Straight, Axis
The Earth does not spin perfectly upright as it circles the Sun; its axis, the imaginary line running from pole to pole, leans over by about 23.5 degrees. Because that tilt stays pointed in roughly the same direction in space throughout the year, different parts of Earth end up leaning toward the Sun at different points in its yearly orbit. When your hemisphere leans toward the Sun, sunlight hits it more directly and for more hours a day, bringing summer, and half a year later, when the same hemisphere leans away, sunlight arrives at a shallower angle for fewer hours, bringing winter.
It Is Not About Distance
A common but incorrect idea is that summer happens because Earth moves closer to the Sun, and winter happens because it moves farther away. In reality, Earth's orbit is only very slightly oval-shaped, and it is actually at its closest point to the Sun, called perihelion, in early January, during winter for the northern hemisphere. If distance from the Sun controlled the seasons, both hemispheres would experience summer and winter at exactly the same time, but they clearly do not: while people in Europe shovel snow in December, people in Australia are at the beach, which only makes sense once axial tilt, not distance, is understood as the real cause.
Opposite Seasons, Same Planet
Because the northern and southern hemispheres lean toward the Sun at opposite times of the year, they experience opposite seasons at exactly the same moment. When the northern hemisphere tilts toward the Sun and enjoys summer, from roughly June to August, the southern hemisphere tilts away and has winter, and the reverse happens around December. Places sitting right on the equator barely tilt toward or away from the Sun at all, which is why they experience only small changes in daylight and temperature across the year, often noticing wet and dry seasons instead of the four seasons familiar further north or south.
Try it yourself
Shine a torch at a tilted globe or a ball with a stick pushed through it at an angle, and slowly walk the torch around it while keeping the tilt pointing the same way. Watch how one half gets more direct light at some points and the other half does later.
Questions children ask
Do seasons happen because Earth gets closer to the Sun?
No, seasons are not caused by Earth's distance from the Sun; they happen because Earth's axis is tilted, so different hemispheres lean toward or away from the Sun through the year. Earth is actually closest to the Sun during northern winter.
Why are seasons opposite in the northern and southern hemispheres?
Seasons are opposite because when one hemisphere tilts toward the Sun, the other automatically tilts away at the same time, due to Earth's fixed axial tilt. That is why it can be summer in Australia while it is winter in Europe.
How tilted is the Earth's axis?
The Earth's axis is tilted at about 23.5 degrees from vertical, relative to its path around the Sun. This tilt is the main reason Earth has four distinct seasons rather than a constant climate all year.