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Why we have seasons

It is not the distance from the Sun: it is the tilt. Straighten Earth’s axis and watch the seasons disappear.

The trip around the Sundrag to turn the view
Seen from above: the Sun in the middle, Earth along its orbit, and the four main positions. In all four, Earth’s axis points in the same direction.
Earth up closedrag to turn the view
Earth seen up close, with its tilted axis and the line between day and night. The Sun is 23.4 degrees above the equator.
At 41° north14h 57mof daylight in a day
At 41° south9h 03mof daylight in a day
The Sun is above23.4°the equator, to the north
Where we are along the orbit June solstice
How much the axis tilts: 23.4 degrees

Above 66.6 degrees of latitude there is at least one day a year when the Sun does not set, and one when it does not rise. That line is the polar circle, and it is not a number to memorize: it is 90 minus the tilt. Move the slider and watch it shift.

Which latitude you are watching from

Why it is not the distance

Earth spins tilted, and its axis always points in the same direction: it is the planet that changes sides, not the axis that bends. For half the year the North Pole leans toward the Sun and days in the north are long; for the other half it leans away and days are short. In the view from above, the four light-colored Earths all have parallel axes: that is the picture to keep in mind.

And the distance from the Sun? It really does change, between 91.4 and 94.5 million miles, but that is only 3 percent. And above all: when it is winter in the United States, it is summer in Australia. The distance from the Sun at that moment is the same for both, so it cannot be what decides.

And one detail settles it: Earth passes closest to the Sun on January 3, in the middle of our winter, and farthest on July 4, in the middle of our summer. If distance made the seasons, ours would be the other way around.

The proof is one slider away: bring the tilt down to zero. Earth keeps moving closer to and farther from the Sun exactly as before, but the seasons disappear.

Open the card on the seasons

The numbers come from the NASA Planetary Fact Sheet, line by line.