Why We Have Seasons

Ask people why summer is warm and many will say: "Because Earth is closer to the Sun." It sounds reasonable — but it's wrong. In fact, Earth is closest to the Sun in early January, in the middle of the Northern Hemisphere's winter.

So what really causes the seasons?

The distance myth

Earth's orbit is almost a circle. At its closest point (perihelion, around January 3) Earth is about 147 million km from the Sun. At its farthest (aphelion, around July 4) it is about 152 million km away. That is only a 3% difference — far too small to explain the gap between a Canadian January and a Canadian July.

And if distance were the cause, both hemispheres would have summer at the same time. They don't: when it's summer in Canada, it's winter in Australia.

The real reason: a tilted Earth

Earth's axis is tilted about 23.4 degrees compared to its orbit. As Earth travels around the Sun, the axis keeps pointing the same direction in space — toward the North Star.

That means for half the year the Northern Hemisphere leans toward the Sun, and for the other half it leans away.

Tilt changes two things

1. How directly sunlight hits the ground. When the Sun is high in the sky, its light hits the ground straight on and concentrates its energy on a small area. When the Sun is low, the same beam of light is spread across a much larger area, so each square metre gets less heat. Think of a flashlight pointed straight down versus shining at a steep angle.

2. How long the day lasts. The hemisphere tilted toward the Sun gets long days and short nights, so the ground has more hours to warm up and fewer hours to cool down. In Vancouver, a June day has about 16 hours of daylight; a December day has about 8.

More direct light plus longer days means summer. Slanted light plus short days means winter.

Four key moments in the year

• June solstice (around June 21): the Northern Hemisphere is tilted most toward the Sun. Longest day in the north, shortest in the south.
• September equinox (around September 22): neither hemisphere leans toward the Sun. Day and night are roughly equal everywhere.
• December solstice (around December 21): the Southern Hemisphere is tilted most toward the Sun. Summer begins in Australia; winter begins in Canada.
• March equinox (around March 20): equal day and night again, and the cycle repeats.

Why the equator and the poles are different

Near the equator, the Sun is high in the sky all year and day length barely changes, so there's little difference between "summer" and "winter" — seasons there are often defined by rainfall instead.

Near the poles, the tilt has an extreme effect: in summer the Sun never sets (the midnight sun), and in winter it never rises (the polar night).

In short

Seasons are not about distance. They happen because Earth's tilted axis changes how directly sunlight strikes each hemisphere and how many hours of daylight it gets. Tilt toward the Sun: summer. Tilt away: winter.