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Notes from the field ·

Why Are Summer Days Longer? Tilt, Latitude and the Midnight Sun

By Ahmed, founder

In midsummer, the sky is still light at 10 p.m. in some places. In midwinter, it’s dark by mid-afternoon. And near the equator, days barely change. Why?

Day length depends on two things

  1. Earth’s tilt (about 23.5°), which stays pointed the same direction as Earth orbits the Sun.
  2. Your latitude, how far you are from the equator.

What tilt does

In June, the Northern Hemisphere leans toward the Sun. As Earth rotates, a place in the north spends more of its day in sunlight than in shadow. In December, the same place leans away, and spends less of the day in the light.

Try it with a ball and a lamp. Tilt the ball toward the lamp, put a sticker on a northern city, and spin the ball slowly. The sticker stays in the light for more than half a turn. Now tilt it away, and the sticker is in the dark for more than half a turn.

What latitude does

  • At the equator: day and night are about 12 hours each, all year.
  • At mid-latitudes (such as 51°N, near Calgary): about 16 hours of daylight in June and about 8 in December.
  • Near the poles: the effect is extreme. North of the Arctic Circle, the Sun doesn’t set around the June solstice, the midnight Sun. In December, it doesn’t rise.

The equinoxes

Twice a year, around March 20 and September 22, day and night are nearly equal everywhere. “Equinox” means “equal night.”

Southern Hemisphere

Everything is reversed. December is long days in Australia, Argentina and South Africa, and June is short.

An activity

Give teams a real city’s day lengths at the equinoxes and solstices. Line up across the room from the Arctic to the far south, and have each team step in order of latitude. Notice how the day lengths change smoothly as you move. That’s the pattern a model needs to explain.

Common mix-ups

  • “Days are longer because the Sun is closer.” It isn’t. Earth is actually farther away in July.
  • “Days get longer because Earth spins slower.” Rotation is steady.

The full lesson is Why are summer days longer? in The Reason for the Seasons. Read more about the cause in our guide to why we have seasons. Sign up for three lessons free.