Tilted, not closer

About this video

In Canada a summer evening is still bright at nine o'clock, and in December the street lights are on by half past four. Why does the same Sun give us such different halves of the year? Almost everyone's first guess is that in summer the Earth is closer to the Sun. Two facts show it can't be. When it is summer in Canada it is winter in Australia, at the same time, on the same Earth. And the Earth is actually closest to the Sun in early January, in the middle of Canada's winter. Its orbit is almost a circle, and its distance from the Sun changes by only about 3 parts in 100 over the year.

The real reason is the tilt. The Earth spins around an imaginary line, its axis, running from the North Pole to the South Pole. The axis doesn't stand straight up: it leans over by about 23 degrees, and it keeps leaning the same way all year as the Earth travels around the Sun. In June the northern half leans toward the Sun: summer in Canada, Europe and China, winter in Australia. In December the Earth is on the other side of the Sun, still leaning the same way, so the northern half leans away.

Leaning toward the Sun does two things for your half of the Earth. Longer days: in southern Canada late June has about fifteen and a half hours of daylight, late December only about nine. And a higher Sun, whose light comes almost straight down onto a small patch of ground, instead of slanting across a bigger one. This video goes with the lesson Seasons in Astronomy Level 1 on Knaph.

Transcript

1. 0:00In Canada a summer evening is still bright at nine o'clock. In December the street lights are on at half past four. Most people guess that in summer, the Earth is closer to the Sun.
2. 0:13But when it is summer in Canada, it is winter in Australia, at the same time, on the same Earth. If closer made summer, the whole Earth would have it together.
3. 0:25And the Earth is closest to the Sun in early January, in the middle of Canada's winter. Its distance changes by only about 3 parts in 100.
4. 0:37The real reason is the tilt. The Earth's axis, the line it spins around, leans over by about 23 degrees, and it keeps leaning the same way all year. In June the northern half leans toward the Sun. In December it leans away. So the north and the south always have opposite seasons.
5. 0:58Leaning toward the Sun gives your half longer days. In southern Canada, late June has about fifteen and a half hours of daylight. Late December, only about nine.
6. 1:12And a higher Sun. Its light comes almost straight down, onto a small patch of ground. The low winter Sun's light comes in at a slant and spreads over a bigger patch, so each bit of ground gets less.
7. 1:30Longer days and stronger sunshine make summer. The seasons come from the tilt, not the distance.