Madrid's circle

About this video

On 21 June, three people in three cities watch the evening sky. In Singapore, near the equator, the Sun goes down a little after seven o'clock, as on every day of the year. In Madrid it is still light at nine, and the Sun goes down at almost ten. In Tromsø, in the far north of Norway, it doesn't go down at all. The Earth takes 24 hours to spin once for all three of them, so why are their days so different?

Half of the Earth is always lit by the Sun, and the spin carries every place through the lit half and then through the dark half. As the Earth spins, a city like Madrid goes once all the way around the Earth's axis: its path is a circle. If the axis stood straight up, the line between day and night would cut that circle into two equal halves, and Madrid would have 12 hours of day and 12 of night every day of the year.

But the axis is tilted, so the line cuts Madrid's circle into two unequal parts. In June the north end of the axis leans toward the Sun, and the bigger part is in the light: about 15 hours of day and 9 of night. Six months later the axis still leans the same way, but now the north end leans away from the Sun, and Madrid gets about 9 hours of day. In the southern half of the Earth it is the other way round: in December places like Melbourne have long days.

The line between day and night always goes through the middle of the Earth, so however the Earth leans it cuts the equator into two equal halves. That is why Singapore has about 12 hours of day and 12 of night all year. The farther a place is from the equator, the bigger the difference: Madrid about 15 and 9 hours, Oslo almost 19 and about 6. Close to the North Pole the circles are small. In June they stay in the light for the whole turn, the midnight sun; in December they stay in the dark, the polar night.

Transcript

1. 0:00On the twenty-first of June, three people in three cities watch the evening sky.
2. 0:06In Singapore, the Sun goes down a little after seven.
3. 0:10In Madrid, it is still light at nine. The Sun sets at almost ten.
4. 0:15In Tromsø, in the far north of Norway, it doesn't set at all.
5. 0:20One spin takes 24 hours for all three. So why?
6. 0:24Follow Madrid as the Earth spins. It goes once around the axis.
7. 0:29Its path is a circle.
8. 0:31If the axis stood straight up, the line between day and night would cut that circle in two equal halves.
9. 0:39Twelve hours of day, twelve of night, every day of the year.
10. 0:46But the axis is tilted.
11. 0:48Now the line cuts Madrid's circle into two unequal parts.
12. 0:53In June the north end leans toward the Sun.
13. 0:57So the bigger part of the circle is in the light.
14. 1:02About fifteen hours of day, and nine of night.
15. 1:12Six months later, the Earth is on the other side of the Sun.
16. 1:17The axis leans the same way, so now the north leans away.
17. 1:22The bigger part of Madrid's circle is in the dark. About nine hours of day.
18. 1:28In the southern half it is the other way round. Melbourne has long days in December.
19. 1:36The equator is different.
20. 1:38The line between day and night always goes through the middle of the Earth.
21. 1:44So however the Earth leans, it cuts the equator into equal halves.
22. 1:49Singapore, near the equator, has about twelve hours of day all year.
23. 1:56The farther from the equator, the bigger the difference.
24. 2:00Singapore, twelve and twelve. Madrid, fifteen and nine.
25. 2:04Oslo, almost nineteen hours, and about six.
26. 2:08And Tromsø. No sunset in summer, no sunrise in winter.
27. 2:20Close to the North Pole, the circles are small.
28. 2:24In June, a small circle stays in the light for the whole turn.
29. 2:29The Sun is up all day and all night. The midnight sun.
30. 2:34In December, the same circle stays in the dark. The polar night.
31. 2:42Half the Earth is always lit. The tilt decides how much of your circle is in the light.