Borrowed light

About this video

Picture driving at night on a country road in Canada, far from any town. There are no street lamps, only forest on both sides and your headlights. On a night with no Moon, everything past the headlights is black. On a night with a full Moon, you can see the road ahead, the trees, the fields, even your own shadow. For almost all of history there were no street lamps anywhere, and on a clear night the Moon was the brightest light there was.

So what is it? The Moon is a ball of rock, about a quarter as wide as the Earth, and it travels around the Earth about once a month.

With your eyes it looks like a smooth, bright ball with a few grey patches. Through a telescope you see that its surface is rough. It is covered in craters, round holes, some wider than a whole city, and it has mountains too.

Here is the surprise: the Moon makes no light of its own. Moonlight is sunlight that has bounced off the Moon's grey rocks and come to us. Like the Earth, the Moon always has one half lit by the Sun and one half dark, which is also why it seems to change shape through the month.

The Moon isn't only a night-time thing either: it is up in the daytime about as often as at night. Next time you are outside in the afternoon, look for a pale Moon in the blue sky.

You can see the lit half for yourself with a lamp for the Sun and an orange for the Moon. In a dark room one half of the orange is bright and one half is dark, and wherever you move it around the lamp, the half facing the lamp is the bright one.

Transcript

1. 0:00Picture a country road in Canada, at night, far from any town.
2. 0:05No street lamps. With no Moon, everything past your headlights is black.
3. 0:10Now a full Moon rises. You can see the road, the trees, even your own shadow.
4. 0:16For most of history, the Moon was the brightest light at night.
5. 0:22So what is it? A ball of rock, about a quarter as wide as the Earth.
6. 0:30It travels around the Earth, about once a month.
7. 0:46With your eyes, it looks smooth, with a few grey patches.
8. 0:51Through a telescope, its surface is rough.
9. 0:54It is covered in craters, round holes, some wider than a whole city. And it has mountains too.
10. 1:10Here is the surprise. The Moon makes no light of its own.
11. 1:16Moonlight is sunlight that bounced off the Moon's grey rocks and came to us.
12. 1:24Like the Earth, the Moon always has one half lit by the Sun, and one half dark.
13. 1:38And it isn't only out at night.
14. 1:41The Moon is up in the daytime about as often as at night.
15. 1:47Next time you are outside in the afternoon, look for a pale Moon.
16. 1:58Try it at home, with a lamp for the Sun and an orange for the Moon.
17. 2:05In a dark room, one half of the orange is bright, and one half is dark.
18. 2:12Move it around the lamp. The half facing the lamp is always the bright one.
19. 2:22The Moon is a ball of rock that lights our nights, with light borrowed from the Sun.