The wandering dot

About this video

Look at the same patch of sky every clear night for a few weeks. The stars of a constellation keep their places: Orion has the same shape year after year. But now and then a bright "star" is in a slightly different place each night, drifting slowly across the constellations. People noticed this thousands of years ago, and the ancient Greeks called these moving lights planets, from their word for wanderers.

With your eyes alone you can see five of them: Mercury, Venus, Mars, Jupiter and Saturn. Venus is often the brightest point in the night sky after the Moon.

The planets are not stars. They are worlds that go around the Sun, as the Earth does. We watch them from the Earth, which is moving along its own orbit too, so night after night we see each planet against a slightly different part of the far-away stars. The stars are so far that they keep their places for our whole lives.

Planets make no light of their own. Like the Moon, a planet is lit by the Sun and we see the sunlight it bounces back. Stars twinkle as the air above us moves; planets usually shine with a steadier light, which is an easy way to tell them apart.

There are eight planets. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The Earth is the third. Uranus and Neptune are too faint for the eye and were found with telescopes, in 1781 and 1846.

The closer a planet is to the Sun, the faster it goes around. Mercury takes 88 days, the Earth a year, and Neptune about 165 years.

Transcript

1. 0:00Watch the same stars for a few weeks. They stay in their places. But now and then, one bright star doesn't.
2. 0:11The ancient Greeks called these moving lights planets. It means wanderers.
3. 0:20With your eyes alone you can see five of them. Mercury, Venus, Mars, Jupiter and Saturn.
4. 0:32Venus is the brightest point in the night sky after the Moon. They look like stars, but they move across the constellations.
5. 0:42Why do they wander? They are not stars. They are worlds going around the Sun, like the Earth.
6. 0:55And we watch them from the Earth, which is moving too. So night after night, we see each planet against different stars.
7. 1:10Planets make no light of their own. Like the Moon, a planet shines with sunlight that it bounces back to us.
8. 1:22Stars twinkle as the air above us moves. Planets usually shine with a steadier light.
9. 1:32There are eight planets. In order from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
10. 1:44The Earth is the third. Uranus and Neptune are too faint for the eye. They were found with telescopes, in 1781 and 1846.
11. 1:58The closer a planet is to the Sun, the faster it goes around. The Earth takes a year.
12. 2:09Mercury takes only eighty-eight days. Neptune takes about a hundred and sixty-five years.
13. 2:22Planets are worlds that go around the Sun and shine with its light. The Earth is one of them.