2. Mr. Harper’s science mini lesson with audio Click on the speaker icon for sound

3. h Our planet is always moving. Earth spins around, or rotates, once every 24 hours. That’s why we have day and night.

4. h About half of each 24 hour period, our part of Earth is turned toward the sun.

5. h h Sunlight shines through our atmosphere and the sky appears to be bright blue.

6. h During the other half of each 24 hour period, our part of Earth is turned away from the sun and the sky is dark. This allows us to see stars and other planets.

7. 365 days means Earth rotates (spins) 365 times during each orbit around the sun. As it rotates, Earth is also revolving (orbiting) around the sun like a ball on a very long string. One orbit takes about 365 days (1 earth year).

8. h The sun’s gravity is the reason Earth and the other planets don’t fly off into space. If Earth slowed down it would fall into the sun. If it went faster it would fly out into space. Earth is moving at exactly the right speed to stay in orbit around the sun.

9. h h Mercury 88 days Venus 244 days Earth 365 days Pluto 90728 days ( 248 years ) Mars 687 days Venus 244 days Jupiter 4332 days ( 12 years ) Uranus 30688 days ( 84 years ) Saturn 10759 days ( 29 years ) Neptune 60182 days ( 168 years ) Just like a shorter string would make a ball go around faster, a smaller orbit makes a planet go around the sun faster.

10. h h Mercury 88 days Venus 244 days Earth 365 days Pluto 90728 days ( 248 years ) Mars 687 days Venus 244 days Jupiter 4332 days ( 12 years ) Uranus 30688 days ( 84 years ) Saturn 10759 days ( 29 years ) Neptune 60182 days ( 168 years ) Each planet orbits at just the right speed to stay in orbit. This picture makes the planets look closer to the sun than they really are.

11. Because they orbit the sun at different rates of speed, the planets seem to come and go every few weeks. When a planet moves to the other side of the sun, we can’t see it. h

12. h But we are also moving in other ways. Our whole solar system is orbiting the center of the Milky Way galaxy. A galaxy is a spinning group of billions of stars. This picture shows the location of our sun, but it’s too small to see. The bright spot in the center is a concentration of several billion stars (suns) not just one. It takes 250 million years for our solar system to complete one orbit around the center of our galaxy.

13. h But our galaxy is moving, too. There are billions of galaxies, all expanding away from the center of the universe.