For more than 1,000 years, most people believed the Earth sat still at the center of everything, with the Sun, Moon and stars circling around it. It certainly looks that way. In 1543, a Polish astronomer named Nicolaus Copernicus published a book that said the opposite: the Earth spins, and it travels around the Sun.
Key idea
In 1609 and 1610, Galileo Galilei built telescopes and looked at the night sky. He saw mountains on the Moon, countless stars in the Milky Way, and four moons circling Jupiter. Those moons showed that not everything circles the Earth. Galileo's ideas brought him into a long conflict with Church authorities, who put him on trial in 1633.
In 1687, Isaac Newton showed that the same force, gravity, makes an apple fall and keeps the Moon going around the Earth. His laws of motion still help engineers build bridges and send spacecraft to other planets.
Worked example: Test it
- Drop two balls of the same size but different weights from the same height at the same time.
- Listen for them to hit the ground. (They land together, unless air slows one a lot.)
Answer: With an experiment: drop them together and watch.
Common mistake
Try it at home
- On a clear night, look at the Moon with binoculars. Can you see mountains and craters, as Galileo did? (Never look at the Sun.)
- Drop a heavy ball and a light ball of the same size together from the same height. Which lands first?