English scientist Isaac Newton discovered gravity while observing how an apple fell from a tree. He realized there must be a gravitational force pulling the apple from the tree to the ground. Likewise, he determined that the same gravitational force was holding the universe together and making the planets orbit the sun. Like his predecessor Kepler, Newton reasoned that the planets rotated in an elliptical orbit, not a circular one.
Can ground-based telescopes compete with space-based telescopes?
Answered by Dr. Gerard van Belle
What did Hubble's law establish about our universe?
Answered by Science Channel
How did Cepheids become measuring posts?
Answered by Discovery Channel