75

Isaac Newton, shown

here, could explain orbital

motion using gravity.

Gravity

5

Guidepost
If only Renaissance astronomers had understood gravity, they wouldn’t have had so
much trouble describing the motion of the planets, but that insight didn’t appear until
three decades after the trial of Galileo. Isaac Newton started from the work of Galileo
and devised a way to explain motion and gravity that allowed astronomers to understand
orbits and tides. Later, in the early 20th century, Albert Einstein found an even better
way to describe gravity.
This chapter is about gravity, the master of the universe. Here you will fi nd answers to
fi ve essential questions:

What happens when an object falls?

How did Newton discover gravity?

How does gravity explain orbital motion?

How does gravity explain tides?

How did Einstein better describe motion and gravity?

Gravity rules. The moon orbiting Earth, matter falling into black holes, and the overall
structure of the universe are dominated by gravity. As you study gravity, you will see
science in action.