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Chapter Introduction

Gravity is the only universal force of attraction in the universe. Despite the magnificent solitude that astronaut Bruce McCandless II experienced floating a football-field length from the space shuttle Challenger, he was being held in orbit around Earth with almost the same amount of gravitational attraction that our planet has on each of us. He is falling toward Earth but continually missing it. Why?

Gravitation and the Motion of the Planets

WHAT DO YOU THINK?

What makes a theory scientific?
What is the shape of Earth’s orbit around the Sun?
Do the planets orbit the Sun at constant speeds?
Do all of the planets orbit the Sun at the same speed?
How does an object’s mass differ when measured on Earth and on the Moon?
Do astronauts orbiting Earth feel the force of gravity from our planet?

Answers to these questions appear in the text beside the corresponding numbers in the margins and at the end of the chapter.

We begin this chapter by examining the nature of science, scientific investigation, and scientific inquiry. With the process of scientific investigation in mind, we explore how scientists came to discover that Earth and the other planets orbit the Sun. Then, in the spirit of scientific inquiry, we ask and answer the question “why do they have these orbits?”

In this chapter you will discover

what makes a theory scientific
the scientific discoveries that revealed that Earth is not at the center of the universe, as previously believed
Copernicus’s argument that the planets orbit the Sun
why the direction of motion of each planet on the celestial sphere sometimes changes
that Kepler’s determination of the shapes and other properties of planetary orbits depended on the careful observations of his mentor Tycho Brahe
how Isaac Newton formulated an equation to describe the force of gravity and how he thereby explained why the planets and moons remain in orbit