The Solar System

76 PART 1^ |^ EXPLORING THE SKY

lead of the Catholic countries, had adopted the Gregorian calen-

dar, but Protestant England continued to use the Julian calendar.

So December 25 in England was January 4 in Europe. If you use

the English date, then Newton was born in the same year that

Galileo Galilei died.

Newton became one of the greatest scientists who ever lived,

but even he admitted the debt he owed to those who had studied

nature before him. He said, “If I have seen farther than other

men, it is because I stood on the shoulders of giants.” In the

previous chapter, you learned about Galileo as the defender of

Copernicanism who made the fi rst use of an astronomical tele-

scope, but he also was the fi rst scientist who carefully studied the

motions of falling bodies. Th at was the key information that led

Newton to understand gravity.

Galileo and Motion

Galileo (■ Figure 5-2) began studying the motion of freely mov-

ing bodies even before he built his fi rst telescope. After the

Inquisition condemned and imprisoned him in 1633, he contin-

ued his study of motion. He seems to have realized that he would

Nature and Nature’s laws lay hid in night:

God said, “Let Newton be!” and all was

light.

— ALEXANDER POPE

I

sn’t it weird that Isaac Newton is said to have “discovered”
gravity in the late 17th century—as if people didn’t have gravity
before that, as if they fl oated around holding onto tree branches?
Of course, everyone experienced gravity without noticing it.
Newton’s insight was to see gravity as a force that both makes things
fall, and keeps moons and planets in their orbits, and that realiza-
tion changed the way people thought about nature (■ Figure 5-1).

Galileo and Newton

Isaac Newton was born in Woolsthorpe, England, on December
25, 1642, and on January 4, 1643. Th is was not a biological
anomaly but a calendrical quirk. Most of Europe, following the

5-1

■ Figure 5-1

Space stations and astronauts, as well as planets, moons, stars, and galaxies, follow paths called orbits that are described by three
simple laws of motion and a theory of gravity fi rst understood by Isaac Newton (1642–1727). Newtonian physics is adequate to send
astronauts to the moon and to analyze the rotation of the largest galaxies. (NASA)