The Astronomy Book

68

G

ravity is the name

given to the force of

attraction between any

two masses. It is the force that

attracts all objects to Earth, giving

them weight. It draws objects

downward, toward the center

of Earth. If the object were on

the moon, a much smaller mass

than Earth, the force would be six

times less and its weight would

be one sixth of its weight on Earth.

English physicist, astronomer, and

mathematician Isaac Newton was

the first person to realize that

gravity is a universal force, acting

on all objects, and that it explains

the movement of planets.

Describing orbits

The shapes of the orbits of the

planets were already well-known

in Newton’s time, based on the

three laws of planetary motion

introduced by Johannes Kepler.

Kepler’s first law stated that these

orbits were ellipses, with the sun

at one focus of each ellipse. The

second law described the way that

planets moved along their orbits

more quickly when they were

close to the sun than when they

were farther away. The third law

described the relation between

the time taken to complete one

orbit and the distance from the

sun: the time taken for one orbit,

squared, was equal to the cube of

the average distance between the

planet and the sun. For instance,

Earth goes around the sun in one

year, while Jupiter is 5.2 times

farther away from the sun than

Earth. 5.2 cubed equals 140, and

the square root of 140 gives the

correct figure for one Jupiter year:

11.86 Earth years.

However, although Kepler

had correctly discovered the

shapes and speeds of planetary

GRAVITATIONAL THEORY

IN CONTEXT

KEY ASTRONOMER

Isaac Newton (16 4 2 –172 6)

BEFORE

1609 Johannes Kepler

shows that Mars has an

elliptical orbit.

AFTER

1798 Henry Cavendish

measures the gravitational

constant for the first time.

1846 French mathematician

Urbain Le Verrier uses

Newton’s laws to calculate

the planet Neptune’s position.

1915 Albert Einstein

introduces general relativity

and explains the gravitational

force as a function of the

curvature of spacetime.

2014 The gravitational constant

is measured by studying the

behavior of atoms. The latest

figure is given as 6.6719 × 10−11

m^3 kg−1 s−2. This is about

1 percent less than the value

Henry Cavendish calculated.

Isaac Newton Isaac Newton was born on a farm

in Woolsthorpe, Lincolnshire, on

December 25, 1642. After school

in Grantham, he attended Trinity

College Cambridge, where he

became a Fellow and taught

physics and astronomy. His book

Principia set out the principle of

gravity and celestial mechanics.

Newton invented the reflecting

telescope; wrote theses on optics,

the prism, and the spectrum

of white light; was one of the

founders of calculus; and studied

the cooling of bodies. He also

explained why Earth was oblate

(a squashed sphere) in shape and

why the equinox moved, and

formalized the physics of the

speed of sound. He spent much

time on biblical chronology and

alchemy. Newton was at various

times President of the Royal

Society, Warden and Master

of the Royal Mint, and member

of parliament for Cambridge

University. He died in 1727.

Key works

1671 Method of Fluxions

1687 Philosphiae Naturalis

Principia Mathematica

1704 Optics

To myself I am only a child

playing on the beach, while

vast oceans of truth lie

undiscovered before me.

Isaac Newton