The Science Book

86

DISCOVERING

NEW PLANETS

WILLIAM HERSCHEL (1738–1822)

I

n 1781, German scientist

William Herschel identified

the first new planet to be seen

since ancient times, although

Herschel himself initially thought it

was a comet. His discovery would

also lead to the discovery of another

planet as a result of predictions

based on Newton’s laws.

By the late 18th century,

astronomical instruments had

advanced significantly—not least

IN CONTEXT

BRANCH

Astronomy

BEFORE

Early 1600s The lens-based

refracting telescope is

invented, but mirror-based

telescopes are not developed

until the 1660s, by Isaac

Newton and others.

1774 French observer

Charles Messier publishes his

astronomical survey, inspiring

Herschel to begin work on a

survey of his own.

AFTER

1846 Unexplained changes to

the orbit of Uranus lead French

mathematician Urbain Le

Verrier to predict the existence

and position of an eighth

planet—Neptune.

1930 US astronomer Clyde

Tombaugh discovers Pluto,

which is initially recognized as

a ninth planet, but now seen

as the brightest member of the

Kuiper Belt of small icy worlds.

through the construction of

reflecting telescopes that used

mirrors rather than lenses to gather

light, avoiding many of the problems

associated with lenses at the time.

This was the age of the first great

astronomical surveys, as astronomers

scoured the sky and identified a

wide variety of “nonstellar”

objects—star clusters and nebulae

that looked like amorphous clouds

of gas or dense balls of light.

Using Newton’s laws, it

was possible to calculate

where to look for the

new planet.

Uranus’s orbit was

irregular, suggesting that

it was being pulled

by the gravity of

another planet.

New telescopes allowed

for more detailed mapping

of the skies.

Better observations showed

a new planet in

orbit around the

Sun—Uranus.

Neptune was discovered.