The Astronomy Book

106

THE PLANET

WHOSE POSITION

YOU HAVE POINTED

OUT ACTUALLY EXISTS

THE DISCOVERY OF NEPTUNE

IN CONTEXT

KEY ASTRONOMER

Urbain Le Verrier (1811–1877)

BEFORE

March 1781 William Herschel

discovers Uranus.

August 1781 Finnish−Swedish

astronomer Anders Lexell finds

irregularities in Uranus’s orbit

and suggests that they are due

to other, undiscovered planets.

1799–1825 Pierre-Simon

Laplace explains perturbations

mathematically.

1821 French astronomer Alexis

Bouvard publishes predictions

of future positions of Uranus.

Subsequent observations

deviate from his predictions.

AFTER

1846 Briton William Lassell

discovers Triton, Neptune’s

largest moon, only 17 days after

the discovery of the planet.

1915 Albert Einstein explains

perturbations in the orbit of

Mercury using relativity.

I

n the months following William

Herschel’s discovery of Uranus

in 1781, astronomers found

irregularities, or perturbations,

in its orbit. Most perturbations in

orbits are caused by the gravitational

effects of other large bodies, but with

Uranus there were no known planets

that could cause the observed

motion. This led some astronomers

to suggest that there must be a

planet orbiting beyond Uranus.

Searching for the invisible

Frenchman Urbain Le Verrier tackled

the problem of the perturbations of

Uranus by assuming the location of

an undiscovered planet and using

Newton’s law of gravity to work out

what its effect might be on Uranus.

This prediction was compared to

observations of Uranus, and the

position was revised according to

the planet’s movements. After many

repetitions of this process, Le Verrier

established the likely position of

an unknown planet. He presented

his ideas before the Académie des

Sciences in 1846, and he also sent

his predictions to Johann Galle

(1812–1910) at the Berlin Observatory.

Galle received Le Verrier’s letter

on the morning of September 23,

1846, and obtained permission to

Calculations of Uranus’s

predicted orbit took into

account the gravitational

effects of the sun, Jupiter,

and Saturn. However, the

observed orbit deviated

from the calculations in a

way that suggested the pull

of another massive body

farther out from the sun.

Sun

Saturn

Jupiter

Uranus

Unknown body

Gravitational

pull