The Science Book

40

THE ORBIT OF

EVERY PLANET

IS AN ELLIPSE

JOHANNES KEPLER (1571–1630)

W

hile the work of Nicolaus

Copernicus on celestial

orbits, published in

1543, made a convincing case for a

heliocentric (Sun-centered) model

of the universe, his system suffered

from significant problems. Unable

to break free from ancient ideas

that heavenly bodies were mounted

on crystal spheres, Copernicus had

IN CONTEXT

BRANCH

Astronomy

BEFORE

150 CE Ptolemy of Alexandria

publishes the Algamest, a

model of the universe built

on the assumption that Earth

lies at its center and the

Sun, Moon, planets and

stars revolve around it in

circular orbits on fixed

celestial spheres.

16th century The idea of

a Sun-centered cosmology

begins to gain followers

through the ideas of

Nicolaus Copernicus.

AFTER

1639 Jeremiah Horrocks uses

Kepler’s ideas to predict and

view a transit of Venus across

the face of the Sun.

1687 Isaac Newton’s laws of

motion and gravitation reveal

the physical principles that

give rise to Kepler’s laws.

This suggests

that heavenly bodies are

not attached to fixed

celestial spheres.

The orbit of every

planet is an ellipse.

The birth of a new

star in a constellation

shows that the heavens

beyond the planets are

not unchanging.

Observations of comets

show that they move

among the planets,

crossing their orbits.

If the planets are not

fixed onto spheres, an

elliptical orbit around the

Sun best explains their

observed motion.

stated that the planets orbited the

Sun on perfect circular paths, and

was forced to introduce a variety

of complications to his model to

account for their irregularities.

Supernova and comets

In the latter half of the 16th century,

Danish nobleman Tycho Brahe

(1546–1601) made observations that