41
See also: Nicolaus Copernicus 34–39 ■ Jeremiah Horrocks 52 ■
Isaac Newton 62–69^
SCIENTIFIC REVOLUTION
would prove vital to resolving the
problems. A bright supernova
explosion seen in the constellation
of Cassiopeia in 1572 undermined
the Copernican idea that the
universe beyond the planets was
unchanging. In 1577, Brahe plotted
the motion of a comet. Comets
had been thought of as local
phenomena, closer than the Moon,
but Brahe’s observations showed
that the comet must lie well beyond
the Moon, and was in fact moving
among the planets. In one stroke,
this evidence demolished the idea
of “heavenly spheres.” However,
Brahe remained wedded to the idea
of circular orbits in his geocentric
(Earth-centered) model.
In 1597, Brahe was invited to
Prague, where he spent his last
years as Imperial Mathematician
to Emperor Rudolph II. Here he
was joined by German astrologer
Johannes Kepler, who continued
Brahe’s work after his death.
Breaking with circles
Kepler had already begun to
calculate a new orbit for Mars from
Brahe’s observations, and around
this time concluded that its orbit
must be ovoid (egg-shaped) rather
than truly circular. Kepler
formulated a heliocentric model
with ovoid orbits, but this still did
not match the observational data.
In 1605, he concluded that Mars
must instead orbit the Sun in an
ellipse—a “stretched circle” with
the Sun as one of two focus points.
In his Astronomia Nova (New
Astronomy) of 1609, he outlined two
laws of planetary motion. The first
law stated that the orbit of every
planet is an ellipse. The second law
stated that a line joining a planet to
the Sun sweeps across equal areas
during equal periods of time. This
means that the speed of the planets
increases the closer they are to the
Sun. A third law, in 1619, described
the relationship of a planet’s year
to its distance from the Sun: the
square of a planet’s orbital period
(year) is proportional to the cube
of its distance from the Sun. So a
planet that is twice the distance
from the Sun than another planet
will have a year that is almost
three times as long.
The nature of the force keeping
the planets in orbit was unknown.
Kepler believed it was magnetic,
but it would be 1687 before Newton
showed that it was gravity. ■Johannes Kepler
Born in the city of Weil der
Stadt near Stuttgart, southern
Germany, in 1571, Johannes
Kepler witnessed the Great
Comet of 1577 as a small
child, marking the start of
his fascination with the
heavens. While studying at
the University of Tübingen,
he developed a reputation as
a brilliant mathematician and
astrologer. He corresponded
with various leading
astronomers of the time,
including Tycho Brahe,
ultimately moving to Prague
in 1600 to become Brahe’s
student and academic heir.
Following Brahe’s death in
1601, Kepler took on the post
of Imperial Mathematician,
with a royal commission to
complete Brahe’s work on the
so-called Rudolphine Tables
for predicting the movements
of the planets. He completed
this work in Linz, Austria,
where he worked from 1612
until his death in 1630.Key works1596 The Cosmic Mystery
1609 Astronomia Nova
(New Astronomy)
1619 The Harmony of
the World
1627 Rudolphine TablesKepler’s laws state
that planets follow
elliptical orbits with
the Sun as one of the
two foci of the ellipse.
In any given time, t,
a line joining the
planets to the Sun
sweeps across
equal areas (A)
in the ellipse.
t ttSunAA APlanetFocus Focus