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THE MEDIEVAL ARISTOTELIAN WORLD VIEW
A geocentric system
The Sun and the
planets
The sphere outside the moon orbit. The heavenly
spheres which comprise the fifth, divine element, the
aether. Here everything moves in perfect circles. No
irregularities exist. All fixed stars and the planets exist
in this sphere.
The sphere inside the moon orbit. The earthly
spheres. The four elements (earth, water, fire and
wind) move towards their natural places (natural
motions).
Earth
(still in the centre)
The Moon
The Ptolemaic system (from 100 AD)
The movement of each planet was explained
separately by its own circle system. The
retrograde movements were explained by so
called epi-cycles.
Some problematic issues at that time
The movements of the planets, especially the inner planets (Mercury, Venus, and
Mars) were difficult to predict by using a system based on perfect circles.
The planets had irregular orbital periods.
The Sun did not appear at the same place next day as the day before. In one year
the Sun had moved place in relation to the fixed stars.
The Ptolemaic system contained errors that increased over time.
Copernicus presented a new, heliocentric system
Sun was put in the centre of the system instead of Earth. All planets moved around
the sun. Also Earth moved around the sun and also around its own axis.
However, the planets (including Earth) were still moving in perfect circles. Epi-circles
were still used to explain the motions of the planet.
A more uniform system, but did not result in better predictions of the motions of the
planets. The system was equally complex as the Ptolemaic system.
Copernicus’ system was published in 1543 (the same year as he died).
Obvious facts that conflicted with the new system
The concept of gravitational force did not exist. Loose objects would be thrown out in
space if the Earth was moving around its on axis. Clouds, birds and other things in
the air would lag behind.
How could the Moon follow Earth if Earth was moving around the Sun?
How could Earth move around the Sun? A tremendous force is needed to cause this
movement. Such a force cannot exist.
There were other astronomical observations that contradicted the new system. These
contradictions could be explained if the distance to the stars was substantially
increased; however, this was seen as an ad-hoc explanation.
But most of all: the system challenged the world view at that time. It questioned the
very basic ideas of cosmos and life.
Despite the contradicting facts the heliocentric idea survived and
developed thanks to many contributors. Some important are:
Tycho Brahe:
Contributed with astronomical observations of e.g. comets (but he
did not accept the heliocentric system).
Johannes Kepler: (a student to Brahe). Accepted heliocentric system and developed
theories about the planetary motions. The circular orbits were
replaced by ellipses.
Galileo Galilei:
Held on to the circular motions. Invented telescopes and observed
many irregularities such as the irregular surface of the moon,
sunspots, Jupiter’s moons, and more. Galilei’s observations
supported the heliocentric system. He also developed laws of
motion and a tide theory that further supported the new system.
He wanted to appeal the common-sense and wrote in Italian direct
to the people (Latin was the scientific language at that time). He
was judged by the inquisition in 1633 to life imprisonment. His
observations were not convincing, e.g. knowledge of optical laws
and the nature of light did not exist at that time.
René Descarte:
Also wrote direct to the people in French. He eliminated the
concept of ”natural motions” and introduced a pure mechanical
model of the universe.
Isaac Newton:
His major work Matematika Principa was published in 1687.
Modern laws of motion and the concept of forces were introduced.
His theories were described in a mathematical way. He also
developed the experimental science.