Examine these two graphics. What is different about them? The same? What is a geocentric model?
The early Greeks were among the first to study the night sky. They observed that objects in the sky were moving.
From this, they assumed that the Earth was motionless, and all other objects they could see were moving
around the Earth. This is understandable because they could not feel the Earth itself moving but could see the
stars moving around it. From this they concluded that the universe must all be centered around the Earth. This
theory is called a geocentric model of the universe, which means 'Earth-centered'. The Roman Catholic Church
even believed in this theory because certain Bible passages suggested that the sun was in constant motion
around the Earth.
What is a geocentric model?
The early Greeks were among the first to study the night sky. They observed that objects in the sky were moving.
From this, they assumed that the Earth was motionless, and all other objects they could see were moving
around the Earth. This is understandable because they could not feel the Earth itself moving but could see the
stars moving around it. From this they concluded that the universe must all be centered around the Earth. This
theory is called a geocentric model of the universe, which means 'Earth-centered'. The Roman Catholic Church
even believed in this theory because certain Bible passages suggested that the sun was in constant motion
around the Earth.
Who was Ptolemy?
One of the most influential Greek astronomers, geographers and mathematicians of
the time, Claudius Ptolemy, supported the geocentric theory of the universe. Around
140 AD Ptolemy documented the most detailed explanation of the structure of the
universe of that time. His detailed theory was known as the Ptolemy System.
This Ptolemy System was based on the belief that all heavenly bodies, including the
stars and planets, were attached to spheres which rotated around the Earth. It was
believed that the order of the planets from the Earth were as follows: the Moon,
Mercury, Venus, the Sun, Mars, Jupiter, Saturn and the stars.
Who was Ptolemy?
One of the most influential Greek astronomers, geographers and
mathematicians of the time, Claudius Ptolemy, supported the geocentric theory
of the universe. Around 140 AD Ptolemy documented the most detailed
explanation of the structure of the universe of that time. His detailed theory was
known as the Ptolemy System.
This Ptolemy System was based on the belief that all heavenly bodies, including
the stars and planets, were attached to spheres which rotated around the Earth.
It was believed that the order of the planets from the Earth were as follows: the
Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn and the stars.
How was Ptolemy's model disproved?
Galileo disproved Ptolemy's model while using his telescope to investigate the planets. During his observations
he discovered that the planet Venus goes through phases, just like our moon, which causes it to appear to
change shape.
Galileo realized that this would not be possible under the Ptolemaic system. This is because the position of
Venus, in relation to the sun and the Earth, could not possibly explain the different reflections that would cause
phases.
The only way that the phases could be explained was if Venus were orbiting the sun instead of the Earth.
How was Ptolemy's model disproved?
Galileo disproved Ptolemy's model while using his telescope to investigate the planets. During his observations
he discovered that the planet Venus goes through phases, just like our moon, which causes it to appear to
change shape.
Galileo realized that this would not be possible under the Ptolemaic system. This is because the position of
Venus, in relation to the sun and the Earth, could not possibly explain the different reflections that would cause
phases.
The only way that the phases could be explained was if Venus were orbiting the sun instead of the Earth.
What is a heliocentric model?
Several others found faults with Ptolemy's model as time went on. Astronomer Nicolaus Copernicus was
convinced that the Earth was a planet just like all the other planets. He believed, therefore, that the Earth also
rotated like other planets.
Copernicus proposed a new model with the sun in the middle and all the other planets moving around it in the
same direction. This model of the universe is called a heliocentric model. Heliocentric means sun-centered.
This heliocentric model remains the model of the universe that we know and accept today.
What is a heliocentric model?
Several others found faults with Ptolemy's model as time went on. Astronomer Nicolaus Copernicus was
convinced that the Earth was a planet just like all the other planets. He believed, therefore, that the Earth also
rotated like other planets.
Copernicus proposed a new model with the sun in the middle and all the other planets moving around it in the
same direction. This model of the universe is called a heliocentric model. Heliocentric means sun-centered.
This heliocentric model remains the model of the universe that we know and accept today.
Galileo
Galileo Galilei (1564-1642) was an Italian astronomer working at a time
when all Scientists believed the earth was the center of the Universe
and all planets orbited it (the geocentric model). Using the newly
invented telescope he discovered that Jupiter had four moons. As these
moons were orbiting Jupiter itself, this showed that not everything
orbited the Earth.
Galileo’s findings put him into conflict with the Catholic Church as they
believed the geocentric model. He spent a large amount of his life under
house arrest as a result of his beliefs.
Galileo
Galileo Galilei (1564-1642) was an Italian astronomer working at a time
when all Scientists believed the earth was the center of the Universe
and all planets orbited it (the geocentric model). Using the newly
invented telescope he discovered that Jupiter had four moons. As these
moons were orbiting Jupiter itself, this showed that not everything
orbited the Earth.
Galileo’s findings put him into conflict with the Catholic Church as they
believed the geocentric model. He spent a large amount of his life under
house arrest as a result of his beliefs.
Copernicus’ model: "Sun-centered," or "heliocentric"
Copernicus thought that the planets orbited the Sun, and that the Moon orbited
Earth. The Sun, in the center of the universe, did not move, nor did the stars.
Copernicus was correct about some things, but wrong about others. The Sun is
not in the center of the universe, and it does move, as do the stars. Also, both
Copernicus and Ptolemy thought the orbits of the planets were circular, but we
now know they are elliptical.
Ptolemy's model: "Earth-centered," or "geocentric"
Ptolemy thought that all celestial objects — including the planets, Sun, Moon, and stars
— orbited Earth. Earth, in the center of the universe, did not move at all.
NOTE: The outer planets, like Uranus and Neptune, are missing from both charts
because they had not been discovered at the time. The planets are lined up to make
the charts easy to read; they never line up this way in nature.