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1. Competing Models Of The Universe
By Eudoxus, Aristotle, Aristarchus,
Ptolemy

2. Eudoxus
A Brief History
Since early times, man has been fascinated with discovering the origins of the cosmos. Similarly, man has often
been influenced by his creationist ideas: that some divine power created the universe and everything in it. For
example, the Ancient Greeks developed some of the earliest recorded theories of the origin of the universe.
Unfortunately, many of these Greek philosophers and astronomers placed the Earth in the center of their models
of the universe. They thought, if the heavens are divine, and the gods created man, well then certainly the
universe must be geocentric, meaning the Earth is the center of the universe.
Ancient societies were obsessed with the idea that God must have placed humans at the center of the cosmos (a
way of referring to the universe). An astronomer named Eudoxus created the first model of a geocentric universe
around 380 B.C. Eudoxus designed his model of the universe as a series of cosmic spheres containing the stars,
the sun, and the moon all built around the Earth at its center. Unfortunately, as the Greeks continued to explore
the motion of the sun, the moon, and the other planets, it became increasingly apparent that their geocentric
models could not accurately nor easily predict the motion of the other planets.

3. An astronomer named Eudoxus created the first model of a geocentric universe around 380 B.C. Eudoxus
designed his model of the universe as a series of cosmic spheres containing the stars, the sun, and the moon
all built around the Earth at its center.

4. The first to present a general, geometrical model of celestial motion, Eudoxos started with five basic principles.
1. The earth is the center of the universe.
2. All celestial motion is circular.
3. All celestial motion is regular.
4. The center of the path of any celestial motion is the same as the center of its motion.
5. The center of all celestial motion is the center of the universe

5. Aristotle
Like early astronomers from around the world, the ancient Greeks struggled to understand the universe. Thales,
often called the father of Greek science and mathematics, asked questions about the universe that were not
based on the actions of gods or demons. It is said that Thales provided the bridge between the world of myth and
the world of reason. He used the astronomical records of the Babylonians and Egyptians to accurately predict a
solar eclipse in the sixth century BC. Thales believed the Earth was flat and floated on water like a log.
Aristotle, who lived from 384 to 322 BC, believed the Earth was round. He thought Earth was the center of the
universe and that the Sun, Moon, planets, and all the fixed stars revolved around it. Aristotle's ideas were widely
accepted by the Greeks of his time. The exception, a century later, was Aristarchus, one of the earliest believers in
a heliocentric or sun-centered universe. In the 100s BC, Hipparchus, the most important Greek astronomer of his
time, calculated the comparative brightness of as many as 1,000 different stars. He also calculated the Moon's
distance from the Earth.

6. The first astronomer to make truly scientific maps of the heavens, Claudius Ptolemaeus (better known as Ptolemy of
Alexandria), came along 300 years later. Like most astronomers before him, he believed the Sun, Moon, and other
planets circled the Earth. He thought that each space body moved in a small circle (an epicycle) that was itself orbiting
Earth. This explained why planets sometimes appeared to travel backward in the sky. The Earth-centered view of the
universe was widely accepted for about 1500 years. It was not seriously challenged until 1543, when the Polish monk
Nicolaus Copernicus suggested that the Sun was at the center of the universe. Because the Church taught that the
Earth was central, Copernicus' theory was regarded as heresy. Perhaps this is why he did not want it published until
after his death. Copernicus' published theory, On the Revolution of the Celestial Spheres, met with great hostility from
the Church. The two events most responsible for eventual acceptance of Copernicus' views were Tycho Brahe's
precise observations of the sky and Galileo's use of the telescope.

7. Why is Aristotle's model of the universe important?
Aristotle's model shows the planets in the celestial realm moving around the Earth in an orderly manner, in
perfect circles and with uniform motion--neither speeding up nor slowing down.

8. Aristarchus, the famous ancient astronomer and mathematician born in Samos: Aristarchus (310 BC-230
BC) was a famous Greek mathematician and astronomer, popular for his theories regarding the
heliocentrism of our solar system. He was the first to say that the Sun, and not the Earth, was the center of
our universe. This theory brought him ridicule during his lifetime.
However, when his works were unearthed and studied around 1800 years later by Copernicus, the
rightness of his theory was proven. Although his works were considered inferior to those of Aristotle and
Ptolemy, he has made many significant contributions to science.
Aristarchus,
Aristarchus was one of the first astronomers to calculate the relative sizes of the Sun, the Moon and the Earth. He did
this by observing the Moon during a lunar eclipse and by estimating the angle and the size of the Earth. He
understood that the Sun, the Moon and the Earth form a near right angle during the last and the first quarter of the
Moon.

9. Based on this, he calculated that the Sun was nineteen times further away
from Earth than the Moon. However, he made a mistake in his calculations:
he took the angle as 87 degrees while the correct angle is 89° 50'. Thus,
the actual distance is 390 times and not nineteen times, as proposed by
Aristarchus. Although the geometric theory is current, the calculations
were wrong due to lack of precise instruments rather than logic.
His theory that the diameters of the Moon and the Sun should be
proportional to their distance from the Earth is also logical but gave wrong
results. Today that the intelligence of Aristarchus and his contribution to
science has been renowned, scientists have given his name to a crater on
the Moon.

10. He gave a model of the universe with a stationary Sun and planets rotating in circular orbits around
the Sun. The stars, which are actually stationary, seemed to be rotating because the Earth rotates on
its own axis.

11. Ptolemy
Model of the universe
Ptolemy placed the Earth at the centre of his geocentric model. Using the data he had, Ptolemy thought that the
universe was a set of nested spheres surrounding the Earth. He believed that the Moon was orbiting on a sphere
closest to the Earth, followed by Mercury, then Venus and then the Sun.

12. Ptolemy carefully studied the work of all the astronomers who had lived before him – particularly the Babylonian and
Greek astronomers. He learnt about all the methods that were used to observe and measure objects in the night sky
using the naked eye. Ptolemy took all of the observations and measurements collected over the previous 800 years and
used his excellent mathematical skills to develop his own model of the universe. Ptolemy shared his model in an
important manuscript called ‘Almagest’ which is the only surviving ancient text on astronomy. He created sets of tables
which could accurately predict the position of any planet in the night sky at any time in the past or future. There were
also tables that could predict the position of the Sun and Moon as well as the rising and setting of the stars. In addition,
he included tables that predicted solar and lunar eclipses.
Model of the universe Ptolemy placed the Earth at the centre of his geocentric model. Using the data he had, Ptolemy
thought that the universe was a set of nested spheres surrounding the Earth. He believed that the Moon was orbiting
on a sphere closest to the Earth, followed by Mercury, then Venus and then the Sun. Beyond the Sun were a further
three spheres on which Mars, then Jupiter and then Saturn orbited the Earth. Finally, the outmost sphere was where all
the stars were located in the 48 constellations that Ptolemy described in his text. It wasn’t until 1543 that Polish
astronomer Nicholas Copernicus (1473-1543) proposed a revised model putting the Sun at the centre – the heliocentric
model of the universe

13. How did the Ptolemaic model explain?
Resulta ng larawan para sa Ptolemy model of the universe
In order to explain the motion of the planets, Ptolemy combined eccentricity with an epicyclic model. In the
Ptolemaic system each planet revolves uniformly along a circular path (epicycle), the centre of which revolves
around Earth along a larger circular path (deferent).
How accurate was the Ptolemaic model?
Ptolemy's model was accurate enough to be very useful for navigators of their time. But it worked well because it
was finely tuned to fit with observed experimental data. But was wrong with Ptolemy's model is that it did not
correctly capture cause and effect.

14. The Copernican Model:
A Sun-Centered Solar System
The Heliocentric System
In a book called On the Revolutions of the Heavenly Bodies (that
was published as Copernicus lay on his deathbed), Copernicus
proposed that the Sun, not the Earth, was the center of the Solar
System. Such a model is called a heliocentric system. The ordering
of the planets known to Copernicus in this new system is
illustrated in the following figure, which we recognize as the
modern ordering of those planets.
Copernican system, in astronomy, model of the solar
system centred on the Sun, with Earth and other planets moving
around it, formulated by Nicolaus Copernicus, and published in
1543. It appeared with an introduction by Rhäticus (Rheticus)
as De revolutionibus orbium coelestium libri VI (“Six Books
Concerning the Revolutions of the Heavenly Orbs”). The
Copernican system gave a truer picture than the older Ptolemaic
system, which was geocentric, or centred on Earth.

15. It correctly described the Sun as having a central position relative to Earth and other planets. Copernicus
retained from Ptolemy, although in somewhat altered form, the imaginary clockwork of epicycles and
deferents (orbital circles upon circles), to explain the seemingly irregular movements of the planets in
terms of circular motion at uniform speeds.

16. Brahe's, Data Collection and Importance of Overlapping Circles
Copernicus had largely based his work on a body of existing
observations of the heavens. Although he did some observational
work, the bulk of his contribution was focused on re-evaluating
existing data from a different perspective. However, Tycho Brahe
had a different approach. Born in 1546, (three years after the
publication of Copernicus' De Revolutionibus) Brahe became a
famous astronomer, well known for his unprecedented collection of
astronomical data. Brahe's contributions to astronomy had
revolutionary impacts in their own right.
In 1563, at age 16, he observed Jupiter overtaking Saturn as the
planets moved past each other. Even with his simple observations
he saw that existing tables for predicting this conjunction were off by
a month, and even Copernicus's model was off by two days. In his
work, he demonstrated that better data could help to create much
more robust models.

17. New Stars and Interpretations of Comets
In November of 1572 Brahe observed a new star in the constellation of
Cassiopeia. With a sextant and cross-staff he was able to measure the
star's position and became convinced that it was in the realm of the
supposed unmoving fixed stars. This observation was inconsistent with
the longstanding belief that the celestial realm was a place of perfect
and unchanging fixed stars.
Alongside this development, the appearance of a comet in 1577
provided additional evidence that things did change and did move in
the celestial sphere. Based on careful measurements, Brahe was
able to identify that the comet was outside the sphere of the moon
and he eventually suggested it was moving through the spheres of
different planets.

18. Brahe's Model of the Cosmos
As a result of these observations, Brahe put forward a new model for the cosmos. In Brahe's model, all of
the planets orbited the sun, and the sun and the moon orbited the Earth. Keeping with his observations of
the new star and the comet, his model allowed the path of the planet Mars to cross through the path of the
sun.
Many scientists have been critical of Brahe's model as a backward step in the progress of science.
However, it is critical to remember the value that Brahe's system offered. This system had the advantage of
resolving the problem of stellar parallax. One of the persistent critiques of Copernicus's model (and even of
Aristarchus model in ancient Greece) was that with a moving Earth one should expect to see parallax
movement of the stars. As the Earth changes position in relationship to that of the stars, one would expect
to see the stars change position relative to each other. Copernicus' answer was that the stars had to be so
distant that it wasn't possible to detect parallax. Still, the distance required to make this work was so
massive as to be a problem for the system.
This was not a problem for Brahe's system because his model allowed for the circles in the heavens to
intersect. Brahe's model was not a step backward; but revolutionary in the sense that it was a competing
way to make sense of the data the heavens provided.

19. Kepler's Harmonies of the Heavens
Johannes Kepler, born in 1571, made major contributions to astronomy as his work mixed sophisticated
mathematics and astronomy with mystical ideas about astrology. Because of this Kepler remains difficult
for contemporary readers to understand. He was excited about the possibilities of developing new
astrology that was grounded in the work he engaged in as an astronomer. Kepler worked for Tycho
Brahe, publishing an extensive amount of Brahe's data in Rudolphine Tables. Although he used much of
that data for his own publications Kepler's work would significantly depart from Brahe's.
Kepler's first major work, Mysterium Cosmographicum (The Cosmographic Mystery, 1596), and his later
work Harmonice Mundi (Harmonies of the World, 1619) are both largely concerned with the order and
geometry of the heavens. In these works, he explored how the different shapes of Platonic solids could be
combined to explain a superstructure for the heavens, and how the movements and patterns of the
heavens could be mapped on to scales. For Kepler, the heavens literally made harmonies through their
movements. He was not afraid to attribute qualities to these harmonies and order that would strike us
today as strange superstitions. He was as interested in bringing together geometry and physics as he was
with bringing together alchemy and astrology.