Different Models of Universe
by EUDOXUS, ARISTOTLE,
ARISTARCHUS, PTOLEMY and
COPERNICUS
Man has often been influenced by his creationist ideas:
that some divine power created the universe and
everything in it.
• 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.
Eudoxus
• Was born in about 410 BCE and was a
student of Ancient Greek philosopher Plato
• The first person known to present a
mathematical theory of the universe.
• Created the first model of a
geocentric universe around 380 B.C.
• 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.
Eudoxus (cont.) > Eudoxus’ model placed
the spherical Earth at
the centre of the
universe.
> The Sun and the
planets were then placed
in giant transparent
spheres that orbit the
Earth.
> The Sun’s sphere
orbits the Earth once
every 24 hours, and the
stars are attached to a
larger sphere beyond
this.
Aristotle (Aristotelian
Universe)
• Extended Eudoxus’ model of the
universe in the 4th century BCE.
• Established a geocentric universe
in which the fixed, spherical
Earth is at the centre, surrounded
by concentric celestial spheres of
planets and stars.
• Considered to be the first
“steady-state” universe.
• A spherical and spatially-infinite
cosmos. Spherical earth is
surrounded by concentric
celestial spheres.
Aristotle (Aristotelian Universe) cont.
Below is an image of the
universe as conceived of by The earth is composed of four
elements: Earth, Water, Fire
Aristotle and Ptolemy. and Air.
It is in the center (notice the
trees, mountains and water).
In this cosmology, the earth
does not revolve around
anything else or rotate around
its own axis.
It is surrounded by ten
concentric spheres made of a
perfectly transparent
substance known as
"quintessence."
• NOTE: These spheres revolve
around the earth, carrying the
other celestial bodies.
Ptolemy (Ptolemaic Model)
• Developed Aristotle’s geocentric
theory of the universe in about 150
CE.
• Ptolemy knew that the planets don’t
appear to orbit in perfect circles
around the Earth. Some planets, like
Mars, even appear to move
backwards before moving forwards
again in large loops.
• Ptolemy suggested that planets like
Mars move in circles as they orbit the
Earth, where the circles are called
epicycles.
Ptolemy (Ptolemaic Model) cont.
• Ptolemy’ universe was
geocentric, with planets
orbiting in epicycles.
• Universe orbits about a
stationary Earth.
• Planets move in circular
epicycles, each having a
center that moved in a
larger circular orbit
(called an eccentric or a
deferent) around a
center-point near the
Earth.
Aristarchus(Aristarchean Universe)
• Was the first person to
attempt to calculate the
relative distance between
the Earth and the Sun in
the 3rd century BCE.
• He did this by measuring
the angle between the
Moon and the Sun during a
half moon, and using
trigonometry.
• The ratio of the distances
is given by the cosine of
the angle.
Aristarchus(Aristarchean Universe) cont.
• Aristarchus concluded that
the Sun is about 20 times
further away than the
Moon and must be about
20 times larger, since the
Moon and Sun appear to
be the same size.
• This is most evident during
solar eclipses, when the
Moon blocks out the Sun
completely.
• Ancient heliocentric
universe.
Copernicus (Copernican Universe)
• Reintroduced the concept of a Sun-
centred, or heliocentric universe to
Europe in On the Revolutions of the
Celestial Spheres, first published in
1543.
• Adapted the geocentric Maragha
model of Ibn al-Shatir to meet the
requirements of the ancient
heliocentric universe of Aristarchus.
• Was essentially a remapping of the
Ptolemaic scheme to make the Sun
the center of the system rather than
the Earth. It retained ideal circular
orbits and still needed epicycles.
HELIOCENTRIC
GEOCENTRI
C • rotate around one
celestial body • everything
• everything • used to try to show revolves
what orbits what in around the
revolves the universe
around the sun
• assumed the planets
• ellipses
earth orbits were spherical
and epicycles not circular
• models of the solar orbits
system
Tyco Brahe (Changing Universe)
• Tycho Brahe Tycho Brahe (1546-
1601) was a Danish astronomer
who also made the planetary
observations.
• In Brahe’s model of the universe,
called the Tychonic System, the
earth was at the center and the sun
and the moon revolve around it,
and all the other planets orbited
the sun. Such a model was a type
of the geoheliocentric system.
Kepler’s Law of Planetary Motion
• Brahe’s Student first extended
Aristotle’s theory of spheres by
arguing that they are separated by
five polyhedrons.
• The figure at the left shows a
depiction of a universe composed
of a series of shapes nested inside
each other.
• He also suggested that the planets
might produce musical notes
because they can be described
with a frequency.
Kepler’s First Law
“The orbit of every planet is an ellipse with the Sun at one
of the two foci”
Kepler’s Second Law
“A line joining a planet and the Sun sweeps out equal
areas during equal intervals of time”.
- states that planets move faster when they are closer to the Sun. This is due to the
conservation of angular momentum.
Kepler’s Third Law
“The square of the orbital period of a planet is directly
proportional to the cube of the semi-major axis of its orbit”.
Astronomical
Phenomena
known to
Astronomers
before the Advent
of The Telescope
The most observable objects in the sky are the
sun and moon.
Babylonian and Egyptian
civilizations used a primitive version
of a sundial, called gnomon, in
systematically observing the motion
of the sun.
By looking at the shadows that the
gnomon casts, they were able to
observe that the sun rises in the
eastern part of the sky, reaches its
highest point in midday, and sets in
the western part of the sky.
Also, they recorded that the
points where the sun rises
and sets on the horizon varies
over a year and these
variations happen
periodically.
They observed that these
variations are related to
weather and so concluded
that seasonal changes in
climate happen during a
course of one year.
Phases of the Moon
Ancient people have
observed that the moon
changes its path and its
appearance within a
period of 29.5 days.
They observed that the
moon changes its
appearance from thin
semi-circular disk to full
circular disk.
NOTE: These phases of
the moon is the basis of
ancient calendars.
Lunar Eclipse Besides their observation in
the different phases of the
moon, they also noticed that
there are times when the
moon or part of it seemed to
be covered by a shadow for a
brief moment.
A lunar eclipse occurs when
the Earth casts its shadow on
the moon when the Earth is
between the Sun and the
Moon.
A phenomenon such as this
is known as a lunar eclipse
wherein the moon changes
into a dark or blood red color.
Solar Eclipse
Aside from lunar
eclipse, the occurrence
of a solar eclipse was
also observed.
Solar eclipse occurs
when the Moon is in
between the Sun and
the Earth.
The moon partially or
completely blocks out
the sun.
The Motion of the Stars
It was also observed that the stars appear to be attached to a
celestial sphere that rotates around an axis in one day.
This axis intersects the celestial sphere at a point in the northern sky
and is presently close to the northern star, Polaris.
Also, the constellations’ positions in the night sky vary depending on
the time of the year.
Visibility of Planets
Astronomers have discovered that Mercury, Venus, Mars, Jupiter, and
Saturn are planets because they have noticed that the stars are in a fixed
position with respect to each other (like how constellations are always
grouped).
But there are very bright stars that change positions periodically. These
“stars” do not belong to any group of constellations in the sky. Thus, they
are called “wanderers” or planetes in Greek terms.
