PHYSICAL SCIENCE
NAME: ____________________________________ DATE:________________________
GRADE & SECTION:__________________________ TEACHER:_____________________
CONCEPT NOTES 3
I. TOPIC: PTOLEMAIC, COPERNICAN, AND TYCHONIC MODELS OF THE UNIVERSE
II. LEARNING GOALS: The students should be able to:
a. Describe the most basic form of the models proposed by the following: Ptolemy, Copernicus and
Tycho Brahe.
b. Demonstrate how the Copernican model of the Universe greatly simplified the explanation of
retrograde motion of the planets.
c. Explain how the Tychonic model of the Universe satisfies both the geo- and heliocentric
frameworks.
III. CONCEPTS
Notable differences in the Model Systems considerable by the 16th century (excludes the
Keplerian system)
Model System Center Orbits Stars
PTOLEMAIC Earth All other celestial bodies orbit Outermost celestial sphere,
the Earth fixed
COPERNICAN Sun All planets including Earth orbits Outermost celestial sphere,
the Sun, only Moon orbits the fixed
Earth.
URSINE/ Earth The Moon and the Sun orbits Outermost celestial sphere,
TYCHONIC the Earth, all other planets orbit fixed
the Sun; debatable whether
some of orbits (or shells)
transect each other; the version
of Tycho involves non-
crystalline orbital spheres of the
planets
Several notes regarding the model systems
a. They are properly called “systems” because of their modeling paradigms since they all
correspond well to the observed astronomical data. These models are made before the advent
of using telescopes to observe the celestial bodies.
b. The Ptolemaic system is a robust system that can be upgraded by adding new epicycles upon
existing epicycles based on the need for improvement as the data dictates. Hence, while it is
accurate, the Ptolemaic system requires inherent increase in complexity requiring
mathematicians and specialists in the area.
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c. The epicycles originated earlier than Ptolemy and are used to explain the apparent reversal
(“retrograde motion”, as observed from Earth) of planets in relation to the background stars
when observed for the whole year (annual motion).
d. The Copernican system is proposed by Nicolaus Copernicus (see the Reference on Copernican
model) to explain the retrograde motion with the argument for simplicity. Due to lack of
physical consistency with the prevailing Aristotelian Physics of his time (as in going against the
physical expectations discussed in the previous lesson PSp-03), the system is considered only
as a “mathematical tool” rather than a physical reality.
e. Tyco Brahe, with his vast amount of more reliable astronomical data (positions of celestial
objects over many years) is convinced that the Ursine model is a possible reality and added his
version based on the data available. There are some subtle differences between Ursine and
Tychonic model but the essence of being geoheliocentric is glaring.
f. The reliability and consistency with the available data are equal for these systems. The debate
is really more of their underlying philosophical foundations and assumptions. Some details of
these philosophical biases are detailed in literature especially that of the blog in the reference
“The Great Ptolemaic Smackdown”.
IV. ACTIVITY/EXERCISES
Direction:
In a coupon pad, or sketch pad draw the three alternative models of the Universe.
V. ASSESSMENT
Direction:
Compare and Contrast the three models of the Universe by;
a. Ptolemaic System
b. Copernican System
c. Ursine/ Tychonic System
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