Transcript Geocentric vs. Heliocentric Models of the Solar System

Astronomy Picture of the Day
Question
The Moon has a(n) ________ orbit meaning ________.
A. synchronous, its orbital period is equal to its rotation period.
B. moderate, its orbital period is equal to the Earth's rotation
period
C. retrograde, it sometimes appears to travel backward across the
sky
D. highly eccentric, it is circular
Question
Solar eclipses occur _____ often than lunar eclipse and
are ______ in duration.
A. more, shorter
B. more, longer
C. less, shorter
D. less, longer
Question
Stellar parallax is ______.
A. a way to measure angles
B. a way to measure the baseline
C. a way to measure distances
D. only useful for astronomical objects greater than a few
hundered light years away.
Parallax
• Triangulation - Measure
angles at points A and B
• Parallax - Know Baseline.
Measure third angle in
triangle made by A, B, and
object in space

Baseline problem
The apparent displacement (shift) of a foreground object relative to the background as the
observer’s location changes is known as parallax.
Scientific Method
Experimentation and observation are central parts
of scientific inquiry.
Also, theory – framework of ideas and
assumptions used to explain a set of obervations
and make predictions.
If it fails even once must be reformulated or
rejected!
If a “theory” makes no predictions at all, it has no
scientific value.
“The scientific method is designed to yield –
eventually – an objective view of the world...”
From Aristotle to Newton
The history of the Solar System (and the universe to
some extent) from ancient Greek times through to the
beginnings of modern physics.
Where do we put the other planets in our picture of
the solar system?
Geocentric vs. Heliocentric Models of the
Solar System
Ancient Greeks knew of Sun, Moon, Mercury, Venus, Mars, Jupiter and
Saturn.
Aristotle vs. Aristarchus (3rd century B.C.)
Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth.
Aristarchus: 1st Heliocentric model
Aristotle: But there's no wind - Doesn’t “feel” like
we are moving. (Actually orbiting sun at 70,000 mph!)

Difficulties with "Geocentric" model
- Retrograde motion of planets
- Phases of Venus
Aside: Aristarchus
- Written in the second century BCE he
calculated the ratio of the distance
between the Earth and Sun to that
between the Earth and the Moon. (His
estimate was more than an order of
magnitude too small, but the fault was
in his lack of accurate instruments
rather than in his method of reasoning.)
-This image compares the line
subtending the arc dividing the light and
dark portions of the Moon in a lunar
eclipse with the relative diameters of the
Moon and Sun.
-Aristarchus also found an improved
value for the length of the solar year.
Geocentric Model of Solar System
(Earth Centered)
What are some reasons that the geocentric model of the
universe seems to make intuitive sense?
• It doesn't feel like we are moving – wouldn't there be a wind or
something?
• Why would things fall down and not towards the center of the
universe?
• Why don't we see stellar parallax?
Retrograde Motion of Planets
Planets sometimes appear to loop
back - retrograde motion
Loops are called "epicycles"
Mars
July
7
*
Earth
7
6
5
Apparent motion of
Mars against "fixed"
stars
*
6
3
4
4
3
1
5
2
2
1
January
*
*
*
*
Geocentric Model
(Earth Centered)
• Fairly good agreement
with retrograde motion
of planets
• Some predictive power
• More precise
measurements showed
errors
Ptolemy's geocentric model (A.D. 140)
(VIDEO
Heliocentric Model
“Rediscovered” by Copernicus in 16th century.
Much simpler was the main attraction for
Copernicus. Simply explains retrograde motion.
(VIDEO CLIP)
Put Sun at center of everything, but
still insisted on circles, thereby retainig
unnecessary complexity.
Opposed by Catholic Church
Copernican revolution – critical
realization that Earth is not at the
center of the universe, only accepted
after his death.
Copernicus 1473-1543
Galileo (1564-1642),
Experimentalist
Built his own telescope.
Discovered four moons orbiting
Jupiter. What does this suggest?
Discovered sunspots. What might
we infer about the Sun from these
observations?
Observed phases of Venus.
Was imprisoned for the last 9 years of his life for his scientific
discoveries.
Heliocentric model easily accounts
for phases of Venus
Geocentric model fails to account for
phases of Venus
The Scientific Method
Geocentric model
abandoned because of
its failure to account
for observations, and
to a lesser extent
because of its
complexity.
(VIDEO CLIPS)