Chapter 2:
The Rise of
Astronomy
Ancient Roots:
Early Homo-Sapiens
Ancient Roots: Egyptian
Ancient Roots: England
Ancient Roots: Mayan
Ancient Roots: Aztec
Ancient Roots: Incan
Ancient Roots: Cambodia
Ancient Roots: Anasazi
Early Ideas: Pythagoras
• Pythagoras in 500
B.C. taught that the
Earth was round
based on the belief
that the sphere is
the perfect shape
used by the gods
Early Ideas: Aristotle
• By 300 B.C.,
Aristotle presented
naked-eye
observations for the
Earth’s spherical
shape:
– Shape of Earth’s
shadow on the Moon
during an eclipse
The Hot Debate!
• A geo-centric cosmology is a theory that
proposes Earth to be at the center of the
universe.
versus
• A helio-centric cosmology is a theory that
proposes the Sun to be at the center of the
universe.
Aristotle’s model of a geo-centric solar
system
(384-322 BC)
But model couldn’t account fully for
retrograde motion of the planet!!!
Retrograde motion is the apparent
“backward” motion of a planet
Ptolemy’s Geo-centric Solar
System
Tried to account
for retrograde
motion, but could
not quite
match
observations
Copernicus devised the first
comprehensive helio-centric
cosmology to successfully
explain retrograde motion
Tycho Brahe’s Golden Nose
Mathematician
Johannes
Kepler created
laws of
planetary
motion
Kepler’s First Law: The orbit of a planet about
the Sun is an ellipse with the Sun at one focus.
Kepler’s Second Law: A line joining a planet and the
Sun sweeps out equal areas in equal intervals of time.
In other words, the closer a planet is in its orbit around
the Sun, the faster its speed is.
Kepler’s Third Law
In other words, the planets
that are
closest to the Sun orbit the
Sun at a faster
speed than planets that are
farther away
.
Summary of Kepler’s Laws
of Planetary Motion
1st
Law
The orbit of a planet about the Sun is an
ellipse with the Sun at one focus
2nd
Law
The closer the planet is in its orbit around the
Sun, the faster its speed is.
3rd
Law
The planets that are closest to the Sun orbit
the Sun at a faster speed than planets that
are farther away.
Italian
scientist
Galileo
made
discoveries
that strongly
supported a
helio-centric
cosmology
“I do not feel obliged to believe that the same
god who has endowed us with sense, reason and
intellect has intended us to forgo their use.” - Galileo
Galileo’s telescope
revealed that
Jupiter had moons
which orbited
Jupiter instead of
Earth.
Galileo’s
telescope
revealed all
phases of
Venus which
could only
occur IF
Venus orbits
the Sun.
Geo-centric or Helio-centric?
Galileo
Ptolemy
Kepler
Copernicus
Aristotle
Sir Isaac Newton
What goes up,
must come down!
Newton’s Universal Law of
Gravitation
• Every body that has mass has
gravity
• The gravity between the two
bodies increase with the mass
• The gravity between the two
bodies increase if they are
closer together.
Mass vs. Weight
• Mass is a measure of the total amount of
material in the object
 remains the same everywhere
• Weight is the force with which an object is
pulled down while on the ground (due to
gravity’s attraction)
 changes depending on the body you are
standing on
Foucault proves that the Earth
rotates!