Download cosmogony

Chapter 2
Gravitation
and the Waltz
of the Planets
What do you think?
• What is the shape of the Earth’s orbit
around the Sun?
• Do the planets orbit the Sun at constant
speeds?
Cosmogony
• A cosmogony is theory about Earth’s place in
the universe.
• A geocentric cosmogony is a theory that
proposes Earth to be at the center of the
universe.
• A heliocentric cosmogony is a theory that
proposes the Sun to be at the center of the
universe.
Which is the geocentric cosmogony and
which is the heliocentric cosmogony?
geocentric (Earth-centered)
heliocentric (Sun-centered)
Planets were often called wandering stars
because they seem to move from one
constellation to the next.
Copernicus devised the first
comprehensive heliocentric
cosmology to successfully
explain retrograde motion
sidereal period is
the time it takes a
planet to orbit the
Sun once.
synodic period is
the time that elapses
between two
successive identical
configurations as
seen from Earth
(e.g., time from
opposition to
opposition)
Scientists use parallax to measure distances.
Tycho Brahe measured distances
using parallax that disproved
ancient ideas about the heavens
• A supernova in 1572 was shown to exist in
the distant heavens; this troubled scholars
who previously thought the heavens were
unchanging.
• He showed that comets were objects that
occurred in the region of the planets, not in
Earth’s atmosphere.
Mathematician
Johannes Kepler
created laws of
planetary motion
that describe the
orbital shapes,
changing speeds,
and the lengths
of planetary
years
Kepler’s First Law: The orbit of a planet about
the Sun is an ellipse with the Sun at one focus.
The distance between the two foci impact the
eccentricity of the ellipse’s shape.
Kepler’s Second Law: A line joining a
planet and the Sun sweeps out equal areas
in equal intervals of time.
Kepler’s Third Law: The square of a planet’s
sidereal period is proportional to the cube of the
length of its orbit’s semimajor axis (p2=a3).
Italian
scientist
Galileo made
discoveries
that strongly
supported a
heliocentric
cosmology
Galileo’s telescope revealed phases of
Venus which could only occur IF
Venus orbits the Sun.
Galileo’s telescope
revealed that Jupiter
had moons which
orbited Jupiter instead
of Earth.
Isaac Newton
formulated
three laws that
describe
fundamental
properties of
physical
reality
Newton’s Three Laws of Motion
• A body remains at rest or moves in a
straight line at a constant speed unless acted
upon by an unbalanced outside force.
• Force = mass x acceleration
• Whenever one body exerts a force on a
second body, the second body exerts an
equal and opposite force on the first body.
Newton’s description of gravity
accounts for Kepler’s laws
• Newton’s universal law of gravitation
states: Two bodies attract each other with a
force that is directly proportional to the
square of the distance between them.
– This law mathematically proves Kepler’s Third
Law (p2=a3).
– This law describes various orbits objects can
take when moving near the Sun.
– This law can be used to predict when comets,
such as Halley’s comet, will pass near Earth.
What did you think?
• What is the shape of the Earth’s orbit
around the Sun?
All planets have elliptical orbits around the Sun.
• Do the planets orbit the Sun at constant
speeds?
The speed varies inversely with distance from the
Sun. The farther a planet is in its elliptical orbit
from the Sun, the slower it moves.
Self-Check
1: Compare and contrast the Ptolemaic and Copernican
cosmologies by explaining a variety of naked-eye
observations, using both models.
2: State Kepler’s three laws of planetary motion and describe the
geometric content and observational consequences of each.
3: List Galileo’s telescopic observations and explain the success
of failure of Ptolemaic and Copernican models in accounting
for them.
4: State and identify examples of Newton’s three laws of motion.
5: State Newton’s law of universal gravitation and identify the
characteristics that explain Kepler’s laws in terms of Newton’s
laws.