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• Planetary Motion
• Early Astronomers
Pg. 72
Pg. 73
Background: don’t write
The Universe – Everything there is; all energy,
space, and matter
Astronomy – The study of the universe beyond
the earth’s atmosphere.
Solar System – the Sun and all the objects that
travel around it due to gravitational force.
Objects = planets, over 60 satellites (moons)
orbiting the planets, thousands of asteroids,
countless meteoroids and comets, and
interplanetary dust.
Asteroid = large rock
1000km to 10m
Meteoroids = rocks less that 10 meters
• Meteor = a meteoroid that is flaming hot from
entering Earth’s atmosphere.
• Meteorite= meteor that survives atmospheric
entry and lands on earth.
Comets = dust, ice, and gases
(dirty snowball)
•Ancient civilizations wanted to learn about
astronomy for religious and societal purposes—
when to plant crops, harvest them, when to offer
sacrifice, etc. Mesoamerican tribes, the Anasazi,
the Babylonians, and other ancient civilizations
also developed calendars based on relatively
sophisticated astronomical observations.
•The history of science involves the history of
astronomy and the controversies involved in
heavenly motions.
Aristotle 384-327 B.C.
• Greek Philosopher
• 1st to suggest the Earthcentered solar system model
(Geocentric).
• Couldn’t explain why
sometimes planets appeared to
move backward, relative to
other planets.
Ptolemy
90-168 AD
Greek Astronomer
Created the earth centered model of the
solar system = geo-centric.
Ptolemy’s model stood unchallenged for
1400 years. This was due to the fact that it
proposed that Earth was the center of the
universe and this matched with the Church’s
teaching that Earth and its inhabitants were
the center the universe. The
geocentric(=earth-centered) system then
become an article of faith, closed to any
scientific scrutiny.
Ptolemy explained planet orbits
and rotations.
• The greatest difficulties he
had to overcome were
explaining the changing
speeds and the occasional
east-to-west, or retrograde,
motion of the planets. He
accomplished this by
having each planet move
along a small circle, called
an epicycle, whose center
traveled along a larger
circle, called a deferent,
with Earth at its center.
Introduced the ellipse
• Although this scheme came close to
accomplishing what he wanted, it still came
up a little short. So Ptolemy made a couple
of refinements. First, he placed Earth
slightly away from the center of the
deferent. (A slightly off-center circle comes
very close to mimicking an ellipse.)
Copernicus- 1473 -1543
As a Polish astronomer, Copernicus
made a great leap forward by realizing
that the motions of the planets could be
explained by placing the Sun at the
center of the universe instead of Earth.
In his view, Earth was simply one of
many planets orbiting the Sun, and the
daily motion of the stars and planets
were just a reflection of Earth spinning
on its axis.
Created the sun-centered
model of the solar system =
Heliocentric.
Tycho Brahe – 1546 -1601
• Brahe was a Danish nobleman
known for:
• accurate and comprehensive
observations of the solar
system.
• He observed a bright “new
star” and named it a
supernova.
Johannes Kepler
1571- 1630
• A German
mathematician,
astronomer, and
astrologer. A key
figure in the 17th
century scientific
revolution, he is best
known for his:
• laws of planetary
motion
Keplar – 1571 1630
Kepler believed firmly in the Copernican
system.
Kepler was forced finally to the realization that
the orbits of the planets were not the circles
demanded by Aristotle and assumed implicitly by
Copernicus, but were instead the "flattened
circles”.
Determined planets did rotate in elliptical paths.
Laws of Planetary Motion
1. Law of ellipses – The orbits of the
planets are ellipses, with the Sun at
one focus of the ellipse.
Laws of Planetary Motion
2. Law of equal areas – an orbiting
object sweeps through equal areas in
equal periods of time. (=speed)
he orbits of the planets are ellipses,
with the Sun at one focus of the
ellipse.
Laws of Planetary Motion
3. Law of periods – average distance of
a planet from the sun(a3) is
proportional to the square of its orbital
period(time it takes to orbit the sun)=p2
This was used to calculate how far
away the planets are from the sun.
Contributions of Isaac Newton
• Mathematics –created calculus
• Astronomy – built first practical
reflecting telescope.
• 3 Laws of Motion
• Light and Color
• Universal laws of gravitation –
gravity held planets in orbit
Galileo Galilei – 1564 - 1642
1564-1642 Pisa Italy
• was an Italian physicist,
mathematician, astronomer,
and philosopher who played
a major role in the scientific
revolution.
A true scientist
• Galileo refined his theories on motion and
falling objects, and developed the universal
law of acceleration, which all objects in the
universe obeyed.
Inventions and improvements:
•
•
•
•
Refracting(light bending) Telescope
Galileo Thermometer
Gravity experiment at the Tower of Piza
Designed mechanism for pendulum clocks
• Soon Galileo began mounting a body of
evidence that supported Copernican theory
and contradicted Aristotle and Church
doctrine.
• There were Mountains on the moon.
• Jupiter was orbited by four moons.
• The sun had spots
• Venus had phases.
Sir Isaac Newton – 1642 - 1727
• English scientist
Contributions of Isaac Newton
• Created calculus
• Astronomy – built first practical
reflecting telescope.
• 3 Laws of Motion
• Studied Light and Color
• Universal laws of gravitation –
gravity held planets in orbit
Planetary Motion Pg. 72
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Geo-Centric
Helio-Centric