Ch. 22: “Origin of Modern
Astronomy”
22.1: “Early Astronomy”
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Ancient Greeks


Astronomy is the science
that studies the universe.
It includes the
observation and
interpretation of celestial
bodies and phenomena.
Early Greek astronomers
used philosophical
arguments to explain
natural events and some
observational data.

Astrolabes were often
used to track the
positions of the sun and
stars.
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Solar System Models

Geocentric Model


Heliocentric Model


The moon, the sun,
and the known
planets – Mercury,
Venus, Mars, and
Jupiter – orbit Earth.
Earth and other
planets orbit the sun.
Ptolemaic System

Planets moved in
circular motion
around a motionless
Earth.

His model
accounted for the
movements of the
planets and was
unchallenged for
nearly 13 centuries.
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Retrograde Motion



It was noticed that the
planets move eastward
among the stars.
However, each planet
then seems to stop,
reverse direction for a
time, and then resume
an eastward motion.
This apparent westward
motion of the planets
with respect to the stars
is called retrograde
motion.
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The Birth of Modern Astronomy

Nicolaus Copernicus


Tycho Brahe


Discovered 3 laws of planetary motion.
Galileo Galilei


Made observations that were far more precise than any
made previously.
Johannes Kepler


Concluded that Earth is a planet. He changed the model
of the solar system to having the sun at the center of the
planets orbiting around it.
Created descriptions of the behavior of moving objects.
Sir Isaac Newton

First to formulate and test the law of universal
gravitation.
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Kepler’s 3 Laws of Planetary Motion

1st Law:


2nd Law:


The orbits of the planets are
elliptical (oval shaped path).
Planets revolve faster when
they are closer to the sun.
3rd Law:

An orbital period is the time
(in Earth years) it takes a
planet to make one revolution
around the sun.


The further the planet is
from the sun, the longer the
orbital period is.
p2 = a3 (planet’s orbital
period squared is equal to its
mean solar distance cubed)
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Newton’s Accomplishments


Proposed that a moving
object will continue to move
at a constant speed and in a
straight line = Inertia.
Universal Gravitation



Gravity decreases with distance.
The greater the mass of an
object, the greater its
gravitational force.
Proved that the force of gravity,
combined with the tendency of a
planet to remain in it’s straightline motion, results in the
elliptical orbits that Kepler
discovered.
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