origins of modern astronomy earth-centered (geocentric) vs. sun-centered (heliocentric) or ?
TRANSCRIPT
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Origins of Modern Astronomy
Earth-Centered (Geocentric) vs.
Sun-Centered (Heliocentric)
OR ?
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Why a Geocentric Universe?
• The Earth does not feel like it is moving.
• All objects in the sky appear to revolve around us.
• It’s comforting to think we are so important.
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A Strong Point in Favor of the Geocentric Model…
there is no observed parallax.
June Dec.
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A Major Problem for
the Geocentric
Model…Retrograde
Motion of the Planets
The Path of MarsMars across the night sky…
Mars Retrograde
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This was explained by Ptolemy, using epicycles,
orbits on top of orbits:
Animation
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Copernicus: The Sun-Centered Universe
The Polish astronomer, Copernicus, believed that Ptolemy’s model was too complicated, so he proposed the heliocentric model of the universe….
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By having planets,
including the Earth, revolving around the Sun,
the epicycles are eliminated.
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In the heliocentric model, retrograde motion is explained by the Earth passing a slower planet.
Animation
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Jupiter’s Retrograde Motion
• It takes Jupiter 12 years to revolve around the sun.
• Every Earth-year it goes through one Zodiac constellation.
• When the Earth passes it, Jupiter “falls backward,” and then resumes its pro-grade motion. June 2007July 2007
August 2007Sept. 2007
Oct. 2007
Nov. 2007
Dec. 2007February 2008
May 2008Sept. 2008Nov. 2008
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Jupiter 2006-2018
• Each year Jupiter moves slowly to a new constellation in the zodiac.
• During this time is goes through a retrograde stage when the Earth passes it.
2006 2007 2008 2009
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Enter Galileo with the first physical evidence of a heliocentric universe!
THE TELESCOPE
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The telescope greatly improved our ability to observe the universe
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There are too many stars to be “pasted on” one sphere.
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Galileo discovered that Jupiter had moons of its own -a mini-Solar System, independent of the Earth
Jupiter 4 moons
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More significantly, he discovered that
Venus goes through phases like the
Moon.
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This is difficult to explain with an earth-centered universe.
Sketch made by Galileo:
The crescent Venus is much larger, since it is closer to Earth
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Tycho’sTycho’s Compromise
• Danish astronomer Tycho Brahe was a superb observer of the heavens.
• He didn’t observe any parallax, so he kept the Earth in the center.
• Other planets revolved around the Sun, as it revolved around us.
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Mathematical Evidence:Johannes Kepler
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Kepler studied the records of planetary positions and came
up with three laws…
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Law #1: All the planets travel around the Sun in
elliptical orbits.
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Law #2: Planets move faster as they get near the Sun, slower when farther away
A1A2
A1 = A2
Animation
Equal Area Ani.
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Law #3: The distance of the planet to the sun cubed is equal
to the period of revolution squared: D3 = P2
For Mars: D = 1.5237 AU and
P = 1.881 years
(1.5237)3 =
3.538
(1.881)2 =
3.538
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Finally, there was Sir Isaac Newton, whose
Laws of Gravitation proved the heliocentric model.
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Newton was able to show how the forces of gravity and the
inertia of moving planets produces a Solar System.
Orbits in a cone
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Quiz!1. Why did the geocentric view hold on
so long?
2. Who plotted out the epicycles for the known planets?
3. How did Copernicus, Galileo, and Kepler help promote the heliocentric model of the universe?
4. What was Newton’s contribution?