Astronomy 100 -- The Astronomical Revolution
"THE ASTRONOMICAL REVOLUTION"
After about 1000 A.D., scientific inquiry resurfaced.
About 1340 A.D., "Occam's Razor" was put forth.
Simply put, "Simplicity is better".
Over the 1200 years from the acceptance of the Ptolemaic Model,
the predictions of the planetary positions became more and more
unsuccessful. This required introducing more and more epicycles.
COPERNICUS [1473 -- 1543]
Published first "Heliocentric" or Sun-centered model of the
But, it still had:
1) All motions being circular.
This Heliocentric model could easily explain:
- Retrograde Motion of Mars.
Fix Figure 4-04
- Why Mercury and Venus are always close to the Sun.
A natural consequence if their orbits are inside the
Fix Figure 4-07
TYCHO BRAHE [1547 - 1601]
Believed in the geocentric solar system.
- Kept extremely accurate planet positions over more than
20 years, with the goal of disproving the heliocentric
- Tycho's "compromise" solar system model, which was not
ever really accepted.
Sun goes around the Earth and other planets go around Sun.
Fix Figure 4-12
JOHANNES KEPLER [1571 - 1630]
Inherited Tycho's planetary position data when Tycho passed
away (BBD). He used these data to come with:
Kepler's Laws of Planetary Motion
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- Each planet moves in an ELLIPSE with the Sun at one
Fix Figure 4-13
How to draw an Ellipse
- The line connecting the Sun and the each planet sweeps
out EQUAL AREAS in EQUAL TIME INTERVALS. (This implies
that planets move faster in their orbits the closer they
are to the Sun)
Fix Figure 4-15
- The ratio of the PERIOD of revolution (P) SQUARED and the
SEMI-MAJOR AXIS (a) CUBED is the same (constant) for all planets!
(Note: mass of the planet does not matter!)
[P2 / a3 = P x P / a x a x a = CONSTANT]
GALILEO GALILEI [1564 -- 1642]
First to use the
- Mountains and valleys on the Moon.
Galileo's Drawings of the Moon
Sunspots on the Sun.
Implication: Sun and Moon were IMPERFECT spheres!
- 4 moons orbiting Jupiter (the Galilean Satellites)
Galileo's Drawings of Jupiter's Moons
Implication: There are some objects NOT orbiting the Earth!
- The Phases of Venus
Geocentric model (left) predicts only "Crescent Venus"
Heliocentric model (right) predicts all phases for Venus.
Fix Figure 4-19
Galileo observed all phases of Venus and noted the
"New Venus" was larger than the "Full Venus". These
observations support the Heliocentric model.
Images of the Phases of Venus
When Galileo was elderly, he was brought to trial by the Church.
Galileo "recanted" the Copernican model, which saved him from
being burned at the stake. Most believe this was done just so that
he could stay alive and not that he believed the Geocentric model.
He had published all his findings, so there was not need to die to
prove his arguments supporting the Heliocentric (Sun-centered)
ISAAC NEWTON [1643 -- 1727]
Considered to be the GREATEST Scientist to have lived. Made
numerous contributions in physics and mathematics. We are only
Newton's Laws of Gravity
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The FORCE (Pull) between two objects is:
- proportional to the Product of the MASSES.
- inversely proportional to the SQUARE of the separation
(written as " d " or " R ")
" G " is the Universal Gravitational Constant,
which is a very small number.
Using this Law of Gravity, Newton was able to derive Kepler's
Third Law of Planetary Motion! A real physical understanding of
the Solar System was accomplished!
Solar System Basics
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