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 Solar System.
But, it still had:
1) All motions being circular.
2) Epicycles.

This Heliocentric model could easily explain:
Retrograde Motion of Mars.
Earth catches and passes Mars in its orbit.
See the Movie!

Fix Figure 4-04

Why Mercury and Venus are always close to the Sun.
A natural consequence if their orbits are inside the Earth's orbit.

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 model.
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 FOCUS.

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]

Check it out!

GALILEO GALILEI [1564 -- 1642]

First to use the TELESCOPE for Astronomy.
His Discoveries:

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) Solar System.

ISAAC NEWTON [1643 -- 1727]
Considered to be the GREATEST Scientist to have lived. Made numerous contributions in physics and mathematics. We are only interested in

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!

Early Astronomy
Solar System Basics
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