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- Claudius Ptolemy was born c. 100 ce and died in c. 170 ce
- Ptolemy supported the geocentric model of our solar system
- The geocentric model is the theory that the universe is a set of nested spheres surrounding the Earth. This theory modeled that the Moon was orbiting on a sphere closest to the Earth, followed by Mercury, then Venus and then the Sun. (All the planets orbited the earth)
- Ptolemy developed his geocentric model based on observations he made with his naked eye

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lived 1546-1601

supported both the geocentric and heliocentric models of our solar system

He spent decades collecting astronomical data using the naked eye and math

What was new about his evidence: he believed the sun orbited earth but other planets orbited the sun

He believed in a model of the Universe with the Sun orbiting the Earth, but the other planets orbiting the Sun.

Laws: he believed in perfect circle and objects revolving around a celestial body

he did not believe in ellipses

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Lived 1571-1630

Model Supported: Heliocentric model of the universe

Evidence: Used Tycho's mathematical data, as he was Tycho's assistant

He came up with mathematical, empirical rules to describe how planetary orbits in a heliocentric system.

Empirical science describes how something works, not why.

Started as assistant for Brahe

Used Mathematical Models

proved planets did not revolve around the sun in perfect circles

Recognized ellipses were the key

Kepler's first law: Planetary orbits are ellipses. Each ellipse has two foci. The Sun is at one focus of a planet's elliptical orbit. There is nothing in space at the other focus; it is merely a mathematical point.

Kepler's second law: Often called the Law of Equal Areas. It says: “The line between the Sun and the planet ‘sweeps’ out equal areas in equal times.”  This law showed that planets do not revolve at a constant speed, A planet will go fastest when closest to the Sun. (“perihelion”) A planet will go slowest when farthest from the Sun. (“aphelion”) This law applies to each planet individually.

Kepler's third law: relates the orbital period to the distance of the orbit.
If you know one, you can figure out the other. You can also infer if faster or slower period based on distance from the sun. Orbital Period squared = radius of Major Axis cubed

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Lived 1473 – 1543

was the first to create a mathematical Heliocentric model with the Sun at the center)

first to use mathematics

his model had circular orbits and realized that the different planets orbited at different speeds

Could estimate relative distances of the planets from the Sun & each other.

not the 1st to put forth the model (but first with evidence)

he wrote a book about his evidence

Limitations: he only had mathematics to back up his ideas

Ideas he supported:

- the earth and all other planets revolve around the sun
- the earth rotates but also spins on a tilted axis
- the distance from the earth to the sun is less than the distance from to earth to the stars

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- Galileo Galilei (1564-1642) was the first scientist to observe the sky with a telescope
- He didn't invent the telescope but did make his own version of it after reading about its invention by a dutch scientist
- he worked as a professor
- invented the military compass
- 10 and 30-power telescopes
- Used his telescope to make observations of the earth's moon, Jupiter and its moons, and the phases of the planet venus
- he saw imperfections on the sun and the moon with his telescope which was evidence that the old "perfect heavens" model was wrong
- saw that venus had phases
- in a geocentric model, venus's phase would not change because venus was always between the sun and the earth
- venus's phases are similar to the moon
- venus is one of the only objects bright enough to be seen during the daytime