Part 1: Historical Models Astronomy! Part 1: Historical Models

What is Astronomy? Astronomy is the study of the universe! (everything we see in the night sky)

Astronomy is very different from astrology: Astronomers are scientists who use telescopes to see the night sky. Astrologers are people who believe the planets and stars can tell your horoscope. (a prediction of your future)

Long ago, people who looked at the night sky realized Earth was not alone in the universe... Earth is part of a greater system including the sun, moon, and stars.

They saw that the sun and moon seemed to move against the background of stars...

and some of the stars also seemed to move... They called these planets! (Which means ‘wandering star’)

In 140 AD, Greek astronomer Claudius Ptolemy created a model of this system. His model only had 5 planets, and it was accepted as true for over 1400 years!

He put Earth at the center since we were thought to be the most important. This is called geocentric (Earth centered).

The planets went in circles around Earth, doing loopty-loops to explain their retrograde motion in the sky!

In the 1500’s, Polish astronomer Nicolai Copernicus proposed a heliocentric (sun centered) system of 6 planets.

His system was rejected, especially by the churches! Earth not the center?!

In the late 1500’s Danish astronomer, Tycho Brahe made detailed observations of the planetary motions. He had a silver nose, because it got cut in a duel over mathematics!

He came up with a slightly different model, a mix of Ptolemy and Copernicus. Earth was still the center of the universe, but other planets went around the sun.

Brahe had to really work to get this model to match what he observed!

In the 1600’s, Italian astronomer Galileo Galilei was the first astronomer to use a telescope to support heliocentricity.

He saw moons orbiting Jupiter, which showed not everything revolves around the Earth.

He also saw that Venus had phases like our moon, which would only happen with the sun at the center, not Earth!

Then the Copernican model was finally accepted as true, and made more sense! Let’s compare...

A man named William of Ockham also helped support the Copernican model. Ockham’s Razor is the theory that the simplest explanation is usually correct.

In the 1600’s, German mathematician Johannes Kepler analyzed Brahe’s data.

He discovered 3 laws of planetary motion that, with Newton’s laws, made the model that we still use today! The planets orbit the sun in ellipses not circles, with the sun off-center making them go faster when closer!

Our Solar System (the system revolving around our sun named Sol!)