Chapter 3.1 – Observing the Solar System

I. Earth at the Center A. Greek Observations 1. Stars When the Greeks observed the sky at night they saw that the stars moved across the night sky but the patterns of the stars were the same. In other words, the stars stayed in a fixed position to one another. The patterns or fixed positions of the stars are called constellations.

Picture of Orion the Hunter

There are certain constellations that are present during certain seasons of the year (such as the constellation Leo during the summer). There are some constellations present all year (such as Ursa Major). Constellations present all year round are called circumpolar constellations.

2. Planets Besides seeing constellations of stars, the Greeks also saw points of light moving in and around the constellations. These moving points of light were called planets, which means 'wanderers" in English. The Greeks made very accurate drawings of the movements of these planets around teh sun. They were only able to see six (6) of the eight (8) planets we know today.

The Romans named the six visible planets based upon their size and color: Mercury (The Messenger of the Gods) Venus (Goddess of Love and Beauty) Mars (God of War) Jupiter (King of all the Gods) Saturn (God of Agriculture)

Based upon the Greek drawings, they incorrectly assumed that the Earth was the center of the universe and stationary. They assumed that everything else in the universe (planets, Sun, and stars) revolved around the Earth. It was known as the Geocentric System (Geo means Earth in Greek).

Geocentric System

B. Ptolemy's Model In 140 AD, the Greek astronomer Ptolemy developed a new model of the universe. He found that the planets moved in small orbiting circles around themselves. However, he believed that the planets and their small orbits are part of one large orbit that had the earth as the center.

Ptolemy’s Model

II. Sun At the Center Not all Greek astronomers believed that the Earth was the center of the universe. From their own drawing of the movement of the planets, stars and Sun, the group of astronomers believed that the Sun was the center of the universe and everything in the universe revolved around it. This view of the universe is called the Heliocentric System (Helios is Greek for Sun).

A. The Copernican Revolution In 1542, a Polish astronomer named Nicolaus Copernicus further refined the Heliocentric System. He added the Heliocentric System the arrangement of the known planets in space and how such planets moved around the sun in perfect circles. Since most powerful institutions such as the Roman Catholic Church at that time believed that the universe was geocentric, Copernicus waited until his death to publish his findings about the universe being heliocentric.

Heliocentric System

B. Galileo's Evidence. Using the compound telescope, Galileo was able to track the exact orbits of the moon, planets and the Sun. His scientific data gathered from his observations of Jupiter and Venus proved that the universe is heliocentric.

1. Jupiter Through the compound telescope, Galileo saw that Jupiter had at least 4 moons. Today we call these 4 observable moons the Galilean Moons. He noticed that the four moons did not orbit the Earth. Instead, the four moons only orbited Jupiter.

2. Venus Galileo also tracked the orbital pattern of Venus. He found that Venus did not orbit the Earth but it did orbit the Sun. Venus's orbital path around the Sun was the same as the Earth's orbit around the Sun.

C. Tycho Brahe's Observations Tycho Brahe was an astronomer who observed the solar system patterns and paths for over 20 years. Brahe confirmed the findings that the universe was heliocentric but he did not see the planets in perfectly circular orbits around the sun. Instead, he suspected that the planets orbit the sun in an elliptical or oval shape, but died before he could test out his theory.

D. Kepler's Calculations Brahe's assistant was Johannes Kepler. Kepler carried on Brahe's work and tested his theory about the orbits of the planets around the sun being elliptical in nature. Kepler took careful mathematical measurements of Mars orbiting the Sun. Looking at the data Kepler realized that the shape of the orbit was an ellipse and not a perfect circle.

III. Modern Discoveries. Today we refer to the universe as the "solar system" - a Sun-centered system. We have also discovered three more planets after Saturn: Uranus, Neptune, and Pluto. Scientists have also discovered other objects in space such as comets, stars and asteroids. Some of these are also in the orbits revolving the sun.

Since 2006, Pluto is no longer considered a planet. It is a planetoid or dwarf planet because of its small size and mass.