Geocentric Model

• A model of space that put the Earth in the very middle of the stars and other planets

• This was the common belief of ancient Chinese and Greeks and lasted until the 1600s

• Most geocentric models only included the planets that were visible with their telescope

Geocentric Model

• A Greek astronomer named Claudius Ptolemy developed his own version of this

• Ptolemy put the Earth in the middle of the ENTIRE universe

Heliocentric Model

• First proposed by ancient Greek scientist, Aristarchus

• His model claimed the Sun was the fixed center of the universe and not the Earth

• Modern heliocentric models show the Sun as the middle of our solar system (SS), and our SS inside a galaxy

Heliocentric Model

• Nicolaus Copernicus tried to make this model more popular in the mid 1500's

• His model worked out the arrangement of the known planets and how they moved around the Sun

Heliocentric Model

• Later on in the 1500's two astronomers, Tycho Brahe and Johannes Kepler, made more accurate observations and calculations

• They discovered the planets do not move in perfect circles, but instead in an ellipse, or an oval shape

Our Solar System is VERY large

• So large that it would be insane to measure it by miles, so instead...

• We measure distance in space by Astronomical Units (AU)

• ONE AU is the average distance from Earth to the Sun

• 1 AU = about 150,000,000 kilometers

• Our SS extends to more than 100,000 AU away from the Sun

What's in our Solar System??

• ONE star (the Sun)

• EIGHT planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)

• FIVE dwarf planets (Pluto, Eris, Ceres, Makemake, Haumea)

• 200+ moons

• and a whole lotta rocks....to be discussed later

Why are some planets called dwarf planets??

• To be considered a "real" planet, it must have THREE characteristics

Planet Criteria

• 1) Be large enough that it is rounded by its own gravity

• 2) It must orbit the Sun

• 3) It must have cleared the path of its orbit

• **Most dwarf planets do not meet #1 or #3

How did the SS form?

• Scientists believe that the solar system formed from a large cloud of hydrogen, helium, rock, ice, and other elements

• This clouds gravity caused it to collapse and pull material towards its center

• The pressure and temperature eventually grew so high that nuclear fusion began and created the Sun

• As the Sun grew, so did it's gravity

• This gravity started pulling together chunks of rock and ice and clouds of gas to form planetesimals

• Eventually these smashed into each other and created all of the planets