1. Observing the Solar System / Earth in Space Chapters 19 & 20 Sections 1 & 2

2. Stars in the Milky Way Galaxy What do you know about the stars in our Galaxy? What do you want to know about the stars In our galaxy?

8. Early Observations Ancient Greeks watched stars move across the night sky – they noticed – Stars seemed to move – Stars stayed in the same position relative to each other – Star patterns stayed the same year after year

9. Early Observations Early Greeks noticed something else Several points of light seemed to move slowly among the stars They named these objects planets – from the Greek word “wanderers” These observations led them to believe that the Earth was the center of the Universe – and all the planets and stars rotate around the Earth Known as the geocentric system Geo means “earth” in Greek “Earth Centered”

10. Earth at the Center In A.D. 140 – the famous Greek astronomer Ptolemy further developed the geocentric model – His model showed how planets moved in small circles that move on bigger circles – he called “epicycles”

12. Ptolemy’s Model Ptolemy’s model was accepted by most for over 1500 years!

13. Sun at the Center Not everyone agreed with the geocentric model The Greek astronomer Aristarchus was the first person to propose a sun centered model for our solar system This model was called the Heliocentric system - Helios is Greek for “Sun” – “Sun centered” Earth and other planets revolve around the sun

14. Rejecting the Geocentric Model The geocentric model was finally proven incorrect through the work of these early astronomers.

15. The Copernican Revolution Nicolaus Copernicus - 1543 – Believed the heliocentric model – Strongly believed that the Earth rotated on its axis and the Earth revolved around the sun – Figured out the arrangement of the known planets and how they moved around the sun – Ideas were controversial and not accepted

16. Tycho Brahe – Late 1500’s Carefully observed and recorded the planets positions for twenty years Suggested planets did not follow a circular orbit Made all his observations without a telescope

17. Johannes Kepler Brahe’s assistant After Brahe’s death he analyzed the 20 years worth of observations Discovered that the orbit of each planet is in the shape of an ellipse Ellipse – an oval shape that can be elongated or nearly circular Discovered the Laws of Planetary Motion

18. Galileo Galilei Used the newly developed telescope to make discoveries that supported the heliocentric model

19. Galileo’s Discoveries New stars – (Milky Way is made up of stars) Mountains and valleys on the moon Four moons orbiting Jupiter Phases of Venus Sunspots rotating around the sun The rings of Saturn

20. Galileo His discoveries showed the problems with the geocentric model Published his work in Italian – a widely known language Was forced to recant his views under threat of torture – and was placed under house arrest for the last 10 years of his life

21. Venus and Jupiter in the morning sky