Chapter 3 The Solar System

1. Chapter 3The Solar System Section 1 Introduction to the Solar System Notes 3-1

2. Modeling the Solar System • Aristotle: Greek philosopher (384 – 322 BC) • Geocentric model: Earth centered • Sun and all other planets revolved around the earth • Didn’t explain the reverse direction that some planet occasionally do • Called retrograde motion

3. Modeling the Solar System • Claudius Ptolemy: Greek astronomer • 500 years after Aristotle • Used epicycles to explain the retrograde motion in geocentric model • As the planet revolved it also moved in circles • Circles are much like links in a chain • Seemed to make the planet look like it was in retrograde

4. Modeling the Solar System • Nicolaus Copernicus: Polish astronomer • 1500’s he challenged the geocentric model • Heliocentric model: Sun centered • Earth and other planets revolved around the sun • Explained that planets are different distances from the sun and move at different speeds • Galileo Galilei: Italian scientist • 1600’s confirmed the heliocentric model by using his newly invented telescope

5. Modeling the Solar System • Tycho Brahe: Danish astronomer • In 1600’s made many detailed observations of the position of the stars and planets • Johannes Kepler: German astronomer • Lived 1571 - 1630 • Hired by Brahe to be his assistant • He explained Brahe’s observations mathematically • Developed three laws

6. Law of Ellipses • Kepler’s first law: • Each planet orbits the sun in a path called an ellipse • Oval shape • Determined by two points in the oval • Points are called foci (singular: focus) • If you draw a line from any point on the oval to the foci; the length of the lines will be the same • Circle is a special ellipse where the two foci are at the same spot

7. Law of Ellipses • Planet is therefore not always the same distance from the sun • Perihelion: closest to the sun • Aphelion: farthest from the sun • Distance from the sun averages the two distances together • Average distance from the earth to the sun is 149.5 million km • Called an astronomical unit (au) • Used to measure the distance from the sun to other planets

8. Circle where both foci are at the same point. Ellipse where the sun is at one focus.

9. Law of Equal Areas • Kepler’s second law: • Area of a long, thin sector is the same as an area of a short, wide sector • Describes the speed planets travel at different spots in their orbits • Earth moves fastest when closest to the sun • Earth moves slower when farther from the sun • The sun is off center in Earth’s orbit • The triangles, made by the sun and two points on the orbit of Earth, will have the same areas

10. Equal Area SUN Equal Area

11. Law of Periods • Kepler’s third law: • Describes the relations between the average distance of a planet to the sun to its orbital period • Orbital period: time it takes for one trip around the sun • K x r3 = p2 • K is a constant equal to 1 when AU’s are used

12. Law of Periods • Example: • Jupiter is 5.2 AU’s from the sun (radius) = r • Its period is 11.9 years (period) = p • K x r3 = p2 • 1 x (5.2)3 = (11.9)2 • 140.6 = 141.6 (error caused by rounding numbers)

13. Newton’s Application • Kepler explained HOW the planets move. • Newton explained WHY the planets move. • An object will move in a straight line at a constant speed until an outside force acts upon it. (Newton’s 1st Law) • Called inertia • Also applies to an object remaining at rest until an outside force acts upon it. • The force acting on planets is called gravity from the sun.