Summary Slide Celestial Objects The Celestial Sphere Development

Summary Slide • • • Celestial Objects The Celestial Sphere Development of the Heliocentric Model The Heliocentric Model Kepler’s Laws of Planetary Motion Galileo Galilei and Sir Issac Newton 1

Astronomy Observing the sky 2

Celestial Objects • any object you can view in the sky that is not in earth’s atmosphere • The sun, stars, planets, comets and moons 3

Celestial motion • • • Objects appear to move in an arc across the sky from east to west at a rate of 15 /hr Arcs are centered on the star Polaris Location of stars changes with time and location. Early observers used celestial objects to find their position and the time. 4

Star Trails 5

“Apparent” vs. “Real” Motion • If an observer is moving while observing a still object, the object will appear to move 6

The Celestial Sphere • Stars move all together, so early astronomers imagined that stars in the sky were attached to a giant sphere surrounding the earth. 7

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We can only ever see half of the celestial sphere. 9

Altitude and Azimuth 10

Horizon and Zenith 11

• • This model explained many observations and is called the geocentric model. Geocentric Model – Earth is the center of the universe and everything goes around the earth. 12

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The Problem with Planets. • • • Early astronomers observed objects in the sky that did not move together with the stars. They called them planets (from the Greek word for “wander”). This motion is called retrograde motion. Retrograde motion – The periodic backward motion of planets in the sky compared to the background stars. 14

Development of the Heliocentric Model Early models: Aristotle and Ptolemy 15

Aristotle (384 -322 BC) • • • Spheres within spheres - objects in the sky that move in different directions or at different rates are located on different spheres surrounding the earth. Geocentric model – earth is the center of the universe The earth must stay still because we do not observe parallax (a change in observation because of a change in 16 position).

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Ptolemy • Ptolemy proposed a model where the planets moved in “epicycles” as they orbited the earth. • Ptolemy’s model did not predict the motions of celestial objects accurately 19

The Heliocentric Model 20

Nicolaus Copernicus • • • Heliocentric model – the sun is the center of the universe, and the planets revolve around the sun, while moons revolve around planets. The lack of parallax is because objects are so far away that we cannot see it. Explains why the planets exhibit retrograde motion (moving backwards in the sky). 21

Tycho Brahe and Johannes Kepler • • Tycho Brahe spent much of his life making and recording celestial observations. When he died, his assistant, Johannes Kepler, reviewed his work Kepler made three important discoveries based on Brahe’s work. These are called Kepler’s Laws of Planetary Motion 22

• Galileo • www. teachersdomain. org 23

Kepler’s Laws of Planetary Motion 24

Kepler’s First Law 25

Kepler’s First Law • Each planet revolves around the Sun in an elliptical orbit with the Sun at one focus • Eccentricity - out-of-roundness • Eccentricity (e) = distance between foci (d) length of the major axis (L) 26

Parts of an Ellipse • Foci- two fixed points in the ellipse (focus singular) • Major Axis- longest straight line distance across an ellipse. It cuts through both foci 27

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Eccentricity of Planet Orbits • Eccentricity- the degree of flatness or “ovalness” of an ellipse – Described as the amount of difference between an ellipse and a circle – Always a number between 0 and 1 with no units (NAKED NUMBER!!!) 29

• Perfect Circle has an eccentricity = 0 • Flat line has an eccentricity = 1 30

Sample Problem • Calculate the eccentricity: – Major Axis: 20 km – Distance Between Foci: 15 km 31

Earth’s Orbit • Aphelion- when the distance from the earth to the sun is the greatest and the sun appears the smallest • Perihelion- when the distance from the earth to the sun is the smallest and the sun appears the largest 32

Earth’s Orbit first law 33

Planetary orbit eccentricities • Mercury. 206 • Venus. 0068 • Earth. 0167 • Mars. 0934 • Jupiter. 0485 • Saturn. 0556 • Uranus. 0472 • Neptune. 0086 • Pluto. 25 34

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Kepler’s Second Law 36

Kepler's second law • The planets do not move at a constant velocity Perihelion – planet moves quickly in orbit Aphelion – planet moves slowly in orbit 37

Kepler’s Third Law 38

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Kepler’s Third Law • This law basically says that the farther you are from the sun, the longer it takes to go around it. Kepler's Laws 40

Galileo Galilei and Sir Issac Newton • Galileo used one of the first telescopes to make observations of the night sky. He discovered the four largest moons of Jupiter 41

• Newton combined the models of Copernicus, Kepler and Galileo with his laws of motion and formed the astronomical model that we use today. 42

Newton’s Law of Gravity G = the gravitational constant m 1= mass of object 1 m 2= mass of object 2 d = radius between objects 43

• Newton explained the forces behind orbital motion. He proposed that an object in orbit has two forces keeping it in orbit: forward motion and gravity. Newton’s Canon 44