Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION

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Stuart Garrett
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1 Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION CHAPTER 1 The Copernican Revolution Lecture Presentation

2 1.0 Have you ever wondered about? Where are the stars during the day? What is the near vertical band of light (in the photo)? What is the light streak (in the photo)?

3 1.1 Discovery: Ancient Astronomy Ancient civilizations observed the skies Many built structures to mark astronomical events Summer solstice sunrise at Stonehenge (constructed by the Druids about 3100 BCE)

4 1.1 Discovery: Ancient Astronomy Summer solstice sunrise at Stonehenge

5 1.1 Discovery: Ancient Astronomy This temple at Caracol (constructed by the Mayan peoples about 1200 BCE), in Mexico, has many windows that are aligned with astronomical events

6 1.1 Discovery: Ancient Astronomy Spokes of the Big Horn Medicine Wheel (constructed by the Plains Native Americans between years ago) in Wyoming, are aligned with eg the rising and setting directions of the Sun at summer solstice.

7 1.1 The Motion of the Planets Ancient astronomers observed: Sun Moon Stars Five planets: Mercury, Venus, Mars, Jupiter, Saturn What about you?

8 1.1 The Motions of the Planets The Sun, Moon, and stars all have simple movements in the sky, consistent with an Earthcentered system. Planets: Move with respect to fixed stars Change in brightness Change speed Have retrograde motion à Are difficult to describe in Earth-centered system! Why are planets so different?

9 1.2 The Birth of Modern Astronomy Copernician revolution 1. Earth is not at the center of everything. 2. Center of earth is the center of moon s orbit. 3. All planets revolve around the Sun. 4. The stars are very much farther away than the Sun. 5. The apparent movement of the stars around the Earth is due to the Earth s rotation. 6. The apparent movement of the Sun around the Earth is due to the Earth s rotation. 7. Retrograde motion of planets is due to Earth s motion around the Sun. 8. But no understanding of why planets revolve around the Sun or the Moon around the Earth!

1.2 The Birth of Modern Astronomy Because we view from the Earth, we call the planets with orbits between the Earth and the Sun Inferior planets: Mercury, Venus and those with orbits beyond the Earth

10 1.2 The Birth of Modern Astronomy Because we view from the Earth, we call the planets with orbits between the Earth and the Sun Inferior planets: Mercury, Venus and those with orbits beyond the Earth s orbit Superior planets: Mars, Jupiter, Saturn This makes a big observational difference! Special names are given to the places in the planet orbits when the planet, the Earth and the Sun are all in a line.

11 1.2 The Birth of Modern Astronomy Matches early observations: Inferior planets never too far from Sun Superior planets not tied to Sun and thus exhibit clear retrograde motion (next page à ) Superior planets brightest at opposition Inferior planets brightest near inferior conjunction Hmmm and what is opposition and what is conjunction? (Words that describe Earth:planet:Sun relative alignments see the previous slide.)

12 1.2 The Birth of Modern Astronomy Retrograde motion of a superior planet in a heliocentric model eg when Earth laps Mars.

13 1.2 The Birth of Modern Astronomy Observations of Galileo with a telescope! The Moon has mountains, valleys, and craters. The Sun has imperfections, and it rotates. Jupiter has moons. Venus has phases. All these were in contradiction to the general belief that the heavens were constant and immutable. This is very scary! What can we think of today that is similar? What about global warming? And is everyone willing to overthrow the existing beliefs so guess what happened to Galileo: in 1633 he was sentenced to life in prison even though he was right!

14 1.2 The Birth of Modern Astronomy The Moon has mountains, valleys, and craters.

15 1.2 The Birth of Modern Astronomy The Sun has imperfections and it rotates!

16 1.2 The Birth of Modern Astronomy Jupiter has moons and they revolve around Jupiter!

17 1.2 The Birth of Modern Astronomy Venus has phases.

18 1.3 The Laws of Planetary Motion Kepler s laws: 1. Planetary orbits are ellipses, with the Sun at one focus. Where is the planet s perihelion and aphelion? 2. FYI: a circle has one parameter: the radius. An ellipse has two parameters: the semi-major axis, a, and the eccentricity, e. Most planet orbits are nearly circles as e is near 0! are

19 1.3 The Laws of Planetary Motion Kepler s laws: 2. Imaginary line connecting Sun and planet sweeps out equal areas in equal times. Because of this planets speeds are greatest at perihelion!

20 1.3 The Laws of Planetary Motion Kepler s laws: 3. Square of period of planet s orbital motion is proportional to cube of semimajor axis. What is an AU? What is the last column of the Table telling us? For this class we call the semimajor axis the orbit radius!

1.3 The Laws of Planetary Motion The dimensions of the solar system The distance from Earth to the Sun is an astronomical unit (AU). Its actual lengt

21 1.3 The Laws of Planetary Motion The dimensions of the solar system The distance from Earth to the Sun is an astronomical unit (AU). Its actual length (e.g. in km) may be determined by bouncing a radar signal off Venus and measuring the signal s travel time. Why? Because distance (e.g. 0.3 AU) = speed of light x time.

22 1.3 The Laws of Planetary Motion Kepler s laws were derived using observations made by Tycho Brahe But with no understanding of why the planets move by Kepler s laws!

23 1.4 Newton s Laws Newton s laws of motion explain how objects interact with the world and with each other. Newton s laws of motion + his law of gravity finally explain why objects eg the Earth revolves around the Sun next few pages!

1.4 Newton s Laws Newton s laws of motion explain how objects interact with the world and with each other. Newton s first law for us this is the most important!

24 1.4 Newton s Laws Newton s laws of motion explain how objects interact with the world and with each other. Newton s first law for us this is the most important! An object at rest will remain at rest (a), and an object moving in a straight line at constant speed will not change its motion (b), unless a net force (green arrow) acts on it (c).

25 1.4 Newton s Laws Aside: if planet orbits are not straight lines, and/or the planet speeds are not constant, then there must be a net force acting on them What could this force be? Its invisible; how is that possible? Newton s second law mostly for astronomers! When a force (F) is exerted on an object, its acceleration (a) is inversely proportional to its mass (m): a = F/m Newton s third law mostly for astronomers! When object A exerts a force on object B, object B exerts an equal and opposite force on object A.

26 1.4 Newton s Laws Can you think of objects not travelling in a straight line that must have invisible forces acting on them? For much of our course the most important force is the force of gravity. On/near Earth s surface, the acceleration due to gravity is approximately constant and directed toward the center of Earth.

27 1.4: Newton s Laws: The Moon is Falling! Newton s insight: same force causes apple to fall and keeps Moon in orbit! But why does one fall and one move in an orbit?

28 1.4 Newton s Laws For two massive objects, the gravitational force is proportional to the product of their masses divided by the square of the distance between them (see formula in figure). Forces have magnitudes and directions. The direction of gravity is along the line joining the two masses and is attractive.

29 1.4 Newton s Laws The magnitude of the gravity force is given by: The constant G is called the gravitational constant; it is measured experimentally and found to be: G = 6.67 x N m 2 /kg 2 but this is too technical for us and is mostly for astronomers.

30 1.4 Newton s Laws The gravitational attraction of the Sun keeps the planets moving in their orbits. (From 3 slides ago: But why does one fall and one move in an orbit? Gravity is doing something! The planet s velocity, shown as the red arrows, is changing.)

31 1.4 (extra): Weighing the Sun Newtonian mechanics tells us that the force keeping the planets in orbit around the Sun is the gravitational force due to the masses of the planet and Sun. This allows us to calculate the mass of the Sun, knowing the the parameters (radius, r, and speed, v) of Earth s orbit: M = rv 2 /G The result is M = 2.0 x kg (!) and we ll do this in Chapter 9; see class web page file: astro_notes_chapt9.pdf

32 Summary of Chapter 1 First models of the solar system were geo (Earth) centric, but they couldn t easily explain retrograde motion. The Helio (Sun) centric model does explain retrograde motion. Galileo s observations supported the heliocentric model. Kepler found three empirical laws of planetary motion from observations.

33 Summary of Chapter 1 (con t) Laws of Newtonian mechanics explained Kepler s observations. Gravitational force between two masses is proportional to the product of the masses divided by the square of the distance between them.

Chapter 2 The Copernican Revolution

Chapter 2 The Copernican Revolution Units of Chapter 2 2.1 Ancient Astronomy 2.2 The Geocentric Universe 2.3 The Heliocentric Model of the Solar System The Foundations of the Copernican Revolution 2.4