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Published on November 13, 2007

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ASTRO 101:  ASTRO 101 Principles of Astronomy Instructor: Jerome A. Orosz (rhymes with “boris”) Contact::  Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: 594-7118 E-mail: orosz@sciences.sdsu.edu WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ Office: Physics 241, hours W TH 3:30-5:00 Coming Up::  Coming Up: Review September 25 (Tuesday) Exam September 27 (Thursday) Extra review session: Wednesday September 26 from 3:30 to 5:30 in PA 216 Questions From Before:  Questions From Before Why do different lamps have different colors? Each tube has a different gas it in, and each element has its own pattern of emission lines. What is the difference between red and blue light? The energy (or wavelength or frequency). Why is argon different from Helium? Each element has a different number of protons in the nuclei of their atoms. Review:  Review Thursday: Exam #1: Chapters 1-4.1, plus 5.2 and 5.4 (e.g. the parts of Chapters 4 and 5 about light). Bring the Scantron No. F-288-PAR-L Breakdown:  Breakdown There will be three types of questions: multiple choice questions (2 pts each) long answer (5 pts each) “fill in the blank” (1 pt each) Highlights:  Highlights Astronomy without a telescope Celestial sphere Stellar coordinates Stellar brightnesses The clockwork of the Universe The day/night cycle The reason for the seasons The phases of the moon Highlights:  Highlights A brief history of Astronomy The geocentric model: Aristotle, Ptolomy The heliocentric model: Copernicus, Galileo, Kepler Isaac Newton Gravitation Physical model Highlights:  Highlights Energy Definition Forms of energy Conservation of energy Light as a form of energy Light Light as particles Light as a wave The electromagnetic spectrum Emission lines and absorption lines The uses of a spectrum Quick Review of Chapters 4.1, 5.2 and 5.4:  Quick Review of Chapters 4.1, 5.2 and 5.4 RECAP Energy is the ability to do work, i.e. the ability to move or change the state of matter. The conservation of energy: Energy is neither created nor destroyed, but may be changed in form.:  RECAP Energy is the ability to do work, i.e. the ability to move or change the state of matter. The conservation of energy: Energy is neither created nor destroyed, but may be changed in form. Light is a form of energy. Why is this important? With very few exceptions, the only way we have to study objects in Astronomy is via the light they emit.:  Light is a form of energy. Why is this important? With very few exceptions, the only way we have to study objects in Astronomy is via the light they emit. What is the nature of light? Light can be thought of as a wave in an electric field or as discrete particles of energy… :  What is the nature of light? Light can be thought of as a wave in an electric field or as discrete particles of energy… How light interacts with matter and the line spectrum.:  How light interacts with matter and the line spectrum. What are Things Made of?:  What are Things Made of? Among other things, chemistry is the study of matter and its composition. Most substances around us can be divided chemically into simpler things: Water --> hydrogen and oxygen Table salt --> sodium and chlorine … At some point, certain things don’t chemically break down into different parts. These are called elements. What are Things Made of?:  What are Things Made of? At some point, certain things don’t chemically break down into different parts. These are called elements. Examples of elements: hydrogen, helium, carbon, oxygen, gold, silver, mercury, uranium, … There are 92 stable and common elements. What are Things Made of?:  What are Things Made of? At some point, certain things don’t chemically break down into different parts. These are called elements. Suppose you took a sample of an element and physically divided the sample into two, and took one of the halves and divided it into two, and so on. Can you go on forever dividing by two? What are Things Made of?:  What are Things Made of? At some point, certain things don’t chemically break down into different parts. These are called elements. Suppose you took a sample of an element and physically divided the sample into two, and took one of the halves and divided it into two, and so on. Can you go on forever dividing by two? No, at some point you reach individual atoms. An atom cannot be split into parts without changing it. How Light Interacts with Matter.:  How Light Interacts with Matter. Atoms are the basic blocks of matter. They consist of heavy particles (called protons and neutrons) in the nucleus, surrounded by lighter particles called electrons. The number of protons determines which element the atom is. How Light Interacts with Matter.:  How Light Interacts with Matter. An electron will interact with a photon. An electron that absorbs a photon will gain energy. An electron that loses energy must emit a photon. The total energy (electron plus photon) remains constant during this process. How Light Interacts with Matter.:  How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Thus, only photons of certain energy can interact with the electrons in a given atom. How Light Interacts with Matter.:  How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Thus, only photons of certain energy can interact with the electrons in a given atom. Image from Nick Strobel (http://www.astronomynotes.com) How Light Interacts with Matter.:  How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Each element has its own unique pattern of energies. How Light Interacts with Matter.:  How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Each element has its own unique pattern of energies, hence its own distinct line spectrum. Image from Nick Strobel (http://www.astronomynotes.com) Emission spectra and absorption spectra.:  Emission spectra and absorption spectra. Emission and Absorption:  Emission and Absorption Image from Nick Strobel (http://www.astronomynotes.com) Tying things together::  Tying things together: The spectrum of a star is approximately a black body spectrum. Hotter stars are bluer, cooler stars are redder. For a given temperature, larger stars give off more energy than smaller stars. Slide28:  In the constellation of Orion, the reddish star Betelgeuse is a relatively cool star. The blue star Rigel is relatively hot. Tying things together::  Tying things together: The spectrum of a star is approximately a black body spectrum. Hotter stars are bluer, cooler stars are redder. For a given temperature, larger stars give off more energy than smaller stars. However, a closer look reveals details in the spectra… The Line Spectrum:  The Line Spectrum Upon closer examination, the spectra of real stars show fine detail. Dark regions where there is relatively little light are called lines. The Line Spectrum:  The Line Spectrum Today, we rarely photograph spectra, but rather plot the intensity vs the wavelength. The “lines” where there is relatively little light show up as dips in the curves. The Line Spectrum:  The Line Spectrum Today, we rarely photograph spectra, but rather plot the intensity vs the wavelength. The “lines” where there is relatively little light show up as dips in the curves. These dips tell us about what elements are present in the star! Atomic Fingerprints:  Atomic Fingerprints Hydrogen has a specific line spectrum. Each atom has its own specific line spectrum. Atomic Fingerprints:  Atomic Fingerprints These stars have absorption lines with the wavelengths corresponding to hydrogen! Atomic Fingerprints.:  Atomic Fingerprints. One can also look at the spectra of other objects besides stars, for example clouds of hot gas. This cloud of gas looks red since its spectrum is a line spectrum from hydrogen gas. Good Review Questions, Chapter 2:  Good Review Questions, Chapter 2 5. In what ways is the celestial sphere a scientific model? 6. Where would you go on Earth if you wanted to be able to see both the north celestial pole and the south celestial pole at the same time? 8. Explain how to make a simple astronomical observation to determine your latitude. 13. Why are the seasons reversed in the southern hemisphere relative to the northern hemisphere? 15. Do the phases of the Moon look the same from everywhere on Earth …? Good Review Questions, Chapter 3:  Good Review Questions, Chapter 3 2. How did the Ptolemaic model explain retrograde motion of the planets? 3. In what ways were the models of Ptolomy and Copernicus similar? 7. Explain how Kepler’s laws contradict uniform circular motion. 9. Review Galileo’s telescope discoveries and explain why they supported the Copernican model and contradicted the Ptolemaic model? 13. Explain why you might describe the orbital motion of the moon with the statement “The moon is falling”. Good Review Questions, Chapter 2:  Good Review Questions, Chapter 2 5. In what ways is the celestial sphere a scientific model? 6. Where would you go on Earth if you wanted to be able to see both the north celestial pole and the south celestial pole at the same time? 8. Explain how to make a simple astronomical observation to determine your latitude. 13. Why are the seasons reversed in the southern hemisphere relative to the northern hemisphere? 15. Do the phases of the Moon look the same from everywhere on Earth …? The Celestial Sphere:  The Celestial Sphere Imagine the sky as a hollow sphere with the stars attached to it. This sphere rotates once every 24 hours. This imaginary sphere is called the celestial sphere. Even though we know it is not the case, it is useful to imagine the Earth as being stationary while the celestial sphere rotates around it. The Celestial Sphere:  The Celestial Sphere The north celestial pole is directly above the north pole on the Earth. The south celestial pole is directly above the south pole on the Earth. The celestial equator is an extension of the Earth’s equator on the sky. The zenith is the point directly over your head. The horizon is the circle 90 degrees from the zenith. The Celestial Sphere:  The Celestial Sphere The celestial poles and the celestial equator are the same for everyone. The zenith and the horizon depend on where you stand. http://www.astronomynotes.com/nakedeye/s4.htm Stellar Coordinates and Precession:  Stellar Coordinates and Precession There are a few ways to specify the location of a star (or planet) on the sky: Altitude/Azimuth: The altitude describes how many degrees the star is above the horizon, the azimuth describes how far the star is in the east-west direction from north. The altitude and azimuth of a star is constantly changing owing to the motion of the star on the sky! Stellar Coordinates and Precession:  Stellar Coordinates and Precession There are a few ways to specify the location of a star (or planet) on the sky: Equatorial system: Lines of longitude on the earth become right ascension, measured in units of time. The RA increases in the easterly direction. Lines on latitude on the earth become declination, measured in units of degrees. DEC=90o at the north celestial pole, 0o at the equator, and -90o at the south celestial pole. http://www.astronomynotes.com/nakedeye/s6.htm Stellar Coordinates and Precession:  Stellar Coordinates and Precession The north celestial pole moves with respect to the stars very slowly with time, taking 26,000 years to complete one full circle. Good Review Questions, Chapter 2:  Good Review Questions, Chapter 2 5. In what ways is the celestial sphere a scientific model? 6. Where would you go on Earth if you wanted to be able to see both the north celestial pole and the south celestial pole at the same time? 8. Explain how to make a simple astronomical observation to determine your latitude. 13. Why are the seasons reversed in the southern hemisphere relative to the northern hemisphere? 15. Do the phases of the Moon look the same from everywhere on Earth …? In Detail::  In Detail: If we do some careful observations, we find: The length of the daylight hours at a given spot varies throughout the year: the Sun is out a longer time when it is warmer (i.e. summer), and out a shorter time when it is colder. On a given day, the length of the daylight hours depends on where you are on Earth, in particular it depends on your latitude: e.g. in the summer, the Sun is out longer and longer the further north you go. In Detail::  In Detail: Near the North Pole, the Sun never sets in the middle of the summer (late June). Likewise, the Sun never rises in the middle of the winter (late December). In Detail::  In Detail: In most places on Earth, the weather patterns go through distinct cycles: Cold weather: winter, shorter daytime Getting warmer: spring, equal daytime/nighttime Warm weather: summer, longer daytime Cooling off: fall, equal daytime/nighttime These “seasons” are associated with the changing day/night lengths. In Detail::  In Detail: When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and the other way around. What Causes the Seasons?:  What Causes the Seasons? Is the Earth closer to the Sun during summer, and further away during winter? (This was the most commonly given answer during a poll taken at a recent Harvard graduation). No! Otherwise the seasons would not be opposite in the northern and southern hemispheres. What Causes the Seasons?:  What Causes the Seasons? The Earth moves around the Sun. A year is defined as the time it takes to do this, about 365.25 solar days. This motion takes place in a plane in space, called the ecliptic. The axis of the Earth’s rotation is inclined from this plane by about 23.5 degrees from the normal. What Causes the Seasons?:  What Causes the Seasons? The axis of the Earth’s rotation points to the same point in space (roughly the location of the North Star). The result is the illumination pattern of the Sun changes throughout the year. What Causes the Seasons?:  What Causes the Seasons? Here is an edge-on view, from the plane of the Earth’s orbit. What Causes the Seasons?:  What Causes the Seasons? Here is a slide from NASA and NOAA. What Causes the Seasons?:  What Causes the Seasons? A slide from Nick Strobel. What Causes the Seasons?:  What Causes the Seasons? Because of the tilt of the Earth’s axis, the altitude the Sun reaches changes during the year: It gets higher above the horizon during the summer than it does during the winter. What Causes the Seasons?:  What Causes the Seasons? Because of the tilt of the Earth’s axis, the altitude the Sun reaches changes during the year: It gets higher above the horizon during the summer than it does during the winter. Also, the length of the daytime hours changes during the year: the daylight hours are longer in the summer and shorter in winter. What Causes the Seasons?:  What Causes the Seasons? The altitude of the Sun matters: when the Sun is near the horizon, it does not heat as efficiently as it does when it is high above the horizon. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/). What Causes the Seasons?:  What Causes the Seasons? Winter: The combination of a short daytime and a Sun that is relatively low above the horizon leads to much less heating in the day, plus a longer period of cooling at night. Overall, it is colder. What Causes the Seasons?:  What Causes the Seasons? Summer: The combination of a long daytime and a Sun that is relatively high above the horizon leads to much more heating in the day, plus a shorter period of cooling at night. Overall, it is warmer. What Causes the Seasons?:  What Causes the Seasons? Spring and Fall: The number of hour of daylight is about equal to the number of nighttime hours, leading to roughly equal times of heating and cooling. Good Review Questions, Chapter 2:  Good Review Questions, Chapter 2 5. In what ways is the celestial sphere a scientific model? 6. Where would you go on Earth if you wanted to be able to see both the north celestial pole and the south celestial pole at the same time? 8. Explain how to make a simple astronomical observation to determine your latitude. 13. Why are the seasons reversed in the southern hemisphere relative to the northern hemisphere? 15. Do the phases of the Moon look the same from everywhere on Earth …? What Causes the Phases of the Moon?:  What Causes the Phases of the Moon? What Causes the Phases of the Moon?:  What Causes the Phases of the Moon? The full Moon always rises just after sunset. The crescent Moon always points towards the Sun. A crescent Moon sets shortly after sunset, or rises just before sunrise. The Moon is illuminated by reflected sunlight. What Causes the Phases of the Moon?:  What Causes the Phases of the Moon? The full Moon always rises just after sunset. A crescent Moon sets shortly after sunset. What Causes the Phases of the Moon?:  What Causes the Phases of the Moon? The full Moon always rises just after sunset. A crescent Moon sets shortly after sunset. What Causes the Phases of the Moon?:  What Causes the Phases of the Moon? The lit side of the Moon always faces the Sun. Because of the motion of the Moon relative to the Sun, we see different amounts of lit and dark sides over the course of a month. What Causes the Phases of the Moon?:  What Causes the Phases of the Moon? The lit side of the Moon always faces the Sun. Because of the motion of the Moon relative to the Sun, we see different amounts of lit and dark sides over the course of a month. Image from Nick Strobel (http://www.astronomynotes.com/) Good Review Questions, Chapter 2:  Good Review Questions, Chapter 2 5. In what ways is the celestial sphere a scientific model? 6. Where would you go on Earth if you wanted to be able to see both the north celestial pole and the south celestial pole at the same time? 8. Explain how to make a simple astronomical observation to determine your latitude. 13. Why are the seasons reversed in the southern hemisphere relative to the northern hemisphere? 15. Do the phases of the Moon look the same from everywhere on Earth …? Good Review Questions, Chapter 3:  Good Review Questions, Chapter 3 2. How did the Ptolemaic model explain retrograde motion of the planets? 3. In what ways were the models of Ptolomy and Copernicus similar? 7. Explain how Kepler’s laws contradict uniform circular motion. 9. Review Galileo’s telescope discoveries and explain why they supported the Copernican model and contradicted the Ptolemaic model? 13. Explain why you might describe the orbital motion of the moon with the statement “The moon is falling”. Aristotle (385-322 B.C.):  Aristotle (385-322 B.C.) Aristotle was perhaps the most influential Greek philosopher. He favored a geocentric model for the Universe: The Earth is at the center of the Universe. The heavens are ordered, harmonious, and perfect. The perfect shape is a sphere, and the natural motion was rotation. Geocentric Model:  Geocentric Model The motion of the Sun around the Earth accounts for the rising and setting of the Sun. The motion of the Moon around the Earth accounts for the rising and setting of the Moon. You have to fiddle a bit to get the Moon phases. Geocentric Model:  Geocentric Model The fixed stars were on the “Celestial Sphere” whose rotation caused the rising and setting of the stars. Slide74:  This is the constellation of Orion Slide75:  The constellations rise and set each night, and individual stars make a curved path across the sky. The curvature of the tracks depend on where you look. Geocentric Model:  Geocentric Model The fixed stars were on the “Celestial Sphere” whose rotation caused the rising and setting of the stars. However, the detailed motions of the planets were much harder to explain… Planetary Motion:  Planetary Motion The motion of a planet with respect to the background stars is not a simple curve. This shows the motion of Mars. Sometimes a planet will go “backwards”, which is called “retrograde motion.” Planetary Motion:  Planetary Motion Here is a plot of the path of Mars. Other planets show similar behavior. Image from Nick Strobel Astronomy Notes (http://www.astronomynotes.com/) Aristotle’s Model:  Aristotle’s Model Aristotle’s model had 55 nested spheres. Although it did not work well in detail, this model was widely adopted for nearly 1800 years. Better Predictions:  Better Predictions Although Aristotle’s ideas were commonly accepted, there was a need for a more accurate way to predict planetary motions. Better Predictions:  Better Predictions Although Aristotle’s ideas were commonly accepted, there was a need for a more accurate way to predict planetary motions. Claudius Ptolomy (85-165) presented a detailed model of the Universe that explained retrograde motion by using complicated placement of circles. Ptolomy’s Epicycles:  Ptolomy’s Epicycles By adding epicycles, very complicated motion could be explained. Ptolomy’s Epicycles:  Ptolomy’s Epicycles Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/). Ptolomy’s Epicycles:  Ptolomy’s Epicycles Ptolomy’s Epicycles:  Ptolomy’s Epicycles Ptolomy’s model was considered a computational tool only. Aristotle’s ideas were “true”. They eventually became a part of Church dogma in the Middle Ages. The Sun-Centered Model:  The Sun-Centered Model Nicolaus Copernicus (1473-1543) proposed a heliocentric model of the Universe. The Sun was at the center, and the planets moved around it in perfect circles. The Sun-Centered Model:  The Sun-Centered Model Nicolaus Copernicus (1473-1543) proposed a heliocentric model of the Universe. These stamps mark the 500th anniversary of his birth. The Sun-Centered Model:  The Sun-Centered Model The Sun was at the center. Each planet moved on a circle, and the speed of the planet’s motion decreased with increasing distance from the Sun. The Sun-Centered Model:  The Sun-Centered Model Retrograde motion of the planets could be explained as a projection effect. The Sun-Centered Model:  The Sun-Centered Model Retrograde motion of the planets could be explained as a projection effect. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/) Copernican Model:  Copernican Model The model of Copernicus did not any better than Ptolomy’s model in explaining the planetary motions in detail. He did work out the relative distances of the planets from the Sun. The philosophical shift was important (i.e. the Earth is not at the center of the Universe). Good Review Questions, Chapter 3:  Good Review Questions, Chapter 3 2. How did the Ptolemaic model explain retrograde motion of the planets? 3. In what ways were the models of Ptolomy and Copernicus similar? 7. Explain how Kepler’s laws contradict uniform circular motion. 9. Review Galileo’s telescope discoveries and explain why they supported the Copernican model and contradicted the Ptolemaic model? 13. Explain why you might describe the orbital motion of the moon with the statement “The moon is falling”. Johannes Kepler (1571-1630):  Johannes Kepler (1571-1630) Kepler was a mathematician by training. He believed in the Copernican view with the Sun at the center and the motions of the planets on perfect circles. Tycho hired Kepler to analyize his observational data. Johannes Kepler (1571-1630):  Johannes Kepler (1571-1630) Kepler was a mathematician by training. He believed in the Copernican view with the Sun at the center and the motions of the planets on perfect circles. Tycho hired Kepler to analyize his observational data. After years of failure, Kepler dropped the notion of motion on perfect circles. Kepler’s Three Laws of Planetary Motion:  Kepler’s Three Laws of Planetary Motion Starting in 1609, Kepler published three “laws” of planetary motion: Kepler’s Three Laws of Planetary Motion:  Kepler’s Three Laws of Planetary Motion Starting in 1609, Kepler published three “laws” of planetary motion: Planets orbit the Sun in ellipses, with the Sun at one focus. Ellipses:  Ellipses An ellipse is a “flattened circle” described by a particular mathematical equation. The eccentricity tells you how flat the ellipse is: e=0 for circular, and e=1 for infinitely flat. Ellipses:  Ellipses You can draw an ellipsed with a loop of string and two tacks. Kepler’s Three Laws of Planetary Motion:  Kepler’s Three Laws of Planetary Motion Starting in 1609, Kepler published three “laws” of planetary motion: Planets orbit the Sun in ellipses, with the Sun at one focus. Kepler’s Three Laws of Planetary Motion:  Kepler’s Three Laws of Planetary Motion Starting in 1609, Kepler published three “laws” of planetary motion: Planets orbit the Sun in ellipses, with the Sun at one focus. The planets sweep out equal areas in equal times. That is, a planet moves faster when it is closer to the Sun, and slower when it is further away. Kepler’s Second Law:  Kepler’s Second Law The time it takes for the planet to move through the green sector is the same as it is to move through the blue sector. Both sectors have the same area. Kepler’s Three Laws of Planetary Motion:  Kepler’s Three Laws of Planetary Motion Starting in 1609, Kepler published three “laws” of planetary motion: Planets orbit the Sun in ellipses, with the Sun at one focus. The planets sweep out equal areas in equal times. That is, a planet moves faster when it is closer to the Sun, and slower when it is further away. Kepler’s Three Laws of Planetary Motion:  Kepler’s Three Laws of Planetary Motion Starting in 1609, Kepler published three “laws” of planetary motion: Planets orbit the Sun in ellipses, with the Sun at one focus. The planets sweep out equal areas in equal times. That is, a planet moves faster when it is closer to the Sun, and slower when it is further away. (Period)2 = (semimajor axis)3 Kepler’s Third Law:  Kepler’s Third Law Good Review Questions, Chapter 3:  Good Review Questions, Chapter 3 2. How did the Ptolemaic model explain retrograde motion of the planets? 3. In what ways were the models of Ptolomy and Copernicus similar? 7. Explain how Kepler’s laws contradict uniform circular motion. 9. Review Galileo’s telescope discoveries and explain why they supported the Copernican model and contradicted the Ptolemaic model? 13. Explain why you might describe the orbital motion of the moon with the statement “The moon is falling”. Heliocentric or Geocentric?:  Heliocentric or Geocentric? The year is around 1610. The “old” school is Aristotle and a geocentric view. The “new” school is the heliocentric view (Copernicus and Kepler). Which one is correct? Observational support for the heliocentric model would come from Galileo. Theoretical support for the heliocentric model would come from Isaac Newton. Galileo Galilei (1564-1642):  Galileo Galilei (1564-1642) Galileo was one of the first to use a telescope to study astronomical objects, starting in about 1609. His observations of the moons of Jupiter and the phases of Venus provided strong support for the heliocentric model. Venus:  Venus Venus, the brightest planet, is never far from the Sun: it sets at most a few hours after sunset, or rises at most a few hours before sunrise. It is never out in the middle of the night. Venus:  Venus Galileo discovered that Venus had phases, just like the Moon. Furthermore, the crescent Venus was always larger than the full Venus. Conclusion: Venus shines by reflected sunlight, and it is closer to Earth when it is a crescent. Venus in the Geocentric View:  Venus in the Geocentric View Venus is always close to the Sun on the sky, so its epicycle restricts its position. In this view, Venus always appears as a crescent. Venus in the Heliocentric View:  Venus in the Heliocentric View In the heliocentric view, Venus orbits the Sun closer than the Earth does. We on Earth can see a fully lit Venus when it is on the far side of its orbit. Venus in the Heliocentric View:  Venus in the Heliocentric View The correlation between the phases and the size is accounted for in the heliocentric view. Good Review Questions, Chapter 3:  Good Review Questions, Chapter 3 2. How did the Ptolemaic model explain retrograde motion of the planets? 3. In what ways were the models of Ptolomy and Copernicus similar? 7. Explain how Kepler’s laws contradict uniform circular motion. 9. Review Galileo’s telescope discoveries and explain why they supported the Copernican model and contradicted the Ptolemaic model? 13. Explain why you might describe the orbital motion of the moon with the statement “The moon is falling”. Newton’s Laws of Motion:  Newton’s Laws of Motion A body in motion tends to stay in motion in a straight line unless acted upon by an external force. The force on an object is the mass times the acceleration (F=ma). For every action, there is an equal and opposite reaction. (For example, a rocket is propelled by expelling hot gas from its thrusters). What is Gravity?:  What is Gravity? Gravity is a force between all matter in the Universe. It is difficult to say what gravity is. However, we can describe how it works. What is Gravity?:  What is Gravity? The gravitational force between larger bodies is greater than it is between smaller bodies, for a fixed distance. What is Gravity?:  What is Gravity? As two bodies move further apart, the gravitational force decreases. The range of the force is infinite, although it is very small at very large distances. Newton’s Laws:  Newton’s Laws Using Newton’s Laws, we can… Derive Kepler’s Three Laws. Measure the mass of the Sun, the Moon, and the Planets. Measure the masses of distant stars in binary systems. Laws of Physics:  Laws of Physics The models of Aristotle and Ptolomy were based mainly on beliefs (i.e. that motion should be on perfect circles, etc.). Starting with Newton, we had a physical model of how the planets moved: the laws of motion and gravity as observed on Earth give a model for how the planets move. All modern models in Astronomy are based on the laws of Physics. Newton’s Laws and Orbits:  Newton’s Laws and Orbits Newton realized that since the Moon’s path is curved (i.e. it is accelerating), there must be a force acting on it. Newton’s Laws and Orbits:  Newton’s Laws and Orbits If you shoot a cannonball horizontally, it follows a curved path to the ground. The faster you launch it, the further it goes. Newton’s Laws and Orbits:  Newton’s Laws and Orbits If you shoot a cannonball horizontally, it follows a curved path to the ground. The faster you launch it, the further it goes. If it goes really far, the Earth curves from under it… Newton’s Laws and Orbits:  Newton’s Laws and Orbits Newton showed mathematically that the expected shape for a closed orbit is an ellipse (i.e. he explained the origin of Kepler’s first law). Newton’s Laws and Orbits:  Newton’s Laws and Orbits A geosynchronous satellite has an orbital period around the Earth of 24 hours (23 hours and 56 minutes actually), which is the rotation period of the Sun. The net effect is that the satellite is always above the same spot. Good Review Questions, Chapter 3:  Good Review Questions, Chapter 3 2. How did the Ptolemaic model explain retrograde motion of the planets? 3. In what ways were the models of Ptolomy and Copernicus similar? 7. Explain how Kepler’s laws contradict uniform circular motion. 9. Review Galileo’s telescope discoveries and explain why they supported the Copernican model and contradicted the Ptolemaic model? 13. Explain why you might describe the orbital motion of the moon with the statement “The moon is falling”.

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