3 YouPlease take out a piece of paper, or compose an to me on your phone/computer, and answer:Name & Student ID# (also for attendance)How many semesters have you completed at MSU?What do you know or have you heard about astronomy?What is your dream career?Why are you taking this class?What do you dread doing? What do you love doing?Can you give me tips on how you learn best?What would you like to know about me?What would you like me to know about you?What can I teach you?What can you teach me?
4 Syllabus & Exams The internet will be used extensively in this class. Clicker- you need one. Get it registered (there is help on my course web page)The internet will be used extensively in this class.Lecture notes will be posted after class.Please let me know if you need helpaccessing the web, or if the web pages are not working.
5 Extra Credit OptionsExtra credit is available and can contribute up to 5-10%. The purpose of extra credit is to find a balance between your interests and the subject material of astronomy. Possible extra credit options must be approved by the instructor, but can include for example:Visit Baker Observatory. Bring back proof - photographic - of your visit. To qualify for the extra credit, please write a one page summary of your visit.Astronomy as art. Many of you are non-science majors, and excel in other areas of specialization - art, writing, music, etc. There is a vast history of art inspired by astronomy. See for an example. For another example, consider constructing a scale model of our galaxy or local group of galaxies.In order to qualify for the extra credit, please create an original work of art, music or writing inspired by astronomy and the material you have learned in class. You may team with up to two other people.ALL EXTRA CREDIT REPORTS ARE DUE BY THE FINAL EXAM, DIRECTLY TO ME.
6 “Group” Projects Sign up for a Thursday to present to the class. Two people will present each week, and there will be a competition for the better presentation. Your classmates will vote!Presentations can be voice only, Powerpoint, Keynote, PDF. Please blog about your presentation here:Presentations are minutes eachFrom each lecture’s topic, pick a subject to go into detail on. Consult with the professor on the topics covered in class well before you prepare your presentation.Will count towards 10% of final grade.
7 Our Modern View of the Universe Our Place in the UniverseOur Modern View of the Universe
8 A Modern View of the Universe Our goals for learning:What is our place in the universe?How did we come to be?Can we know what the universe was like in the past?Can we see the entire universe?
9 AstronomyThe branch of science dedicated to the study of everything in the Universe that lies above Earth’s atmosphere
10 What is our place in the universe? 80 yr agoWhat is our place in the universe?Here at the beginning of the semester, our main goal is simply to make sure that students understand the basic hierarchy of structure from planet to solar system to galaxy to universe (clustering is less important at this stage). You might also wish to note that this slide shows a lot about human history. E.g., we first learned that Earth is a sphere some 2,500 years ago; we learned that Earth is a planet going around the Sun only about 400 years ago; and we learned that the Millky Way is only one of many galaxies with the work of Hubble some 80 years ago…400 yr2500 yr
11 StarA large, glowing ball of mostly hydrogen gas that generates heat and light through nuclear fusion at its centerContext: “gas” in astronomy usually means H, or H2, but could include other atoms and molecules in the gas state.Our star – the Sun
12 PlanetMarsNeptuneA moderately large object that orbits (goes around) a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition.
13 Moon (or satellite) An object that orbits a planet. The terms moon and satellite are often used interchangeably, but artificial objects (spacecraft) are usually called satellites and not moons…Ganymede (orbits Jupiter)
14 AsteroidA relatively small and rocky object which orbits a star.
15 Comet A relatively small and icy object that orbits a star. Also worth noting: (1) the basic difference between an asteroid and a comet is composition; (2) comets have tails ONLY when they come close to the Sun, not when they are much farther away.
16 Solar (Star) SystemA star and all the material that orbits it, including its planets and moons.Note: planets and orbits are not to scale; planets are tiny compared to their orbits.
17 of hydrogen gas and/or tiny smoke-like particles called “dust” NebulaAn interstellar cloudof hydrogen gas and/or tiny smoke-like particles called “dust”Note: We do not include “nebula” in our list of basic definitions in ch. 1, because it is a less important term at this point in the course. However, you may find yourself talking about nebulae (e.g., Orion Nebula) if you are doing any early-term observing with your students, in which case you may find this slide useful.
18 GalaxyA great island of stars in space, all held together by gravity and orbiting a common centerRemember that one of the most common student problems is confusion between the terms “solar system” and “galaxy.” You can use these slides of basic definitions to help combat this problem.M31, The Great Galaxy in Andromeda
19 UniverseThe sum total of all matter and energy; that is, everything within and between all galaxies
20 How can we know what the universe was like in the past? The key: light travels at a finite speed300,000 km/s, 186,000 miles/s, 670 million miles per hourYou can circle Earth 8 times in 1 secondMore on the Speed of Light when we discuss RelativityDestinationLight travel timeMoon1 secondSun8 minutesSirius8 yearsAndromeda Galaxy2.5 million yearsPoint out how fast the speed of light is: could circle Earth 8 times in one second….Also note that the speed of light is always the same…Thus, we see objects as they were in the past:The farther away we look in distance, the further back we look in time.
21 Light-year The distance light can travel in one year. About 10 trillion km (6 trillion miles).A light-year is NOT a unit of time!Can also talk about light-seconds.At great distances we see objects as they were when the universe was much younger.
22 How far is a light-year?1 year = 31.5 million seconds
23 How far is a light-year?Approximately 10 trillion kilometers; exact value is 9.46 trillionIs this a large distance?Yes! The distance from the Earth to the Sun is 150,000,000 km, also known as 1 Astronomical Unit (AU). So, 1 light-year is over 63,000 AU.Emphasize that a light-year is a unit of distance NOT a unit of time. Remind students of Common Misconception box in text (p. 6). Equation is optional: it is not given in the book, but should be easy for most students to follow.
24 What have we learned? What is our physical place in the universe? Earth is part of the Solar System, which is in the Milky Way galaxy, which is a member of the Local Group of galaxies in the Local SuperclusterHow can we know that the universe was like in the past?When we look to great distances we are seeing events that happened long ago because light travels at a finite speedCan we see the entire universe?No, the observable portion of the universe is about 13.7 billion light-years in radius because the universe is about 13.7 billion years old. (We may round this number to 14 billion for convenience but the best modern measurements give 13.7 billion years since the time from the Big Bang.)
25 The Scale of the Universe Our goals for learning:How big is Earth compared to our solar system?How far away are the stars?How big is the Milky Way Galaxy?How big is the universe?How do our lifetimes compare to the age of the universe?It is very important to grasp the huge distances and enormous time spans that we deal with in astronomy. The way to do this is to create a SCALE MODEL.
26 The Moon is 384 million m from Earth (~250,000 miles). The easy questionsHow far away are the Sun and Moon?The Moon is 384 million m from Earth (~250,000 miles).The Sun is 1.5x1011m from Earth (150 billion m, or ~93 million miles). This distance is called 1 astronomical unit (AU).
27 How big is Earth compared to our Solar System How big is Earth compared to our Solar System? Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter or about 5.6 inches; 2.54 cm = 1 inch). How big is Earth on this scale?an atoma ball pointa marblea golf ballThis slide begins our discussion of the scale of the solar system, introducing the 1-to-10 billion scale used in the book. As shown here, a good way to start is to show students the size of the Sun on this scale, then ask them to guess the size of Earth in comparison.Suggestion: Bring a large grapefruit (or similar size ball) to class to represent the Sun; also have a 1 mm ball bearing to represent Earth for the correct answer to the multiple choice question.Radius of the Sun = 700,000 kmDiameter Sun = 1.4 x 1011 cmDivide by 1010 to get 14 cm
28 Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter). How big is Earth on this scale?an atoma ball pointa marblea golf ballHere is the answer to the question. Emphasize the incredible difference in size; then ask students to think about how far from the Sun they think Earth would be on this scale…How far apart would our model Sun and Earth be on this scale?
29 The scale of the solar system On a 1-to-10 billion scale:Sun is size of a large grapefruit (14 cm)Earth is size of a ball point, 15 meters away15 meters is about 107 grapefruitsWhich means in the real solar system you could fit about107 Suns into the Earth –Sun distanceUse this slide/tool to introduce the Voyage scale (Voyage is the name of the model solar system using this scale on the National Mall in Washington, DC). At this point,we suggest that you stick to the solar system in using this tool; we’ll move out to the stars in the next couple of slides. Other points to emphasize:This tool taken from the Scale of the Universe tutorial on the web site; encourage students to try it for themselves.Voyage shows a straight line, but remember that planets orbit. In class, put the grapefruit for the Sun on a table, then walk 15 meters to Earth position, and ask students to imagine it going around Sun once a year.A major lesson from all this is that the solar system is almost entirely made of nearly empty space (that’s why they call it “space”!).
30 Answer: D, the distance across the U.S. How far away are the stars? On our 1-to-10 billion scale, it’s just a few minutes walk to Pluto. [See model of Jefferson Ave.] How far would you have to walk to reach the nearest star to the Sun - Alpha Centauri?1 mile10 miles100 milesthe distance across the U.S. (2500 miles)Now we continue outward to the stars. Try asking this question of your students; most are quite surprised at how far away the stars are…Answer: D, the distance across the U.S.
31 a few weeks a few months a few years a few thousand years Thought Question Suppose you tried to count the more than 100 billion (1011) stars in our galaxy, at a rate of one per second… How long would it take you?a few weeksa few monthsa few yearsa few thousand yearsAsk this question to get students thinking about the vast number of stars in our galaxy…
32 Suppose you tried to count the more than 100 billion stars in our galaxy, at a rate of one per second… How long would it take you?a few weeksa few monthsa few yearsa few thousand years (100 billion seconds is nearly 3,200 years. Why? Because there are about 30 million seconds in a year, so 1011/3x107 = 0.33x104 = 3.3 x 103)Here is the answer to the question. Worth noting:a simple calculation shows that 100 billion seconds is nearly 3,200 yearsFun to note that younger kids often say they could count FASTER than 1 per second.. But can they still when they get to, say, 54,537,691,702? How fast can they say THAT number, and can they even remember what comes next?So the rules if you want to count to 100 billion: start soon, limit your breaks, and most importantly: don’t die!Also remind students that the years is just to COUNT the stars, not even to give them names, study them to see if they have planets, or etc.
33 How big is the (observable) Universe? The Milky Way is one of about 100 billion galaxies.(1011 stars/galaxy) x (1011 galaxies) = 1022 starsThis slide gives our favorite way of giving students a sense of the size of the observable universe; be sure to note we are talking about the OBSERVABLE universe, since we do not know the extent of the ENTIRE universe.As many stars as grains of (dry) sand on all Earth’s beaches…
34 How do human lifetimes compare to the AGE of the Universe? The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year!Our favorite way to present the scale of time: a modified version of Carl Sagan’s Cosmic Calendar. Worth noting
35 How do human lifetimes compare to the age of the Universe? The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year. 1 day represents about 40 million years; 1 second represents about 440 years.Our favorite way to present the scale of time: a modified version of Carl Sagan’s Cosmic Calendar. Worth noting:Since we are compressing the 14 billion-year history of the universe into one calendar year, 1 month represents about 1.2 billion real years, 1 day represents about 40 million years; 1 second represents about 440 years.the universe already 2/3 of the way through its history before our solar system even formed.dinosaurs arose the day after Christmas, died yesterday.All of (recorded) human history is in the last 30 seconds.You and I were born about 0.05 seconds before midnight, Dec. 31.
36 What have we learned? How big is Earth compared to our solar system? The distances between planets are huge compared to their sizes—on a scale of 1-to-10 billion, Earth is the size of a ball point and the Sun is 15 meters awayHow far away are the stars?On the same scale, the stars are thousands of km awayHow big is the Milky Way galaxy?It would take more than 3,000 years to count the stars in the Milky Way Galaxy at a rate of one per second, and they are spread across 100,000 light-years
37 What have we learned? How big is the universe? The observable universe is almost 14 billion light-years in radius and contains over 100 billion galaxies with a total number of stars comparable to the number of grains of sand on all of Earth’s beachesHow do our lifetimes compare to the age of the universe?On a cosmic calendar that compresses the history of the Universe into one year, human civilization is just a few seconds old, and a human lifetime is a fraction of a second
38 Spaceship Earth Our goals for learning: How is Earth moving in our solar system?How is our solar system moving in the Galaxy?How do galaxies move within the Universe?Are we ever sitting still?
39 How is Earth moving in our solar system? Contrary to our perception, we are not “sitting still.”We are moving with the Earth in several ways, and at surprisingly fast speeds…The Earth rotates around its axis once every day.Our first motion is ROTATION.Point out that most of us are moving in circles around the axis at speeds far faster than commercial jets travel, which is why jets cannot keep up with the Sun when going opposite Earth’s rotation…The spin rate at the Equator is ~1000 mph, twice as fast as a commercial airliner.
40 Earth orbits the Sun (revolves) once every year: at an average distance of 1 AU ≈ 150 million kmwith Earth’s axis tilted by 23.5º (pointing to Polaris)and rotating in the same direction it orbits, counter-clockwise as viewed from above the North Pole.Our second motion is ORBIT. Point out the surprisingly high speed of over 100,000 km/hr.
41 … And orbits the center of the Milky Way galaxy Our Sun moves relative to the other stars in the local Solar neighborhood…typical relative speeds of more than 70,000 km/hrbut stars are so far away that we cannot easily notice their motion… And orbits the center of the Milky Way galaxyevery 230 million years.Our third and fourth motions are MOTION WITH THE LOCAL SOLAR NEIGHBORHOOD and ROTATION OF THE MILKY WAY GALAXY.
42 More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in modern astronomyMost of Milky Way’s light comes from disk and bulge …Although we won’t discuss dark matter until much later in the course, you might wish to mention it now to whet students’ appetites……. but most of the mass is in a huge and DARK halo.
43 How do galaxies move within the universe? The Universe is expanding.In the 1920s Edwin Hubble discovered that galaxies are carried along with the expansion of the Universe.But how did Hubble figure out that the universe is expanding?Describe the raisin cake analogy, and have students work through the numbers with you to make the table. (E.g., “How far away is Raisin 1 at the beginning of the hour? [1 cm] How far is it at the end of the hour? [3 cm] So how far would you have seen it move during the hour? [2 cm] So how fast is it moving away from you? [2 cm/hr]”
44 Hubble discovered that: All galaxies outside our Local Group are moving away from us.The more distant the galaxy, the faster it is racing away.Conclusion: We live in an expanding universe.Now relate the raisin cake analogy to the real universe…
45 What have we learned? How is Earth moving in our solar system? It rotates on its axis once a day and orbits the Sun at a distance of 1 A.U. = 150 million kmHow is our solar system moving in the Milky Way galaxy?Stars in the Local Neighborhood move randomly relative to one another and orbit the center of the Milky Way in about 230 million years
46 What have we learned? How do galaxies move within the universe? All galaxies beyond the Local Group are moving away from us with expansion of the Universe: the more distant they are, the faster they’re movingAre we ever sitting still?No!OKAY, NOW WE HAVE A GOOD OVERALL PERSPECTIVE. NEXT WE NEED SOME MORE DETAILS
48 Patterns in the Night Sky Our goals for learning:What does the universe look like from Earth?Why do stars rise and set?Why do the constellations we see depend on latitude and time of year?
49 The Celestial SphereStars at different distances all appear to lie on the Celestial Sphere.Ecliptic is Sun’s apparent path through the celestial sphere.The illusion of stars all lying at the same distance in the constellations allows us to define the celestial sphere. It doesn’t really exist, but it’s a useful tool for learning about the sky. When discussing this slide, be sure to explain:North celestial poleSouth celestial poleCelestial equatorEclipticIt’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model.
50 The Celestial SphereThe 88 official constellations cover the celestial sphere.It is important to realize that these named patterns have no relation to life on Earth and the stars in a given constellation are often not connected with each other physically.If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…
51 The Milky WayFish Eye lens viewA band of faint light making a circle around the celestial sphere.What is it?Our view into the “plane” of our spiral galaxy.On the previous slide or your model, you can point out that the celestial sphere is also painted with the Milky Way. Many students may never have seen the Milky Way in the sky (especially if they live in a big city), so the photo here is also worth showing. Key points to emphasize:We use the term Milky Way in two ways: for the band of light in the sky and as the name of our galaxy.(2) The two meanings are closely related. We like to use the following analogy: Ask your students to imagine being a tiny grain of flour inside a very thin pancake (or crepe!) that bulges in the middle and a little more than halfway toward the outer edge. Ask, “What will you see if you look toward the middle?” The answer should be “dough.” Then ask what they will see if they look toward the far edge, and they’ll give the same answer. Proceeding similarly, they should soon realize that they’ll see a band of dough encircling their location, but that if they look away from the plane, the pancake is thin enough that they can see to the distant universe.
52 On the previous slide or your model, you can point out that the celestial sphere is also painted with the Milky Way. Many students may never have seen the Milky Way in the sky (especially if they live in a big city), so the photo here is also worth showing. Key points to emphasize:We use the term Milky Way in two ways: for the band of light in the sky and as the name of our galaxy.(2) The two meanings are closely related. We like to use the following analogy: Ask your students to imagine being a tiny grain of flour inside a very thin pancake (or crepe!) that bulges in the middle and a little more than halfway toward the outer edge. Ask, “What will you see if you look toward the middle?” The answer should be “dough.” Then ask what they will see if they look toward the far edge, and they’ll give the same answer. Proceeding similarly, they should soon realize that they’ll see a band of dough encircling their location, but that if they look away from the plane, the pancake is thin enough that they can see to the distant universe.
53 The Local SkyAltitude (above horizon) Azimuth (along horizon) specifies locationZenith: The point directly overheadHorizon: All points 90° away from zenithMeridian: Line passing through zenith from N to S pointsNow we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object:It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.
54 We measure the sky using angles Full circle = 360º1º = 60 (arcminutes)1 = 60 (arcseconds)Point out that in general we have no way of judging true (physical) sizes and distances of objects in the sky -- like the illusion of stars lying on the celestial sphere, this is due to our lack of depth perception in space. Thus, we can measure only angular sizes and distances. Use these diagrams as examples. Optional: You can show how angular sizes depend on distance by having students sitting at different distances from you in the class use their fists to estimate the angular size of a ball you are holding. Students in the back will measure a smaller angular size.
55 Angular SizeAn object’s angular size appears smaller if it is farther awayUse this slide if you want to review the definitions of arc minutes and arc seconds.Aside: We can define a new unit of angular measure called a radian such that1 radian = 360/2π = 57.3 degrees
56 Why do stars rise and set? The answer to the question is very simple if we look at the celestial sphere from the “outside.” But of course, we are looking from our location on Earth, which makes the motions of stars look a little more complex…Earth rotates west to east, so stars appear to circle from east to west.
57 The sky varies with latitude but not longitude Use this interactive figure to explain the variation in the sky with latitude. Show how the altitude of the NCP equals your latitude (for N. hemisphere)…
58 The sky varies as Earth orbits the Sun As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic.At midnight, the stars on our meridian are opposite the Sun in the sky.Use this interactive figure to explain how the constellations change with the time of year.
59 The Reason for Seasons Our goals for learning: What causes the seasons?How do we mark the progression of the seasons?How does the orientation of Earth’s axis change with time?
60 What causes the seasons? Seasons depend on how Earth’s axis affects the directness of sunlightMisconceptions about the cause of the seasons are so common that you may wish to go over the idea in more than one way. We therefore include several slides on this topic. This slide uses the interactive version of the figure that appears in the book; the following slides use frames from the Seasons tutorial on the Astronomy Place web site.
61 Axis tilt changes directness of sunlight during the year This tool is taken from the Seasons tutorial on the Astronomy Place web site. You can use it to reinforce the ideas from the previous slide. As usual, please encourage your students to try the tutorial for themselves.
62 Sun’s altitude in the sky also changes with seasons Sun’s position at noon in summer: higher altitude means more direct sunlight.This tool is taken from the Seasons tutorial on the MasteringAstronomy web site. You can use it to reinforce the ideas from the previous slide. As usual, please encourage your students to try the tutorial for themselves.Sun’s position at noon in winter: lower altitude means less direct sunlight.
63 How do we mark the progression of the seasons? We define four special points:summer solsticewinter solsticespring (vernal) equinoxfall (autumnal) equinoxHere we focus in on just part of Figure 2.13 to see the four special points in Earth’s orbit, which also correspond to moments in time when Earth is at these points.
64 We can recognize solstices and equinoxes by Sun’s path across sky Summer solstice: Highest path, rise and set at most extreme north of due east.Winter solstice: Lowest path, rise and set at most extreme south of due east.Equinoxes: Sun rises precisely due east and sets precisely due west.Of course, the notes here are true for a N. hemisphere sky. You might ask students which part written above changes for S. hemisphere. (Answer: highest and lowest reverse above, but all the rest is still the same for the S. hemisphere; and remind students that we use names for the N. hemisphere, so that S. hemisphere summer actually begins on the winter solstice…)
65 How does the orientation of Earth’s axis change with time? Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years.Polaris won’t always be the North Star.Positions of equinoxes shift around orbit; e.g., spring equinox, once in Aries, is now in Pisces!Earth’s axis precesses like the axis of a spinning topPrecession can be demonstrated in class in a variety of ways. E.g., bring a top or gyroscope to class, or do the standard physics demonstration with a bicycle wheel and rotating platform.You may wish to go further with precession of the equinoxes, as in the Common Misconceptions box on “Sun Signs” --- this always surprises students, and helps them begin to see why astrology is questionable (to say the least!). Can also mention how Tropics of Cancer/Capricorn got their names from constellations of the solstices, even though the summer/winter solstices are now in Gemini/Sagittarius.
66 The Moon, Our Constant Companion Our goals for learning:Why do we see phases of the Moon?What causes eclipses?
67 Phases of Moon Half of Moon is illuminated by Sun and half is dark. NOT caused by Earth’s shadow!We see a changing combination of the bright and dark faces as Moon orbitsYou may want to do an in-class demonstration of phases by darkening the room, using a lamp to represent the Sun, and giving each student a Styrofoam ball to represent the Moon. If you lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads.
68 We see only one side of Moon Synchronous rotation: the Moon rotates exactly once with each orbit of ~28 daysThat is why only one side is visible from EarthUse this tool from the Phases of the Moon tutorial to explain rise and set times for the Moon at various phases. As usual, please encourage your students to try the tutorial for themselves.
69 What causes eclipses? The Earth and Moon cast shadows. When either passes through the other’s shadow, we have an eclipse.This slide starts our discussion of eclipses. Use the figure to explain the umbra/penumbra shadows.
70 When can eclipses occur? Lunar eclipses can occur only at full moon.Lunar eclipses can be penumbral, partial, or total.August 21st, 2017Use the interactive figure to show the conditions for the 3 types of lunar eclipse.
71 When can eclipses occur? Solar eclipses can occur only at new moon.Solar eclipses can be partial, total, or annular.Use the interactive figure to show the conditions for the 3 types of solar eclipse.
72 Why don’t we have an eclipse at every new and full moon? The Moon’s orbit is tilted 5° to ecliptic plane…So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.Use this pond analogy to explain what we mean by nodes and how we get 2 eclipse seasons each year (roughly).Note: You may wish to demonstrate the Moon’s orbit and eclipse conditions as follows. Keep a model “Sun” on a table in the center of the lecture area; have your left fist represent the Earth, and hold a ball in the other hand to represent the Moon. Then you can show how the Moon orbits your “fist” at an inclination to the ecliptic plane, explaining the meaning of the nodes. You can also show eclipse seasons by “doing” the Moon’s orbit (with fixed nodes) as you walk around your model Sun: the students will see that eclipses are possible only during two periods each year. If you then add in precession of the nodes, students can see why eclipse seasons occur slightly more often than every 6 months.
73 Planets Known in Ancient Times Mercurydifficult to see; always close to Sun in skyVenusvery bright when visible; morning or evening “star”Marsnoticeably redJupitervery brightSaturnmoderately brightThis slide explains what students can see of planets in the sky.
74 What was once so mysterious about planetary motion in our sky? Planets usually move slightly eastward from night to night relative to the stars.But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motionThe diagram at left shows Jupiter’s path with apparent retrograde motion in The photo composite shows Mars at 5-8 day intervals during the latter half of 2003.
75 We see apparent retrograde motion when we pass by a planet in its orbit. We also recommend that you encourage students to try the apparent retrograde motion demonstration shown in the book in Figure 2.33a, since seeing it for themselves really helps remove the mystery…
76 Explaining Apparent Retrograde Motion Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus laps us)But very difficult to explain if you think that Earth is the center of the universe!In fact, ancients considered but rejected the correct explanation
77 Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor.ptan p = 1AU/d (AU)For small angles:p = 1/dIf the angle p = 1 second of arc then d is defined as 1 parsec (= AU).d1 AUNot to scale
78 Earth does not orbit the Sun; the Earth is the center of the universe The Greeks knew that the lack of observable parallax could mean one of two things:Stars are so far away that stellar parallax is too small to notice with the naked eyeEarth does not orbit the Sun; the Earth is the center of the universeWith rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far awayThus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.In fact, the nearest stars have parallax angles less than 1 arcsecond, far below what the naked eye can see. Indeed, we CAN detect parallax today, offering direct proof that Earth really does go around the Sun…
79 Length of a DaySidereal day: Earth rotates once on its axis relative to the distant stars in 23 hrs, 56 min, and 4.07 sec.
80 Length of a DaySolar day: The Sun makes one circuit around the sky in 24 hours (by definition)
81 Why are the two different? Solar day is longer than a sidereal day by about 1/360 because Earth moves about 1° in orbit each day
82 Length of a Month Sidereal month: Moon orbits Earth in 27.3 days. Earth & Moon travel 30° around Sun during that time (30°/360° = 1/12)Synodic month: A cycle of lunar phases; therefore takes about 29.5 days, 1/12 longer than a sidereal monthSynodic means “meeting”
83 Length of a YearSidereal year: Time for Earth to complete one orbit of SunTropical year: Time for Earth to complete one cycle of seasonsTropical year is about 20 minutes (1/26,000) shorter than a sidereal year because of Earth’s precession.
84 Planetary PeriodsPlanetary periods can be measured with respect to stars (sidereal) or to apparent position of Sun (synodic).
85 Planetary PeriodsDifference between a planet’s orbital (sidereal) and synodic period depends on how far planet moves in one Earth yearfor outer planets
86 Planetary PeriodsDifference between a planet’s orbital (sidereal) and synodic period depends on how far planet moves in one Earth yearfor inner planets
87 How do we tell the time of day? Apparent solar time depends on the position of the Sun in the local skyA sundial gives apparent solar time
88 Mean Solar TimeLength of an apparent solar day changes during the year because Earth’s orbit is slightly elliptical.Mean solar time is based on the average length of a day.Noon is average time at which Sun crosses meridianIt is a local definition of time
89 Standard Time & Time Zones Rapid train travel required time to be standardized into time zones (time no longer local)
90 Universal TimeUniversal time (UT) is defined to be the mean solar time at 0° longitude.It is also known as Greenwich Mean Time (GMT) because 0° longitude is defined to pass through Greenwich, EnglandIt is the standard time used for astronomy and navigation around the world
91 Local Sidereal Time = RA + Hour Angle Time by the StarsSidereal time is equal to right ascension that is passing through the meridianThus, the local siderial time is 0h0m when the spring equinox passes through the meridianA star’s hour angle is the time since it last passed through the meridianLocal Sidereal Time = RA + Hour Angle
92 When and why do we have leap years? The length of a tropical year is about days.In order to keep the calendar year synchronized with the seasons, we must add one day every four years (February 29).For precise synchronization, years divisible by 100 (e.g., 1900) are not leap years unless they are divisible by 400 (e.g., 2000).
93 Celestial Coordinates Right ascension: Like longitude on celestial sphere (measured in hours with respect to spring equinox).Declination: Like latitude on celestial sphere (measured in degrees above celestial equator)
94 Celestial Coordinates of Vega Right ascension: Vega’s RA of 18h35.2m (out of 24h) places most of the way around celestial sphere from spring equinox.Declination: Vega’s dec of +38°44’ puts it almost 39° north of celestial equator (negative dec would be south of equator)
95 Celestial Coordinates of Sun The Sun’s RA and dec change along the ecliptic during the course of a yearSun’s declination is negative in fall and winter and positive in spring and summer
96 How can you determine your latitude? Latitude equals altitude of celestial poleAltitude and declination of star crossing meridian also gives latitude.You can determine Sun’s declination from the day of the yearMeasuring the Sun’s altitude when it crosses meridian can tell you latitude
97 How can you determine your longitude? In order to determine your longitude from the sky, you need to know time of day because of Earth’s rotationYou also need to know day of year because of Earth’s orbitAccurate measurement of longitude requires an accurate clock.
98 GPS NavigationThe Global Positioning System (GPS) uses a set of satellites in Earth orbit as artificial starsGPS devices use radio signals to determine your position relative to those satellitesGPS satellites correct for General Relativity!
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