From the previous class…

From the previous class…
What is smaller a typical planet or a light second? Light second correspond approximately to a Moon-Earth distance. Therefore, the answer is a typical planet. ( by the way radius of the Earth is 6,378 km) Why the plane travel to Europe is shorter? As you guessed the answer is the wind… © 2005 Pearson Education Inc., publishing as Addison-Wesley

Chapter 2 Discovering the Universe for Yourself
We had the sky, up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made, or only just happened. Mark Twain (1835 – 1910) American author, from Huckleberry Finn © 2005 Pearson Education Inc., publishing as Addison-Wesley

© 2005 Pearson Education Inc., publishing as Addison-Wesley
In chapter 2 we will study the motions of the stars, sun, moon and planets across our sky. © 2005 Pearson Education Inc., publishing as Addison-Wesley

2.1 Patterns in the Night Sky
Our goals for learning: What are constellations? How do we locate objects in the sky? Why do stars rise and set? Why don’t we see the same constellations throughout the year? © 2005 Pearson Education Inc., publishing as Addison-Wesley

What are constellations?
A constellation is a region of the sky. (every point in the sky belongs to some constellation; it is to the sky as a state is to the US) 88 constellations fill the entire sky. Official borders set in 1928, by IAU. Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation. Most of the names of northern constellations date back to ancient middle eastern civilizations; the names of southern are mostly given by European explorers in 17th century. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Thought Question The brightest stars in a constellation…
All belong to the same star cluster. All lie at about the same distance from Earth. May actually be quite far away from each other. You can use this question both to check student understanding of the idea of a constellation and as a way of leading into the concept of the celestial sphere that follows. © 2005 Pearson Education Inc., publishing as Addison-Wesley

The brightest stars in a constellation…
All belong to the same star cluster. All lie at about the same distance from Earth. May actually be quite far away from each other. Worth pointing out that two stars in the same constellation can actually be farther apart than stars on opposite sides of our sky, depending on their distances… E.g.,: Procyon and Alpha Centauri are much farther apart than Procyon and Betelgeuse; But in space, Alpha Cen and Procyon are both relatively nearby stars (their distances are 11.4 and 4.4 light-years, respectively) which means they are not more than about a dozen light-years apart, while Betelgeuse is several hundred light-years away from Procyon (distance to Betelgeuse is about 430 light-years). © 2005 Pearson Education Inc., publishing as Addison-Wesley

The Celestial Sphere We lack depth perception when we look into space…it seems like all the stars are surrounding us on a sphere. The ancient Greeks believed that stars actually lie on a celestial sphere. There are four special elements on the celestial sphere: north celestial pole south celestial pole celestial equator ecliptic 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 pole South celestial pole Celestial equator Ecliptic It’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model. © 2005 Pearson Education Inc., publishing as Addison-Wesley

The Celestial Sphere The sphere shows: star patterns, borders of 88 constellations, ecliptic, celestial equator and poles. 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… Question: where would the Earth be on this sphere? Question: Explain the shape of Milky way. © 2005 Pearson Education Inc., publishing as Addison-Wesley

The Milky Way A band of light making a circle around the celestial sphere. What is it? Our view into the plane of our 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

How do we locate objects in the sky?
Now we move from celestial sphere to the local sky. Question: why does the local sky look like a dome? Key features on the local sky: Horizon: boundary between earth and sky Zenith: the point overhead. Meridian: half circle which contains south, zenith and north. Now 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. We can locate any object by its altitude (above horizon) and direction (along horizon) © 2005 Pearson Education Inc., publishing as Addison-Wesley

If we want to measure sizes of objects and distances we do it in angles... 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. Question: why we do not measure actual sizes and distances on the sky? (hint: angular sizes of Moon and Sun are about the same) © 2005 Pearson Education Inc., publishing as Addison-Wesley

Angle measurements: Full circle = 360º 1º = 60 (arcminutes) 1 = 60 (arcseconds) Use this slide if you want to review the definitions of arc minutes and arc seconds. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Thought Question The angular size of your finger at arm’s length is about 1°. How many arcseconds is this? 60 arcseconds 600 arcseconds 60  60 = 3,600 arcseconds This is a quick test of whether students understand what we mean by arcseconds. © 2005 Pearson Education Inc., publishing as Addison-Wesley

The angular size of your finger at arm’s length is about 1°. How many arcseconds is this? 60 arcseconds 600 arcseconds 60  60 = 3,600 arcseconds Now can point out that 1 arc second is only about 1/3600 of the width of your finger against the sky --- yet modern telescopes routinely resolve features smaller than this…. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Why do stars rise and set?
Earth rotates east to west, so stars appear to circle from west to east. 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… © 2005 Pearson Education Inc., publishing as Addison-Wesley

Our view from Earth (local sky):
Stars near the north celestial pole are circumpolar and never set. We cannot see stars near the south celestial pole. All other stars (and Sun, Moon, planets) rise in east and set in west. Question: would you see more circumpolar stars if you would live in Canada or in the US? Now explain the basic motion of the sky seen from Earth. Celestial Equator Your horizon © 2005 Pearson Education Inc., publishing as Addison-Wesley

Thought Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator This question will check whether students understand the pattern they see in this time exposure photograph. © 2005 Pearson Education Inc., publishing as Addison-Wesley

What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator You might now want to ask this second question: Which way are the stars moving around the north celestial pole: clockwise or counterclockwise? (answer: ccw) © 2005 Pearson Education Inc., publishing as Addison-Wesley

Why don’t we see the same constellations throughout the year?
Depends on whether you stay travel: Constellations vary with latitude. Depends on time of year: Constellations vary as Earth orbits the Sun. These are the two basic reasons that the visible constellations vary; next we’ll explore each one. © 2005 Pearson Education Inc., publishing as Addison-Wesley

1) Let’s review how we locate coordinates on the Earth
Latitude: position north or south of equator Longitude: position east or west of prime meridian (runs through Greenwich, England) Use this for a brief review of latitude and longitude; it’s also useful to bring in a real globe to class for this purpose. The photo at right is the entrance to the Old Royal Greenwich Observatory (near London); the line emerging from the door marks the Prime Meridian. © 2005 Pearson Education Inc., publishing as Addison-Wesley

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)… If the south celestial pole appears in your sky at an altitude of 34° above your south horizon, your latitude is 34° - important for navigation! © 2005 Pearson Education Inc., publishing as Addison-Wesley

altitude of the celestial pole = your latitude
Star motion: Show students how to locate the NCP and SCP, and how the sky moves around them. (You might wish to repeat the time exposure photo of the sky at this point to re-emphasize what we see.) Can also ask students where they’d find the north celestial pole in their sky tonight… © 2005 Pearson Education Inc., publishing as Addison-Wesley

Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you? You are on the equator. You are at the North Pole. You are at latitude 50°N. You are at longitude 50°E. You are at latitude 50°N and longitude 50°E. This question just makes sure they understand the altitude = latitude idea… © 2005 Pearson Education Inc., publishing as Addison-Wesley

The North Star (Polaris) is 50° above your horizon, due north. Where are you? You are on the equator. You are at the North Pole. You are at latitude 50°N. You are at longitude 50°E. You are at latitude 50°N and longitude 50°E. © 2005 Pearson Education Inc., publishing as Addison-Wesley

2) The (night) 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 the in the sky. Use this interactive figure to explain how the constellations change with the time of year. © 2005 Pearson Education Inc., publishing as Addison-Wesley zodiac: constellations along the ecliptic!

Special Topic: How Long is a Day?
Solar day = 24 hours Sidereal day (Earth’s rotation period) = 23hr, 56min This slide goes with the optional Special Topic box in the text. Note: rather than simply discussing parts (a) and (b), you can do the actual demonstration in class… © 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned? What are constellations? A region of the sky; every position on the sky belongs to one of 88 constellations. How do we locate objects in the sky? By its altitude above the horizon and its direction along the horizon. © 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned? Why do stars rise and set? Because of Earth’s rotation. Why don’t we see the same constellations throughout the year? The sky varies with latitude. The night sky changes as Earth orbits the Sun. © 2005 Pearson Education Inc., publishing as Addison-Wesley

2.2 The Reason for Seasons Our goals for learning: What causes the seasons? How do we mark the progression of the seasons? Does the orientation of Earth’s axis change with time? © 2005 Pearson Education Inc., publishing as Addison-Wesley

Thought Question TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. A good way to begin discussion of seasons is by posing this question about the most common season misconception. © 2005 Pearson Education Inc., publishing as Addison-Wesley

TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. Hint: When it is summer in the U.S., it is winter in Australia. This hint should make students realize that distance from the Sun CANNOT be the explanation for seasons: if it were, the entire Earth should experience the same seasons at the same time. Note: Some students think the axis tilt makes one hemisphere closer to the Sun than the other; you can show them why this is not the case by revisiting the 1-to-10 billion scale model solar system from Ch. 1. When you remind them that the ball point size Earth orbits 15 meters from the grapefruit size Sun, it’s immediately obvious that the 2 hemispheres cannot have any significant difference in distance. © 2005 Pearson Education Inc., publishing as Addison-Wesley

TRUE OR FALSE! Earth is closer to the Sun in summer and farther from the Sun in winter. • Seasons are opposite in the N and S hemispheres, so distance cannot be the reason. The real reason for seasons involves Earth’s axis tilt. Now that you’ve answered the T/F question, we can go on to explore the real reason for seasons. Note: You might optionally mention that, in fact, Earth is closest to the Sun during N. hemisphere winter… © 2005 Pearson Education Inc., publishing as Addison-Wesley

What causes the seasons?
Sunlight strikes the northern hemisphere at steeper angle (warmer) and it is higher in the sky (longer days). Misconceptions 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Axis tilt causes uneven heating by sunlight throughout the year.
Question: Jupiter has an axis tilt of about 3° and nearly circular orbit around the Sun. Does it have seasons? 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Summary: The Real Reason for Seasons
Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun. Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Why doesn’t distance matter?
Only small variation of Earth-Sun distance — about 3%; this small variation overwhelmed by effects of axis tilt. Note: Some planets have greater distance variation that DOES affect their seasons — notably Mars, Pluto. The two notes should be considered optional. If you cover the first note, you might point out that since Earth is closer to the Sun in S. hemisphere summer and farther in S. hemisphere winter, we might expect that the S. hemisphere would have the more extreme seasons, but it does not because the distance effect is overwhelmed by the geographical effect due to the distribution of oceans. Question: If the southern and north hemisphere get the same amount of sunlight over the year, how come that seasons are milder in the southern part? © 2005 Pearson Education Inc., publishing as Addison-Wesley

How do we mark the progression of the seasons?
Ancient men used to track the change of seasons by observing the Sun’s position in the sky… To do it now, we define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox around March 21st Jun 21st December 21st Here 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. Question: at what latitude the Sun is overhead during the summer solstice? (hint: tropic of cancer. September 22nd © 2005 Pearson Education Inc., publishing as Addison-Wesley

We can recognize solstices and equinoxes by Sun’s path across sky:
Equinoxes: Sun rises precisely due east and sets precisely due west. The day is exactly 12 hours long. 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. 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…) The beginnings of seasons coincide with the days of equinox and solstice, as they are seen from northern hemisphere. Question: How about the Southern hemisphere? And the region of Equator, how do the seasons look like there? © 2005 Pearson Education Inc., publishing as Addison-Wesley

Seasonal changes are more extreme at high latitudes
Other points worth mentioning: Length of daylight/darkness becomes more extreme at higher latitudes. The four seasons are characteristic of temperate latitudes; tropics typically have rainy and dry seasons (rainy seasons when Sun is higher in sky). Equator has highest Sun on the equinoxes. Optional: explain Tropics and Arctic/Antarctic Circles. Path of the Sun on the summer solstice at the Arctic Circle – Sun becomes circumpolar. Question: how does the winter solstice look like at the Arctic Circle? © 2005 Pearson Education Inc., publishing as Addison-Wesley

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. All spinning objects precess, i.e. spinning top. Precession 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” on p 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Question: Is Polaris really a “North Star”? No! Polaris has been a good North Star only for couple of centuries. In 13,000 years Vega will be a North Star. The axis does not point near any bright star during most of its cycle! Question: are equinoxes always on the same position in the orbit? © 2005 Pearson Education Inc., publishing as Addison-Wesley

Question: Is Polaris really a “North Star”? No! Polaris has been a good North Star only for couple of centuries. In 13,000 years Vega will be a North Star. The axis does not point near any bright star during most of its cycle! Question: are equinoxes always on the same position in the orbit? No. For example summer equinox used to be in Aries (2000 years ago), but now it is in Pisces. So, if you are born on March 21st, your astrological sign would appear to be Aries, even though, the Sun was really in Pisces when you were born. (horoscope is based on the star sky mapped by Ptolemy, in old Greece). Also, summer solstice is not in Cancer anymore, it is in Gemini. © 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned? What causes the seasons? The tilt of the Earth’s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter. How do we mark the progression of the seasons? The summer and winter solstices are when the Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight. © 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned? Does the orientation of the Earth’s axis change with time? Yes. The tilt remains about 23.5 degrees (so the season pattern is not affected), but Earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of the Earth’s axis. © 2005 Pearson Education Inc., publishing as Addison-Wesley

2.3 The Moon, Our Constant Companion
Our goals for learning: Why do we see phases of the Moon? What causes eclipses? © 2005 Pearson Education Inc., publishing as Addison-Wesley

Why do we see phases of the Moon?
The Moon is always half illuminated by Sun and half dark Depending on the angle under which see the moon we see some combination of the bright and dark faces You 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Phases of the Moon You can use this tool from the Phases of the Moon tutorial to present the idea behind phases in another way. As usual, please encourage your students to try the tutorial for themselves. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Moon Rise/Set by Phase Different phases of the Moon rise and set at different times above our horizon. Question: Is it true that we see moon only during the night? Use 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Phases of the Moon: day cycle Months is actually a “moonths” new crescent first quarter gibbous full last quarter } waxing Moon visible in afternoon/evening. Gets “fuller” and rises later each day. } waning Moon visible in late night/morning. Gets “less” and sets later each day. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Thought Question It’s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? First quarter Waxing gibbous Third quarter Half moon This will check whether students have grasped the key ideas about rise and set times. © 2005 Pearson Education Inc., publishing as Addison-Wesley

It’s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? First quarter Waxing gibbous Third quarter Half moon If anyone chose “half moon,” remind them that there is no phase with that name… (and that first and third quarter refer to how far through the cycle of phases we are…) © 2005 Pearson Education Inc., publishing as Addison-Wesley

What causes eclipses? The Earth and Moon cast shadows. When either passes through the other’s shadow, we have an eclipse. Lunar eclipse: when Earth lies directly between Sun and Moon. Solar eclipse: when Moon lies between Sun and Earth, then people living where the Moon shadow falls can not see the Sun. This slide starts our discussion of eclipses. Use the figure to explain the umbra/penumbra shadows. Light partially blocked Light completely blocked © 2005 Pearson Education Inc., publishing as Addison-Wesley

Lunar eclipses Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total. The full moon darkens only slightly. Use the interactive figure to show the conditions for the 3 types of lunar eclipse. Lasts about one hour. The Moon appears red because Earth’s atmosphere bends some of the red light toward the Moon. © 2005 Pearson Education Inc., publishing as Addison-Wesley

No! 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Solar Eclipses Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular. Moon’s umbra touches the Earth, covering about 270 km in diameter and traveling at a speed of 1,700 km/hr. Eclipse lasts for few minutes. Use the interactive figure to show the conditions for the 3 types of solar eclipse. If the Moon is farther and the shadow does not reach the Earth we see annular eclipse - a ring of sunlight surrounds the disk of the Moon. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Summary: Two conditions must be met to have an eclipse:
It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes). © 2005 Pearson Education Inc., publishing as Addison-Wesley

Predicting Eclipses We have about two solar eclipses each year, but since positions of the eclipse seasons slowly move around the orbit, eclipses do not occur in fixed parts of the year. Eclipses actually recur with the 18 yr, 11 1/3 day saros cycle, but type (e.g., partial, total) and location may vary. Any two eclipses separated by one saros cycle share very similar geometries. They occur at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Because the saros period is not equal to a whole number of days, its biggest drawback is that subsequent eclipses are visible from different parts of the globe. Since there are two to five solar eclipses every year, there are approximately forty different saros series in progress at any one time. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Paths of totality for solar eclipses till 2030.
Point out that even though some ancient civilizations recognized the saros cycle, precise prediction still eluded them. Use the colored bands in the figure to illustrate the saros cycle (e.g., red bands for 2009 and 2027 eclipses are 18 yr, 11 1/3 days apart). © 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned? Why do we see phases of the Moon? Half the Moon is lit by the Sun; half is in shadow. The appearance of the Moon to us is determined by the Sun, Earth, and Moon positions. What causes eclipses? Lunar eclipse: Earth’s shadow on the Moon. Can be penumbral, partial, or total. Solar eclipse: the Moon’s shadow on Earth. Can be partial, total, or annular. Tilt of Moon’s orbit means eclipses occur during two periods each year. Eclipses recur with the 18 yr, 11 1/3 day saros cycle © 2005 Pearson Education Inc., publishing as Addison-Wesley

Astronomy Picture of the Day for October 16th, 2006. “The robotic Casini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipse Sun. Cassini saw a view unlike any other. First, the night side is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn and slightly scattering sunlight, in the above exaggerated color image.” Taken from the Nasa site: © 2005 Pearson Education Inc., publishing as Addison-Wesley

2.4 The Ancient Mystery of the Planets
Our goals for learning: What was once so mysterious about the movement of planets in our sky? Why did the ancient Greeks reject the real explanation for planetary motion? © 2005 Pearson Education Inc., publishing as Addison-Wesley

Planets Known in Ancient Times
Mercury difficult to see; always close to Sun in sky Venus very bright when visible — morning or evening “star” Mars noticeably red Jupiter very bright Saturn moderately bright This slide explains what students can see of planets in the sky. © 2005 Pearson Education Inc., publishing as Addison-Wesley

What was once so mysterious about planetary motion in our sky?
Planets usually move eastward from night to night relative to the stars. You cannot see this motion on a single night; rather, planets rise in the east and set in the west. But sometimes they go westward for a few weeks or months: apparent retrograde motion The 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. © 2005 Pearson Education Inc., publishing as Addison-Wesley

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 2.28a, since seeing it for themselves really helps remove the mystery… © 2005 Pearson Education Inc., publishing as Addison-Wesley

Explaining Apparent Retrograde Motion
Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap 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… © 2005 Pearson Education Inc., publishing as Addison-Wesley

Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor. The Greeks new that stellar parallax should occur if Earth orbits the Sun. You should have students witness parallax for themselves by having them hold up one finger and alternately open and close one eye. (Note: since the Greeks thought all stars lied at the same distance on the celestial sphere, they expected to see a shift in the angular separations of stars s they viewed them from different distances, rather than parallax as shown in this diagram. But we prefer not to bother students with this subtlety unless they ask about it.) © 2005 Pearson Education Inc., publishing as Addison-Wesley

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 eye Earth does not orbit Sun; it is the center of the universe With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away… Thus 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… © 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned? What was so mysterious about planetary motion in our sky? Like the Sun and Moon, planets usually drift eastward relative to the stars from night to night; but sometimes, for a few weeks or few months, a planet turns westward in its apparent retrograde motion. Easy for us to explain: occurs when Earth passes a planet by (“laps it”) in its orbit. But difficult to explain if you think Earth is the center of the universe. © 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned? Why did the ancient Greeks reject the real explanation for planetary motion? They could not detect stellar parallax. Most Greeks concluded that Earth must be stationary, because they thought the stars could not be so far away as to make parallax undetectable. © 2005 Pearson Education Inc., publishing as Addison-Wesley

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