1. Discovering the Universe for Yourself
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2. Chapter 2 Review Questions 9 – 14, 23 – 26, and 34 – 36
Unit 2 Chapter 2 – 2.2, 2.3 & 2.4
Chapter 2 Review Questions
9 – 14, 23 – 26, and 34 – 36
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3. Big Projects Moon Observations
Star Gazing Journal
You will be observing the night sky for two months.
You will fill in your observations in your journal two nights a week.
Use the Sky Wheel to find what constellations you see.
Journal and Star Wheel
Moon Observations
You will be observing the moon for one month.
You will fill in the time, date, and shape of the moon.
Journal
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4. Observing from Earth
Stars seen in the Northern Hemisphere are NOT the same as those seen in the Southern Hemisphere
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5. 12 constellations along the zodiac, mostare named after animals.
Groups of stars that people associate name with. There are 88 constellations that are known.
12 constellations along the zodiac, mostare named after animals.
Different cultures assigned different names to the same group of stars.
Latin – Dragon, but Egyptian – Hippopotamus
Latin – Orion, but Chinese – White Tiger
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6. Celestial Sphere For observations – we can ignore distances
Assume all objects are at the same distance
Everything is on a big sphere around the Earth…giving us the Celestial Sphere
Path of the Sun is NOT along the celestial equator.
Since the Earth’s rotation axis is tilted at 23.5 degrees, the Sun follows a path which is tilted 23.5 degrees from the equator.
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7. Celestial Sphere
Path of the sun on the celestial sphere is called the Ecliptic.
Two points exist where the Sun’s path crosses the celestial equator.
Sun is directly overhead at Earth’s equator.
12 hours of day and 12 hours of night.
Call these points the Equinoxes
Equinoxes means equal nights
We use the vernal equinox as the origin of a coordinate system for locating the stars.
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8. Motions The motion of the stars is really motion of the Earth
We observe three basic motions when we observe the night sky:
A very long term drift of the North celestial pole
From wobble in Earth’s rotation
Daily Motion
Earth rotation on its axis
Yearly Motion
Earth revolving around the Sun
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9. If nothing else moved, then motions are simple.
Daily Motion
Arise from the rotation (spin) of Earth, causes the rising and setting each day of the Sun, moon, and stars.
If nothing else moved, then motions are simple.
But – Earth is also moving around the Sun.
It takes a little more than one full rotation of Earth for Sun to rise again.
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10. One rotation of the Earth?
Daily Motion
So how long is a day?
Sunrise to Sunset?
Solar Day
One rotation of the Earth?
Siderial Day
Average solar day is about 4 minutes longer than the sideral day.
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11. Yearly Motion
Arise from the orbit of Earth around the Sun, causes different stars to be visible at different times of the year in our night sky.
Stars which are behind the sun in space are in the daytime sky and are not visible from Earth.
Combined with the tilt of the Earth’s rotation axis, causes seasons.
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12. 2.2 The Reason for Seasons Our goals for learning:
What causes the seasons?
How does the orientation of Earth's axis change with time?
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13. 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.
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14. Thought Question
TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter.
Hint: When it is summer in America, 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.
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15. Thought Question 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…
Also, students who know of the Earth's axis tilt may think that the difference in distance between the Sun - Northern hemisphere and the Sun – Southern hemisphere is the key. What part of 150 Mkm is a 1000km?
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16. What causes the seasons?
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 MasteringAstronomy web site.
Seasons depend on how Earth's axis affects the directness of sunlight.
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17. Direct light causes more heating.
This applet is taken from the Seasons tutorial on the Mastering Astronomy. You can use it to reinforce the ideas from the previous slide. As usual, please encourage your students to try the tutorial for themselves.
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18. Axis tilt changes directness of sunlight during the year.
This applet is taken from the Seasons tutorial on MasteringAstronomy.com. You can use it to reinforce the ideas from the previous slide. As usual, please encourage your students to try the tutorial for themselves.
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19. Sun's altitude also changes with seasons.
Sun's position at noon in summer: Higher altitude means more direct sunlight.
Sun's position at noon in winter: Lower altitude means less direct sunlight.
This applet 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.
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20. Seasons
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21. Seasons Why do we have temperature changes? Distance from Sun changes?
No, distance changes very little
During summer, Sun’s rays strike more directly
More efficient heating
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22. Length of daylight is longer in summer More time to heat up
Seasons
Length of daylight is longer in summer
More time to heat up
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23. 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.
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24. Why doesn't distance matter?
Variation of Earth–Sun distance is small—about 3%; this small variation is overwhelmed by the effects of axis tilt.
Variation in any season of each hemisphere-Sun distance is even smaller!
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.
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25. How do we mark the progression of the seasons?
We define four special points:
summer (June) solstice
winter (December) solstice
spring (March) equinox
fall (September) equinox
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.
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26. We can recognize solstices and equinoxes by Sun's path across sky:
Summer (June) solstice: highest path; rise and set at most extreme north of due east
Winter (December) 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…)
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27. Seasons
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28. Seasonal changes are more extreme at high latitudes.
Path of the Sun on the summer solstice at the Arctic Circle
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.
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29. Solstice vs. Equinox
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30. 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 top
Precession can be demonstrated in class in a variety of ways. 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.
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31. 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.
We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon.
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.
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32. Mean Solar Time 24 Standard Time Zones Each 15 degrees Apart
Pacific-Mountain- Central-Eastern
Crossing Time Zones
Traveling Eastward > Add 1 Hour
Traveling Westward > Subtract 1 Hour
Daylight Savings Time
Spring “Ahead”, Fall “Back”
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33. International Date Line 180 Degree Meridian Crossing the Dateline
Mean Solar Time
International Date Line
180 Degree Meridian
Crossing the Dateline
Traveling Eastward > Subtract a day
Traveling Westward > Add a day
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34. Based on the phases of the moon, 29.5 days Sidereal Month
The Month
Synodic Month
Based on the phases of the moon, 29.5 days
Sidereal Month
Based on the moon’s position in relation to a star, days
For the moon:
1 Rev = 1 Rot
Eq. One day = One month on the moon
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35. What have we learned?
How does the orientation of Earth's axis change with time?
The tilt remains about 23.5 (so the season pattern is not affected), but Earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of Earth's axis.
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36. 2.3 The Moon, Our Constant Companion
Our goals for learning:
Why do we see phases of the Moon?
What causes eclipses?
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37. The Moon
The moon is not a planet, but it is another object in the solar system, which we know a lot about.
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38. General Properties of the Moon
Size
About ¼ the diameter of the Earth
Age
About 4.5 billion years old from radioactive dating of rocks
Density
3.3 grams/cm^3
Less than Earth –less metal
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39. General Properties of the Moon
Reflection of Light 7%
Does not reflect light well – rocky
Why does it look so bright?
Just because it is so close!
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40. Moon does rotate slowly, probably no liquid in core
Interior of the Moon
No magnetic field
Moon does rotate slowly, probably no liquid in core
Moon is smaller – cools more quickly – all solidified now
Very little evidence of seismic activity
Few “moonquakes”
No volcanoes
No continental drift
Flow of heat from interior – warm interior
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41. Observe Mountains Craters Canyons Surface of the Moon
Higher than on Earth
No erosion, less surface gravity
Craters
Impacts of meteors
Canyons
From old lava flows
Not water
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42. We classify the land as: Maria (seas)
Surface of the Moon
We classify the land as:
Maria (seas)
Dark color
From lava flow
15% of moon (Mainly on the near side)
Age: 3 billion years
Highlands
Lightly colored
Heavily cratered
85% of the moon
Age: 4.5 billion years
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43. Surface ages from 3-4.5 billion years
Surface of the Moon
Surface ages from billion years
Has not changed much since formation
Mainly silicate rocks (like Earth)
No water in the rocks (but maybe some ice in craters?)
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44. Weak gravity – everything escapes
Atmosphere of the Moon
Almost none
Weak gravity – everything escapes
Observe some hydrogen, helium, argon passing through from outgassing of rocks
No weather, no wind, no rain – equals no erosion
Moon’s surface changes slowly
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45. History of the Moon
4.5 billion years ago, a planetisimal or very large rock or collection of rocks hit a young Earth at 18 km/sec
Silicate debris from collision collect to form the moon, debris continues to come down leaving craters on the surface. Lava starts to flow over parts (3.5 billion) of the surface, the side with the lava is more massive, so it is attracted to the Earth. (3 billion)
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46. At this moment the moon is 18,000 km away.
History…
At this moment the moon is 18,000 km away.
Now, the moon is 380,000 km.
The moon is moving away at 2-3 cm per a year.
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47. Force of gravity depends on distance.
Moon
Earth-Moon System: Tidal forces of gravity
Tides of Earth
Synchronous rotation of moon
Force of gravity depends on distance.
Force is stronger on the near side of Earth and the moon.
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48. Forces of Gravity
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49. Tides on Earth
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50. Tides on Earth
So we have TWO high tides each day, but not of equal size.
The Earth turns “under” the bulges of water.
Spring Tides – Occur when the moon, Earth, and Sun are all in alignment (New and Full Moon)
Neap Tides – Occur when the moon and Sun are at right angles to the Earth (1st and Last Quarter)
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51. Motion of the Moon Moon always keeps the same side towards the Earth
It does rotate, but rotational period = orbital period, which is 28 days
Moon rotates once each orbit = “Synchronous Rotation”
Caused by gravity and tidal forces
Moves Eastward relative to stars/Sun each day
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52. Phases of the Moon New Moon Waxing Crescent First Quarter
Waxing Gibbous
Full Moon
Waning Gibbous
Last Quarter
Waning Crescent
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55. Phases of the Moon Waxing Growing bright Waning Growing dimmer
Crescent
Near Sun
Gibbous
Far from Sun
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56. Why do we see phases of the Moon?
Lunar phases are a consequence of the Moon's 27.3-day orbit around Earth.
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 your lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads.
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57. Phases of the Moon
Half of Moon is illuminated by Sun and half is dark.
We see a changing combination of the bright and dark faces as Moon orbits.
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.
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58. Phases of the Moon
You can use this applet 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.
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59. Moon Rise/Set by Phase
Use this applet 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.
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60. Phases of the Moon: 29.5-day cycle
Waxing
Moon visible in afternoon/evening
Gets "fuller" and rises later each day
Waning
Moon visible in late night/morning
Gets "less full" and sets later each day
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61. Thought Question
It's 9 a.m. 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.
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62. Thought Question
It's 9 a.m. 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…)
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63. We see only one side of Moon
Synchronous rotation: the Moon rotates exactly once with each orbit.
That is why only one side is visible from Earth.
Use this applet 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.
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64. 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.
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65. Occurs when the Earth’s shadow blacks our view of the moon (Full Moon)
Eclipses
Eclipses occur when the moon, Earth, and Sun are all on the same plane (ecliptic)
Lunar Eclipses
Occurs when the Earth’s shadow blacks our view of the moon (Full Moon)
Solar Eclipses
Occur when the moon’s shadow blocks our view of the sun (New Moon)
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66. Lunar Eclipse
This interactive applet goes through lunar eclipses. Use it instead of or in addition to the earlier slides on eclipses.
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67. Penumbral, Partial, and Total Lunar Eclipses
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70. Lunar Eclipse
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71. When can eclipses occur?
Lunar eclipses can occur only at full moon.
Lunar eclipses can be penumbral, partial, or total.
Use the interactive figure to show the conditions for the 3 types of lunar eclipse.
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72. Lunar Eclipse Visible from the entire night side of the Earth
Lasts 1-2 hours
Occurs every 6 months to a year
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73. Solar Eclipse
This interactive applet goes through the solar eclipses. Use it instead of or in addition to the earlier slides on eclipses.
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74. 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.
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75. Partial Solar Eclipses Annular Solar Eclipses
Total Solar Eclipses
Partial Solar Eclipses
Annular Solar Eclipses
Apparent size of the moon is sometimes slightly smaller than the apparent size of the Earth, it leaves a ring of sunlight.
Only a small part of the Earth will see it
Last 3-7 minutes and is as dark as night
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76. Solar Eclipse
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77. Solar Eclipse
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78. 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.
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79. 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
The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).
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80. Predicting Eclipses
Eclipses recur with the 18-year, 11 1/3-day saros cycle, but type (e.g., partial, total) and location may vary.
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).
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82. What have we learned? Why do we see phases of the Moon?
Half the Moon is lit by the Sun; half is in shadow, and its appearance to us is determined by the relative positions of Sun, Moon, and Earth.
What causes eclipses?
Lunar eclipse: Earth's shadow on the Moon
Solar eclipse: Moon's shadow on Earth
Tilt of Moon's orbit means eclipses occur during two periods each year.
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83. 2.4 The Ancient Mystery of the Planets
Our goals for learning:
What was once so mysterious about planetary motion in our sky?
Why did the ancient Greeks reject the real explanation for planetary motion?
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84. 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.
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85. 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 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.
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86. 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…
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87. 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.
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88. Why did the ancient Greeks reject the real explanation for planetary motion?
Their inability to observe stellar parallax was a major factor.
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89. 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 the 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, the stage was set 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…
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90. 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.
Why did the ancient Greeks reject the real explanation for planetary motion?
Most Greeks concluded that Earth must be stationary, because they thought the stars could not be so far away as to make parallax undetectable.
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