1. Chapter 2 Discovering the Universe for Yourself

2. What does the universe look like from Earth?
With the naked eye, we can see more than 2,000 stars as well as the Milky Way.
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.

3. Constellations A constellation is a region of the sky.
88 constellations fill the entire sky.
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.

4. Asterisms
A group of stars that forms a pattern in the sky that is not a constellation.
May be part of a single constellation
Big Dipper or Little Dipper
May be composed of several constellations.
The Summer Triangle

7. Thought Question The brightest stars in a constellation or asterism…
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.

8. The Celestial Sphere
Stars at different distances all appear to lie on 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 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.

9. The Celestial Sphere
The 88 official constellations cover the celestial sphere.
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…

10. North celestial pole is directly above Earth’s North Pole.
South celestial pole is directly above Earth’s South Pole.
Celestial equator is a projection of Earth’s equator onto sky.
Ecliptic is Sun’s apparent path through the celestial sphere.
The Celestial Sphere

11. The Local Sky
An object’s altitude (above horizon) and direction or azimuth (along horizon) specifies its location in your local sky
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.

12. The Local Sky Zenith: The point directly overhead
Horizon: All points 90° away from zenith
Meridian: Line passing through zenith and connecting N and S points on horizon
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.

13. We measure the sky using 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.

14. More Approximate angular separations
Width of little finger… ~10
Fist…~100
Caution!!Be careful where you use #4 .

15. Angular 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.

16. Angular Size
An object’s angular size appears smaller if it is farther away
Use this slide if you want to review the definitions of arc minutes and arc seconds.

17. Why do stars rise and set?
Earth rotates west to east, so stars appear to circle from east to west.
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…

18. Our view from Earth:
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.
A circumpolar star never sets
Now explain the basic motion of the sky seen from Earth.
Celestial Equator
This star never rises
Your Horizon

19. What is the arrow pointing to? the zenith the north celestial pole
Thought Question
What is the arrow pointing to?
the zenith
the north celestial pole
the celestial equator
This question will check whether students understand the pattern they see in this time exposure photograph.

20. 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)…

21. Altitude of the celestial pole = your latitude
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…

22. Diurnal (Daily) Motion
Earth’s Orbital Motion
The 24 hour Solar Day , from noon to noon, is the basic social time unit. By definition 1 solar day is 24 hours.
The daily progress of the Sun and other stars across the sky is called diurnal motion.
Each night the celestial sphere shifts a little across the sky.
… a day measured by the star positions is called a sidereal day.

23. Sidereal vs Solar Day Sidus ,latin for star.
The Earth rotates on its axis and revolves around the Sun.
The solar day is 3.9 minutes longer than the sidereal day.A sidereal day is 23h56m long.

24. 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.

25. 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 Astronomy Place web site.
Seasons depend on how Earth’s axis affects the directness of sunlight

26. 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.

27. 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 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.

28. 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.

29. 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.
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.

30. How do we mark the progression of the seasons?
We define four special points:
summer solstice
winter solstice
spring (vernal) equinox
fall (autumnal) 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.

31. 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…)

32. 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

33. 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. 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.

34. North Celestial Pole Moves but slowly
The precession has a year cycle.
4800 years ago the pole was close to Thuban. The pole will be closest to Polaris in ~2100 .

35. The Moon

36. 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.

37. Phases of 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.

38. } } Phases of the Moon: 29.5-day cycle waxing waning 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.

39. Thought Question First quarter Waxing gibbous Third quarter Half moon
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.

40. Moon Rise/Set by Phase
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.

41. Thought Question First quarter Waxing gibbous Third quarter Half moon
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…)

42. 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 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.

43. 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.

44. Lunar Eclipse
This interactive tool goes through lunar eclipses. Use it instead of or in addition to the earlier slides on eclipses.

45. 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.

46. Solar Eclipse
This interactive tool goes through the solar eclipses. Use it instead of or in addition to the earlier slides on eclipses.

47. Annular Solar Eclipses
The angular sizes of the moon and the sun vary, depending on their distance from Earth.
Perigee
Apogee
Aphelion
Perihelion
When Earth is near perihelion, and the moon is near apogee, we see an annular solar eclipse.

48. 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.

49. 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 full moon and solar eclipse at new 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.

50. Conditions for Eclipses (I)
The moon’s orbit is inclined against the ecliptic by ~ 5º.
A solar eclipse can only occur if the moon passes a node near new moon.
A lunar eclipse can only occur if the moon passes a node near full moon.

51. 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).

52. Predicting Eclipses
Solar eclipses recur with the 18 yr, 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).

54. 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.

55. 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 superior planets 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.

56. 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…

57. 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

58. Why did the ancient Greeks reject the real explanation for planetary motion?
Their inability to observe stellar parallax was a major factor.

59. Earth does not orbit Sun; it 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 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…

60. 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 farther from the sun than us appears to turn 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