1. Chapter 2 Discovering the Universe for Yourself

2. 2.1 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?

3. 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.

4. Rotation - Apparent
Apparent motion = Sun, Moon, stars appear to move counter-clockwise
East to West
Rise in East
Set in West 4

5. 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. 5

6. Earth orbits the Sun (revolves) once every year:
at an average distance of 1 AU ≈ 150 million km.
with 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.

7. Constellations A constellation is a region of the sky.
88 constellations fill the entire sky.
~12 constellations along the ecliptic
(Zodiac, and technically 13)
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.

8. 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.

9. 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).

10. Annual Motion – Apparent
As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic.
At midnight, the stars overhead are opposite the Sun in the sky.
Use this interactive figure to explain how the constellations change with the time of year.

11. 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.

12. The Milky Way
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.

13. The Local Sky
An object’s altitude (above horizon) and direction (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.

14. We measure the sky using angles
Full circle = 360º
1º = 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.

15. 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.

16. 60 arcseconds 600 arcseconds 60  60 = 3,600 arcseconds
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 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….

17. 2.2 The Reason for the Seasons

18. CLOSER means MORE right?

19. CLOSER means MORE right?
Heat
The closer you are the hotter it is

20. CLOSER means MORE right?
Heat
The closer you are the hotter it is
Sound
The closer you get, the louder it is

21. CLOSER means MORE right?
Heat
The closer you are the hotter it is
Sound
The closer you get, the louder it is
Light
The closer you get, the brighter it is

22. Complete this statement!
When the Sun is ______ it is summer, and when the Sun is _______ it is winter.

23. Complete this statement!
When the Sun is CLOSER it is summer, and when the Sun is FARTHER it is winter.

24. When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER

25. When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER

26. When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER
When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER

27. 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.

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. 2.3 The Moon, Our Constant Companion
Our goals for learning:
Why do we see phases of the Moon?
What causes eclipses?

34. The changing phases of the Moon inspired the concept of the month

35. 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

36. Although the Moon is always ½ lit by the Sun, we see different amounts of the lit portion from Earth depending on where the Moon is located in its orbit.

37. Animations at links from our website
Moon is illuminated (always ½) by Sun
We see a changing combination of the bright and dark faces as Moon orbits the 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 you lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads.
Animations at links from our website

38. The full moon rises at approximately:
Midnight
Sunset
Sunrise
9 or 10 p.m.
3 or 4 a.m.
Answer: B

39. If you were on the Moon, would the Earth,
Show no phases
Show phases the same as the moon (when it is full Moon it is full Earth, etc.)
Show phases opposite to the Moon (when it is full Moon it is new Earth, etc.)
Make a sketch to decide!
Answer: C

40. 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.

41. Eclipses

42. 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.

43. 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.

44. 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.

45. 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.

46. 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).

47. Predicting Eclipses
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).

48. 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?

49. 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.

50. 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
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.

51. Heliocentric
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…

52. Geocentric

53. The most sophisticated geocentric model was that of Ptolemy (A. D
The most sophisticated geocentric model was that of Ptolemy (A.D ) — the Ptolemaic model:
Sufficiently accurate to remain in use for 1,500 years.
Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”)
Ptolemy

54. 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

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

56. 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…