Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 35
Chapter 2. Discovering the Universe for Yourself
This chapter introduces major phenomena of the sky, with emphasis on these
topics:
The concept of the celestial sphere.
The basic daily motion of the sky, and how it varies with latitude.
The cause of seasons.
Phases of the Moon and eclipses.
The apparent retrograde motion of the planets and how it posed a problem
for ancient observers.
Key Changes for the 8th Edition: For those who have used earlier editions of
our textbook, please note the following significant changes in this chapter:
Continued effort has been made to make discussions more friendly to
Southern Hemisphere users as well as to those in the Northern Hemisphere.
Further clarification of a few of the more subtle concepts of precession,
Teaching Notes (by Section)
Section 2.1 Patterns in the Night Sky
This section introduces the concepts of constellations and the celestial sphere, as
well as horizon-based coordinates and daily and annual sky motions.
Stars in the daytime: You may be surprised at how many of your students
actually believe that stars disappear in the daytime. If you have a campus
daytime opportunity to point the telescope at some bright stars, showing
the students that they are still there.
36 Bennett, Donahue, Schneider, Voit
Note that in our jargon-reduction efforts, we do not introduce the term
asterism, but instead speak of patterns of stars in the constellations. We also
avoid the term azimuth when discussing horizon-based coordinates. Instead,
Section 2.2 The Reason for Seasons
This section focuses on seasons and why they occur.
In combating misconceptions about the cause of the seasons, we
recommend that you follow the logic in the Common Misconceptions box.
That is, begin by asking your students what they think causes the seasons.
As a follow-up on the above note: Some students get confused by the fact
that season diagrams (such as our Figure 2.15) cannot show the Sun Earth
distance and size of Earth to scale. Thus, unless you emphasize this point
Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 37
Section 2.3 The Moon, Our Constant Companion
This section
including the lunar phases and eclipses.
Though it may seem to be an easy concept, many students find it
remarkably difficult to understand the phases of the Moon. You may want
to do an in-class demonstration of phases by darkening the room, using a
As a follow-up to the above note, it is virtually impossible for students to
understand phases from a flat figure on a flat page in a book. We have
therefore
find in almost every other text which shows the Moon in eight different
positions around Earth because s
moons confuse them. Instead, our Figure 2.21 shows how students can
conduct a demonstration that will help them understand the phases. The
Phases of the Moon tutorial on the MasteringAstronomy website has also
proved very successful at helping students understand phases.
Note about the appearance of lunar phases: We have often heard
instructors describe the appearance of the lunar phases in terms of the
orbit. Keep a model Sun on a table in the center of the lecture area; have
your left fist represent 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
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
same type of eclipse (e.g., partial versus total) does not recur in each cycle
Section 2.4 The Ancient Mystery of the Planets
This section covers the ancient mystery of planetary motion, explaining the
motion, how we now understand it, and how the mystery helped lead to the
development of modern science.
We have chosen to refer to the westward movement of planets in our sky
as apparent retrograde motion, in order to emphasize that planets only
Answers/Discussion Points for Think About It/See It for Yourself
Questions
The Think About It and See It for Yourself questions are not numbered in the
book, so we list them in the order in which they appear, keyed by section number.
Section 2.1
(p. 28) No. We can only describe angular sizes and distances in the sky, so
physical measurements do not make sense. This is a difficult idea for
many children to understand, but should come easily for college students!
Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 39
laxies.
Second, many students have trouble visualizing galaxies as fixed objects
on the celestial sphere like stars, perhaps because they try to see them
Section 2.2
(p. 36) Jupiter does not have seasons because of its lack of appreciable
axis tilt. Saturn, wit
Section 2.3
(p. 40
third-quarter, since the third-quarter moon rises around midnight and sets
around noon.
Section 2.4
(p. 48 eters
apart, or about 30 meters on the 1-to-10-billion scale used in Chapter 1.
40 Bennett, Donahue, Schneider, Voit
Solutions to End-of-Chapter Problems (Chapter 2)
Visual Skills Check
Review Questions
1. A constellation is a section of the sky, like a state within the United States.
They are based on groups of stars that form patterns that suggested shapes to
2. If we were making a model of the celestial sphere on a ball, we would
definitely need to mark the north and south celestial poles, which are the
3. No, space is not really full of stars. Because the distance to the stars is very
4. The local sky looks like a dome because we see half of the full celestial
sphere at any one time.
5. We can measure only angular size or angular distance on the sky because we
lack a simple way to measure distance to objects just by looking at them. It
6. Circumpolar stars are stars that never appear to rise or set from a given
location, but are always visible on any clear night. From the North Pole,
7.
Longitude measures angular distance east or west of the Prime Meridian.
8. The zodiac is the set of constellations in which the Sun can be found at different
9. arth would not have significant seasons because
10. The June solstice is the day when the Northern Hemisphere gets the most
direct sunlight and the Southern Hemisphere the least direct. Also, on the June
solstice the Sun is as far north as it ever appears on the celestial sphere. On
11.
over the centuries, and we call this change precession. Because of this
12. , when the Moon is nearest the
side is facing us and because the dim light we might otherwise see from the
night side (reflected light from Earth) is overwhelmed by the bright daytime
sky. The waxing phases in which we see a gradually increasing amount of
13. We always see the same face of the Moon because the Moon displays
14. While the Moon must be in its new phase for a solar eclipse or in its full
15.
behaviors when they sometimes stop moving eastward relative to the stars
and move westward for a few weeks or months. While the ancients had to
16. Stellar parallax is the apparent movement of some of the nearest stars
relative to more distant ones as Earth goes around the Sun. This is caused by
our slightly changing perspective on these stars through the year. The shift
Does It Make Sense?
17.
18. When I looked into the dark lanes of the Milky Way with my binoculars,
19. Last night the Moon was so big that it stretched for a mile across the sky.
20. I live in the United States, and during a trip to Argentina I saw many
This statement makes sense
21. Last night I saw Jupiter in the middle of the Big Dipper. (Hint: Is the Big
Dipper part of the zodiac?) This statement does not make sense because
22. Last night I saw Mars move westward through the sky in its apparent
retrograde motion. This statement does not make sense because the apparent
23. Although all the known stars rise in the east and set in the west, we might
24. This
25. Because of precession, someday it will be summer everywhere on Earth at
26. This morning I saw the full moon setting at about the same time the Sun was
Quick Quiz
Process of Science
37. (a) This is consistent with the Earth-centered view, simply by having the stars
rotate around Earth. (b) This is consistent with the Earth-centered view by
having the Sun actually move slowly among the constellations on the path of
the ecliptic so that its position north or south of the celestial equator is thought
38. The shadow shapes are wrong. For example, during gibbous phase, the dark
portion of the Moon has the shape of a crescent, and a round object could
Group Work Exercise (no solution provided)
Short Answer/Essay Questions
41. Answers will vary with location; the following is a sample answer for
Boulder, Colorado.
42. a. When you see a full Earth, people on Earth must have a new moon.
b. At full moon as viewed from Earth, you would see new Earth from your
43. You would not see the Moon go through phases if you were viewing it from
44. If the Moon were twice as far from Earth, its angular size would be too
Quantitative Problems
47. a. There are 360 60 = 21,600 arcminutes in a full circle.
48. To solve this problem, we turn to Cosmic Calculations 2.1, where we learn
that the physical size of an object, its distance, and its angular size are
related by this equation:
49. To solve this problem, we use the equation relating distance, physical size,
and angular size given in Cosmic Calculations 2.1:
In this case, we are given the distance to Betelgeuse as 600 light-years and
the angular size as 0.05 arcsecond. We have to convert the angular size to
he wrong units: light-years are too
50. a. Rearranging the formula from Cosmic Calculations 2.1 gives us:
360
angular size physical size 2 distance
b.
diameter (1,390,000 kilometers) and minimum and maximum distances
c. When both objects are at their maximum distances from Earth, both