Discovering the 1
Night Sky
Chapter Outline
1-1 Astronomical distances are, well, astronomical
1-2 Constellations make locating stars easy
1-3 The celestial sphere aids in navigating the sky
1-4 Earth’s rotation creates the day-night cycle and its revolution defines a year
1-5 The seasons result from the tilt of the Earth’s rotation axis combined with its
revolution around the Sun
1-6 Clock times based on the Sun’s location created scheduling nightmares
1-7 Calendars based on equal-length years also created scheduling problems
1-8 Precession is a slow, circular motion of the Earth’s axis of rotation
1-9 The phases of the Moon originally inspired the concept of the month
1-10 Eclipses occur only when the Moon crosses the ecliptic during the new or full phase
1-11 Three types of lunar eclipses occur
1-12 Three types of solar eclipses also occur
1-13 Frontiers yet to be discovered
In this chapter students will discover
• how astronomers organize the night sky to help them locate objects in it
• that the Earth’s spin on its axis causes day and night
• how the tilt of the Earth’s axis of rotation and the Earth’s motion around the Sun combine to
create the seasons
• that the Moon’s orbit around the Earth creates the phases of the Moon and lunar and solar
eclipses
• how the year is defined and how the calendar was developed
Suggested Learning Objectives
At the end of this chapter, the student should be able to
1. Explain the importance of distance measurements in astronomy.
2. Describe the nature and use of constellations.
3. Define the elements of the equatorial coordinate system on the celestial sphere.
4. Define two solstices and two equinoxes.
5. Explain the orientation of the ecliptic on the celestial sphere and how it produces seasons
on the Earth.
6. Describe the daily and yearly motions of the Earth.
7. Describe what precession is, what effect it has on our observations of stars, and why it
occurs.
8. Draw a diagram explain how lunar phases are controlled by the relative positions of the
Sun and the Moon.
9. Explain when and why solar and lunar eclipses occur and why there are not such eclipses
every month.
,6
Teaching Hints and Strategies
This chapter covers many of the topics that students associate with astronomy. It provides
opportunities for a number of both short- and long-range observing projects. Some of these projects
may be completed during one night or one day; others may extend over a few weeks or span the entire
course. Such projects contribute to students’ understanding of modern observations as well as
historical ones. Modern astrophysics depends heavily upon geometric reasoning and is communicated
substantially through geometric representations. Yet typical undergraduate students possess relatively
undeveloped capacities for geometric visualization and manipulation, especially in three dimensions.
Thus, the effectiveness of our introductory astronomy teaching is enhanced by any success in
cultivating the geometry skills of our students. Finally, observational projects can be fun (especially if
they involve group work) and are generally well received by the students. They have been used
effectively in large classes and require only modest equipment and time.
A review of angular measurements is an excellent opportunity to emphasize that astronomy is
an observational rather than an experimental science, principally because of the vast distances
between celestial objects. While the space program is very important in the advancement of modern
astronomy, humans have visited only one other celestial object (the Moon) and have landed
spacecraft on only four others (Mars, Venus, Titan and the asteroid Eros). Much of the space program
has been devoted to using remote observatories that are still passive observers and do not conduct
active experiments beyond the solar system.
The distances between astronomical objects are vast, so their true sizes and distances from
one another are frequently unknown or poorly known. Astronomers are able to measure angular
separations and sizes directly. The presentation of light travel times to such celestial objects as our
Moon, the Sun, the nearest star, the center of our galaxy, the nearest external galaxies, and the most
remote quasars clearly illustrates these distances.
The introduction of scientific notation (Appendix A-1) can be justified by noting that the
science of astronomy spans the entire dimensional scale from the truly vast to the truly minute. Most
people recognize the large numbers of and the large sizes of celestial objects. It is helpful to remind
the students that the observational (passive) nature of astronomical investigations means that all direct
information about the physical conditions existing on celestial objects and about their past and future
conditions must be extracted from an understanding of the nature of atoms and the constituent parts of
atoms, the smallest entities of the universe. Thus, astronomers must frequently deal with very small
numbers as well as very large ones. This is a primary motivation for becoming familiar with scientific
notation. The film Powers of Ten illustrates the range of astronomical sizes dramatically but very
rapidly. Be sure to provide some time for mastery of the concepts of the powers-of-ten notation
before expecting students to experience and interpret this film as a quantitative document.
The use of different systems of distance units in astronomy serves to emphasize the role of
units in physical science and to illustrate the justification for establishing different systems of units.
Many students fail to associate units with numbers. They have dealt with numbers in mathematics
courses but have little experience with the measuring activities that lie at the heart of the physical
sciences. Accordingly, a discussion of how units give meaning to numbers is important to aid student
understanding. By using a variety of length units in astronomy, we are able to compare sizes and
distances of the same order of magnitude with whole numbers and simple fractions. Although any
system of units can be used with powers-of-ten notation, it is not as easy to visualize the relative
magnitudes with very large or very small numbers. Comparisons of the sizes of and distances
between objects of vastly different dimensions are more readily accomplished in powers-of-ten
notation. It is useful to encourage students to view the exponent in the context of order-of-magnitude
comparisons.
Discovering the Night Sky
, 7
It is always a good idea to emphasize (repeatedly) that light-years are units of length and not
units of time. During a topical overview, it is worthwhile to point out that astronomers must
constantly deal with the past as they study remote objects. The light from celestial objects requires a
finite amount of time to reach us. Astronomers thus see things not as they are, but as they were when
the light left them. We see the Moon as it was 1.5 seconds ago, the Sun as it was 8.3 minutes ago,
Pluto as it was about 4 or 5 hours ago, the nearest star as it was 4 years ago, the center of our galaxy
as it was 25,000 years ago, the Andromeda galaxy as it was 2.6 million years ago, distant galaxies as
they were 8 to 10 billion years ago, and remote quasars as they were 10 to 15 billion years ago.
Astronomers can thus attempt to investigate systematic changes that occur with time and try to
explain the results of those observations. The study of the impact of stellar evolution on the
observable properties of stellar populations in galaxies is an example.
Constellations are used as aids in “navigating” the night sky. This discussion can be used as
the basis for modest but very useful observing assignments designed to consolidate a student’s
understanding of angles and apparent positions in the sky.
The naked-eye observational focus of the chapter provides an important opportunity for those
instructors who choose to highlight the character of scientific knowledge. Knowing little about the
process of science, most students are able to recognize only the “incorrectness” of ancient astronomy
but not its successes. Most are astounded to learn (for instance, in a discussion of Chapter 2) that it is
possible to account for all naked-eye observations with a geocentric model of the universe. So, the
immediate successors of Copernicus were confronted with two competing scientific models that were
equally successful in matching the available observations. It is important for students to appreciate
that a given set of data does not automatically specify a unique hypothesis; science, like art, requires
creative leaps. That appreciation is most easily acquired through substantial experience with a
concrete example. An instructor who hopes to use the comparison between Ptolemaic and Copernican
models must develop the observational groundwork in Chapter 1. Useful resources for the instructor
in this context include Kuhn, The Copernican Revolution (Harvard University Press, 1957) and
Crowe, Systems of the World from Ptolemy to Copernicus (Dover, 1991).
The three primary motions of the Earth are rotation, revolution, and precession. Many of the
geometric relationships among observed quantities are more easily displayed in the geocentric model
(hence its use in celestial navigation texts). For example, the dependence of the time and azimuth of
sunrise on the solar declination and latitude of observation is easily recognized on a celestial globe
equipped with a horizon ring. The complementary use of heliocentric and geocentric models is
initially surprising to many students, but it is an illuminating intellectual exercise. This example can
be used to initiate a more general discussion of the use of conceptual models in our attempts to
understand and describe nature. Such a discussion can help students to understand the role and
limitations of models. The fact that a model has only a finite range of applicability does not detract
from its usefulness within that range. (This theme is useful later in discussing the advances in our
understanding of mechanics and the properties of light and matter.)
The rotation of the Earth on its axis is the basis for the geographic coordinate system (latitude
and longitude), which is a natural tie-in to the celestial sphere and celestial coordinates. Most students
have had some exposure to the geographic system. Students should be encouraged to go out at night
and observe diurnal motion. The role of astronomers in the development of our time systems and their
connection with the Earth’s rotation should be noted here as an example of an early application of
astronomy. Students who have used a GPS receiver may be familiar with the metric-based UTM/UPS
coordinate system, an easier system to use than latitude and longitude. Consult the U.S. Naval
Observatory Web site for the correct time. A GPS receiver will also report highly accurate time.
Many clocks, watches, and home weather stations receive signals from WWVB and automatically
synchronize to the correct time.
Discovering the Night Sky
, 8
A planetarium presentation can be very helpful here, if one is available. Computer sky simu-
lators provide an excellent vehicle for quantitative demonstrations and student exercises. It is critical
to remember, however, that students’ experiences of the real sky are very different from their
experiences of a planetarium dome. A small flat sky map or computer screen is an even more abstract
(arguably denatured) representation of the sky. To benefit from these learning aids, a student must
develop a nontrivial mapping from an initially unfamiliar experience to an unfamiliar representation.
This takes time. It is not difficult, however, to devise a set of observation exercises that span and
complement several weeks of lecture material (see, for instance, the suggestions in later chapters).
This strategy allows most students enough time to gain a satisfying level of understanding.
In our experience, the most rapid and substantial learning occurs if we are able to provide
each student with access in lab or discussion class to a concrete “hands-on” model, such as a celestial
globe, and to a computer program such as Starry Night. (If student access to computers is limited,
exercises can be built around carefully selected computer-generated diagrams. The loss of individual
interactive experience is a significant but not a crippling limitation.)
When discussing time keeping, it should be emphasized that the calendar requires a whole
number of days and that a day requires a whole number of hours. These are human needs, and the
Earth is under no obligation to provide them. Therefore, we have to make adjustments to how we
define these time periods so as to agree as closely as possible with the true astronomical year and the
variable duration of the solar day throughout the year. For more information on calendars, consult the
U.S. Naval Observatory Web site.
The causes of seasons are misunderstood by a very large number of students. The effects of
the varying altitude of the Sun at noon can be demonstrated by using a flashlight held at different
angles to a wall. The variation in the length of day is well known to everyone. However, many stu-
dents look at elliptical orbits and conclude that seasons are caused by the varying distance of the
Earth from the Sun. Be sure to point out that the Earth is closest to the Sun in January and farthest
away in July. Also point out that the northern and southern hemispheres have opposite seasons and
the names of the equinoxes and solstices reflect the northern hemispheric seasons only. Geometric
links among day/night duration, observer’s latitude, midday solar altitude, and the sunrise/sunset
azimuths and solar ecliptic longitude are easily recognized by using a celestial globe or armillary
sphere. A computerized sky simulator such as Starry Night can be used to represent the observations
at different latitudes in each hemisphere.
A long-term project to observe the rising and setting azimuths of the Sun and/or its altitude at
noon can help to make some of the patterns more clear. As a complementary observation, one can
(once per month or so) make hourly observations of the length and azimuth of the shadow cast by a
gnomon. Simulations of such gnomon observations at different seasons and latitudes can be done
with a globe and a small square of cardboard perforated by a thumbtack. Tape the gnomon assembly
to the globe at various latitudes and use the beam of a slide projector or overhead projector to cast the
shadow of the thumbtack.
The underlying physics of precession is best demonstrated by the actions of a toy top. The
necessity of rotation for precession is easy to show, and the precise nature of the motion is obvious. It
can be instructive to discuss what summer and winter constellations are, how they are defined, and
how they will appear in 13,000 years. It is also interesting to discuss the impact of the presence of a
pole star on celestial navigation in the northern and southern hemispheres and the role astronomy has
played in navigation in general. Until the past few years, during which satellite navigation has
become common, transoceanic navigation was possible only on the basis of an understanding of the
sky positions of celestial objects and the variation of those positions with time. This is another
example of astronomy as an applied science. The applied nature of astronomy has dramatically
diminished with time. Modern astronomers are generally more involved in basic research than with
Discovering the Night Sky
Night Sky
Chapter Outline
1-1 Astronomical distances are, well, astronomical
1-2 Constellations make locating stars easy
1-3 The celestial sphere aids in navigating the sky
1-4 Earth’s rotation creates the day-night cycle and its revolution defines a year
1-5 The seasons result from the tilt of the Earth’s rotation axis combined with its
revolution around the Sun
1-6 Clock times based on the Sun’s location created scheduling nightmares
1-7 Calendars based on equal-length years also created scheduling problems
1-8 Precession is a slow, circular motion of the Earth’s axis of rotation
1-9 The phases of the Moon originally inspired the concept of the month
1-10 Eclipses occur only when the Moon crosses the ecliptic during the new or full phase
1-11 Three types of lunar eclipses occur
1-12 Three types of solar eclipses also occur
1-13 Frontiers yet to be discovered
In this chapter students will discover
• how astronomers organize the night sky to help them locate objects in it
• that the Earth’s spin on its axis causes day and night
• how the tilt of the Earth’s axis of rotation and the Earth’s motion around the Sun combine to
create the seasons
• that the Moon’s orbit around the Earth creates the phases of the Moon and lunar and solar
eclipses
• how the year is defined and how the calendar was developed
Suggested Learning Objectives
At the end of this chapter, the student should be able to
1. Explain the importance of distance measurements in astronomy.
2. Describe the nature and use of constellations.
3. Define the elements of the equatorial coordinate system on the celestial sphere.
4. Define two solstices and two equinoxes.
5. Explain the orientation of the ecliptic on the celestial sphere and how it produces seasons
on the Earth.
6. Describe the daily and yearly motions of the Earth.
7. Describe what precession is, what effect it has on our observations of stars, and why it
occurs.
8. Draw a diagram explain how lunar phases are controlled by the relative positions of the
Sun and the Moon.
9. Explain when and why solar and lunar eclipses occur and why there are not such eclipses
every month.
,6
Teaching Hints and Strategies
This chapter covers many of the topics that students associate with astronomy. It provides
opportunities for a number of both short- and long-range observing projects. Some of these projects
may be completed during one night or one day; others may extend over a few weeks or span the entire
course. Such projects contribute to students’ understanding of modern observations as well as
historical ones. Modern astrophysics depends heavily upon geometric reasoning and is communicated
substantially through geometric representations. Yet typical undergraduate students possess relatively
undeveloped capacities for geometric visualization and manipulation, especially in three dimensions.
Thus, the effectiveness of our introductory astronomy teaching is enhanced by any success in
cultivating the geometry skills of our students. Finally, observational projects can be fun (especially if
they involve group work) and are generally well received by the students. They have been used
effectively in large classes and require only modest equipment and time.
A review of angular measurements is an excellent opportunity to emphasize that astronomy is
an observational rather than an experimental science, principally because of the vast distances
between celestial objects. While the space program is very important in the advancement of modern
astronomy, humans have visited only one other celestial object (the Moon) and have landed
spacecraft on only four others (Mars, Venus, Titan and the asteroid Eros). Much of the space program
has been devoted to using remote observatories that are still passive observers and do not conduct
active experiments beyond the solar system.
The distances between astronomical objects are vast, so their true sizes and distances from
one another are frequently unknown or poorly known. Astronomers are able to measure angular
separations and sizes directly. The presentation of light travel times to such celestial objects as our
Moon, the Sun, the nearest star, the center of our galaxy, the nearest external galaxies, and the most
remote quasars clearly illustrates these distances.
The introduction of scientific notation (Appendix A-1) can be justified by noting that the
science of astronomy spans the entire dimensional scale from the truly vast to the truly minute. Most
people recognize the large numbers of and the large sizes of celestial objects. It is helpful to remind
the students that the observational (passive) nature of astronomical investigations means that all direct
information about the physical conditions existing on celestial objects and about their past and future
conditions must be extracted from an understanding of the nature of atoms and the constituent parts of
atoms, the smallest entities of the universe. Thus, astronomers must frequently deal with very small
numbers as well as very large ones. This is a primary motivation for becoming familiar with scientific
notation. The film Powers of Ten illustrates the range of astronomical sizes dramatically but very
rapidly. Be sure to provide some time for mastery of the concepts of the powers-of-ten notation
before expecting students to experience and interpret this film as a quantitative document.
The use of different systems of distance units in astronomy serves to emphasize the role of
units in physical science and to illustrate the justification for establishing different systems of units.
Many students fail to associate units with numbers. They have dealt with numbers in mathematics
courses but have little experience with the measuring activities that lie at the heart of the physical
sciences. Accordingly, a discussion of how units give meaning to numbers is important to aid student
understanding. By using a variety of length units in astronomy, we are able to compare sizes and
distances of the same order of magnitude with whole numbers and simple fractions. Although any
system of units can be used with powers-of-ten notation, it is not as easy to visualize the relative
magnitudes with very large or very small numbers. Comparisons of the sizes of and distances
between objects of vastly different dimensions are more readily accomplished in powers-of-ten
notation. It is useful to encourage students to view the exponent in the context of order-of-magnitude
comparisons.
Discovering the Night Sky
, 7
It is always a good idea to emphasize (repeatedly) that light-years are units of length and not
units of time. During a topical overview, it is worthwhile to point out that astronomers must
constantly deal with the past as they study remote objects. The light from celestial objects requires a
finite amount of time to reach us. Astronomers thus see things not as they are, but as they were when
the light left them. We see the Moon as it was 1.5 seconds ago, the Sun as it was 8.3 minutes ago,
Pluto as it was about 4 or 5 hours ago, the nearest star as it was 4 years ago, the center of our galaxy
as it was 25,000 years ago, the Andromeda galaxy as it was 2.6 million years ago, distant galaxies as
they were 8 to 10 billion years ago, and remote quasars as they were 10 to 15 billion years ago.
Astronomers can thus attempt to investigate systematic changes that occur with time and try to
explain the results of those observations. The study of the impact of stellar evolution on the
observable properties of stellar populations in galaxies is an example.
Constellations are used as aids in “navigating” the night sky. This discussion can be used as
the basis for modest but very useful observing assignments designed to consolidate a student’s
understanding of angles and apparent positions in the sky.
The naked-eye observational focus of the chapter provides an important opportunity for those
instructors who choose to highlight the character of scientific knowledge. Knowing little about the
process of science, most students are able to recognize only the “incorrectness” of ancient astronomy
but not its successes. Most are astounded to learn (for instance, in a discussion of Chapter 2) that it is
possible to account for all naked-eye observations with a geocentric model of the universe. So, the
immediate successors of Copernicus were confronted with two competing scientific models that were
equally successful in matching the available observations. It is important for students to appreciate
that a given set of data does not automatically specify a unique hypothesis; science, like art, requires
creative leaps. That appreciation is most easily acquired through substantial experience with a
concrete example. An instructor who hopes to use the comparison between Ptolemaic and Copernican
models must develop the observational groundwork in Chapter 1. Useful resources for the instructor
in this context include Kuhn, The Copernican Revolution (Harvard University Press, 1957) and
Crowe, Systems of the World from Ptolemy to Copernicus (Dover, 1991).
The three primary motions of the Earth are rotation, revolution, and precession. Many of the
geometric relationships among observed quantities are more easily displayed in the geocentric model
(hence its use in celestial navigation texts). For example, the dependence of the time and azimuth of
sunrise on the solar declination and latitude of observation is easily recognized on a celestial globe
equipped with a horizon ring. The complementary use of heliocentric and geocentric models is
initially surprising to many students, but it is an illuminating intellectual exercise. This example can
be used to initiate a more general discussion of the use of conceptual models in our attempts to
understand and describe nature. Such a discussion can help students to understand the role and
limitations of models. The fact that a model has only a finite range of applicability does not detract
from its usefulness within that range. (This theme is useful later in discussing the advances in our
understanding of mechanics and the properties of light and matter.)
The rotation of the Earth on its axis is the basis for the geographic coordinate system (latitude
and longitude), which is a natural tie-in to the celestial sphere and celestial coordinates. Most students
have had some exposure to the geographic system. Students should be encouraged to go out at night
and observe diurnal motion. The role of astronomers in the development of our time systems and their
connection with the Earth’s rotation should be noted here as an example of an early application of
astronomy. Students who have used a GPS receiver may be familiar with the metric-based UTM/UPS
coordinate system, an easier system to use than latitude and longitude. Consult the U.S. Naval
Observatory Web site for the correct time. A GPS receiver will also report highly accurate time.
Many clocks, watches, and home weather stations receive signals from WWVB and automatically
synchronize to the correct time.
Discovering the Night Sky
, 8
A planetarium presentation can be very helpful here, if one is available. Computer sky simu-
lators provide an excellent vehicle for quantitative demonstrations and student exercises. It is critical
to remember, however, that students’ experiences of the real sky are very different from their
experiences of a planetarium dome. A small flat sky map or computer screen is an even more abstract
(arguably denatured) representation of the sky. To benefit from these learning aids, a student must
develop a nontrivial mapping from an initially unfamiliar experience to an unfamiliar representation.
This takes time. It is not difficult, however, to devise a set of observation exercises that span and
complement several weeks of lecture material (see, for instance, the suggestions in later chapters).
This strategy allows most students enough time to gain a satisfying level of understanding.
In our experience, the most rapid and substantial learning occurs if we are able to provide
each student with access in lab or discussion class to a concrete “hands-on” model, such as a celestial
globe, and to a computer program such as Starry Night. (If student access to computers is limited,
exercises can be built around carefully selected computer-generated diagrams. The loss of individual
interactive experience is a significant but not a crippling limitation.)
When discussing time keeping, it should be emphasized that the calendar requires a whole
number of days and that a day requires a whole number of hours. These are human needs, and the
Earth is under no obligation to provide them. Therefore, we have to make adjustments to how we
define these time periods so as to agree as closely as possible with the true astronomical year and the
variable duration of the solar day throughout the year. For more information on calendars, consult the
U.S. Naval Observatory Web site.
The causes of seasons are misunderstood by a very large number of students. The effects of
the varying altitude of the Sun at noon can be demonstrated by using a flashlight held at different
angles to a wall. The variation in the length of day is well known to everyone. However, many stu-
dents look at elliptical orbits and conclude that seasons are caused by the varying distance of the
Earth from the Sun. Be sure to point out that the Earth is closest to the Sun in January and farthest
away in July. Also point out that the northern and southern hemispheres have opposite seasons and
the names of the equinoxes and solstices reflect the northern hemispheric seasons only. Geometric
links among day/night duration, observer’s latitude, midday solar altitude, and the sunrise/sunset
azimuths and solar ecliptic longitude are easily recognized by using a celestial globe or armillary
sphere. A computerized sky simulator such as Starry Night can be used to represent the observations
at different latitudes in each hemisphere.
A long-term project to observe the rising and setting azimuths of the Sun and/or its altitude at
noon can help to make some of the patterns more clear. As a complementary observation, one can
(once per month or so) make hourly observations of the length and azimuth of the shadow cast by a
gnomon. Simulations of such gnomon observations at different seasons and latitudes can be done
with a globe and a small square of cardboard perforated by a thumbtack. Tape the gnomon assembly
to the globe at various latitudes and use the beam of a slide projector or overhead projector to cast the
shadow of the thumbtack.
The underlying physics of precession is best demonstrated by the actions of a toy top. The
necessity of rotation for precession is easy to show, and the precise nature of the motion is obvious. It
can be instructive to discuss what summer and winter constellations are, how they are defined, and
how they will appear in 13,000 years. It is also interesting to discuss the impact of the presence of a
pole star on celestial navigation in the northern and southern hemispheres and the role astronomy has
played in navigation in general. Until the past few years, during which satellite navigation has
become common, transoceanic navigation was possible only on the basis of an understanding of the
sky positions of celestial objects and the variation of those positions with time. This is another
example of astronomy as an applied science. The applied nature of astronomy has dramatically
diminished with time. Modern astronomers are generally more involved in basic research than with
Discovering the Night Sky
