1. Roger A. Freedman • William J. Kaufmann III
Universe
Eighth Edition
CHAPTER 2
Knowing the Heavens

2. Read Chapter 3 (Eclipses and the Motion of the Moon)
Homework 2
Read Chapter 3 (Eclipses and the Motion of the Moon)
Online quiz from Chapter 2
DUE Friday 5 pm
REMINDER: No class on Monday 9/6 (Labor Day)
The Earth’s rotation makes stars appear to trace out circles in the sky.
(Gemini Observatory)

3. The Earth’s rotation makes stars appear to trace out circles in the sky.
(Gemini Observatory)

4. By reading this chapter, you will learn
2-1 The importance of astronomy in ancient civilizations around the world
2-2 That regions of the sky are divided around groups of stars called constellations
2-3 How the sky changes from night to night
2-4 How astronomers locate objects in the sky
2-5 What causes the seasons
2-6 The effect of changes in the direction of Earth’s axis of rotation
2-7 The role of astronomy in measuring time
2-8 How the modern calendar developed

5. Figure 2-1 The Sun Dagger at Chaco Canyon
On the first day of winter, rays of
sunlight passing between stone slabs bracket a spiral stone carving, or
petroglyph, at Chaco Canyon in New Mexico. A single band of light
strikes the center of the spiral on the first day of summer. This and
other astronomically aligned petroglyphs were carved by the ancestral
Puebloan culture between 850 and 1250 A.D. (Courtesy Karl Kernberger)

6. Figure 2-2 Three Views of Orion
The constellation Orion is easily seen on nights
from December through March. (a) This photograph of Orion shows many
more stars than can be seen with the naked eye. (b) A portion of a
modern star atlas shows the distances in light-years (ly) to some of the
stars in Orion. The yellow lines show the borders between Orion and its
neighboring constellations (labeled in capitals). (c) This fanciful drawing
from a star atlas published in 1835 shows Orion the Hunter as well as
other celestial creatures. (a: Luke Dodd/Science Photo Library/Photo
Researchers; c: Courtesy of Janus Publications)

7. Most evidence suggests ancient astronomers were inspired to look at the sky because
they wanted to create scientific theories of the world in which they lived.
observation of star positions allowed calendars to be created.
ancient civilizations associated star patterns with gods and mystical figures.
Both a and c.
Both b and c.
Q2.2

8. Most evidence suggests ancient astronomers were inspired to look at the sky because
they wanted to create scientific theories of the world in which they lived.
observation of star positions allowed calendars to be created.
ancient civilizations associated star patterns with gods and mystical figures.
Both a and c.
Both b and c.
A2.2

9. Figure 2-3 Day and Night on the Earth
At any moment, half of the Earth is
illuminated by the Sun. As the Earth rotates from west to east, your
location moves from the dark (night) hemisphere into the illuminated
(day) hemisphere and back again. This image was recorded in 1992 by
the Galileo spacecraft as it was en route to Jupiter. (JPL/NASA)

10. Figure 2-4 Why Diurnal Motion Happens
The diurnal (daily) motion of the stars,
the Sun, and the Moon is a consequence of the Earth’s rotation. (a) This
drawing shows the Earth from a vantage point above the north pole. In
this drawing, for a person in California the local time is 8:00 P.M. and the
constellation Cygnus is directly overhead.

11. Figure 2-4 Why Diurnal Motion Happens
(b) Four hours later, the Earth
has made one-sixth of a complete rotation to the east. As seen from
Earth, the entire sky appears to have rotated to the west by one-sixth of
a complete rotation. It is now midnight in California, and the constellation
directly over California is Andromeda.

12. Figure 2-5
Why the Night Sky Changes During the Year
As the Earth orbits around the Sun, the nighttime side of the Earth gradually turns toward
different parts of the sky. Hence, the particular stars that you see in the
night sky are different at different times of the year. This figure shows
which constellation is overhead at midnight local time—when the Sun is
on the opposite side of the Earth from your location—during different
months for observers at midnorthern latitudes (including the United
States). If you want to view the constellation Andromeda, the best time
of the year to do it is in late September, when Andromeda is nearly
overhead at midnight.

13. Figure 2-9 The Celestial Sphere
The celestial sphere is the apparent
sphere of the sky. The view in this figure is from the outside
of this (wholly imaginary) sphere. The Earth is at the center of the
celestial sphere, so our view is always of the inside of the sphere. The
celestial equator and poles are the projections of the Earth’s equator and
axis of rotation out into space. The celestial poles are therefore located
directly over the Earth’s poles.

17. Figure 2-10 The View from 35° North Latitude
To an observer at 35° north latitude
(roughly the latitude of Los Angeles, Atlanta, Tel Aviv, and Tokyo), the
north celestial pole is always 35° above the horizon. Stars within 35° of
the north celestial pole are circumpolar; they trace out circles around the
north celestial pole during the course of the night, and are always above
the horizon on any night of the year. Stars within 35° of the south
celestial pole are always below the horizon and can never be seen from
this latitude. Stars that lie between these two extremes rise in the east
and set in the west.

19. In the southern hemisphere
stars rise in the east and set in the west.
stars rise in the west and set in the east.
all stars are circumpolar.
no stars are circumpolar.
a or b, depending on the time of day.
Q2.4

20. In the southern hemisphere
stars rise in the east and set in the west.
stars rise in the west and set in the east.
all stars are circumpolar.
no stars are circumpolar.
a or b, depending on the time of day.
A2.4

21. Figure 2-11 The Apparent Motion of Stars at Different Latitudes
As the Earth rotates, stars appear to rotate around us along
paths that are parallel to the celestial equator. (a) As shown in this long
time exposure, at most locations on Earth the rising and setting motions
are at an angle to the horizon that depends on the latitude.
(David Miller/DMI)

22. Figure 2-11 The Apparent Motion of Stars at Different Latitudes
(b) At the north pole (latitude 90° north) the stars appear to move parallel to the horizon.

23. Figure 2-11 The Apparent Motion of Stars at Different Latitudes
(c) At the equator (latitude 0°) the stars rise and set along vertical paths.

24. Summer occurs in the northern hemisphere of the Earth in June, July, and August because
the northern hemisphere of Earth is closer to the Sun than the southern hemisphere.
days are longer in the northern hemisphere than in the southern hemisphere.
the sunlight strikes the northern hemisphere of the Earth at an angle closer to the vertical.
the Earth is closer to the Sun.
Both b and c.
Q2.5

25. Summer occurs in the northern hemisphere of the Earth in June, July, and August because
the northern hemisphere of Earth is closer to the Sun than the southern hemisphere.
days are longer in the northern hemisphere than in the southern hemisphere.
the sunlight strikes the northern hemisphere of the Earth at an angle closer to the vertical.
the Earth is closer to the Sun.
Both b and c.
A2.5

26. Figure The Seasons
The Earth’s axis of rotation is inclined 231⁄2° away from the
perpendicular to the plane of the Earth’s orbit. The north pole is aimed at
the north celestial pole, near the star Polaris. The Earth maintains this
orientation as it orbits the Sun. Consequently, the amount of solar
illumination and the number of daylight hours at any location on Earth
vary in a regular pattern throughout the year. This is the origin of the
seasons.

27. Figure 2-13 Solar Energy in Summer and Winter
At different times of the year, sunlight strikes the ground at
different angles. (a) In summer, sunlight is
concentrated and the days are also longer, which
further increases the heating. (b) In winter the
sunlight is less concentrated, the days are short, and
little heating of the ground takes place. This
accounts for the low temperatures in winter.

28. Box 2-1 Celestial Coordinates
Your latitude and longitude describe where on the Earth’s
surface you are located. The latitude of your location denotes
how far north or south of the equator you are, and the
longitude of your location denotes how far west or east you
are of an imaginary circle that runs from the north pole to the
south pole through the Royal Observatory in Greenwich, England.
In an analogous way, astronomers use coordinates called
declination and right ascension to describe the position of a
planet, star, or galaxy on the celestial sphere.
Declination is analogous to latitude. As the illustration
shows, the declination of an object is its angular distance north
or south of the celestial equator, measured along a circle passing
through both celestial poles. Like latitude, it is measured
in degrees, arcminutes, and arcseconds (see Section 1-5).
Right ascension is analogous to longitude. It is measured
from a line that runs between the north and south celestial
poles and passes through a point on the celestial equator called
the vernal equinox (shown as a blue dot in the illustration).
This point is one of two locations where the Sun crosses the
celestial equator during its apparent annual motion, as we discuss
in Section 2-5. In the Earth’s northern hemisphere, spring
officially begins when the Sun reaches the vernal equinox in
late March. The right ascension of an object is the angular distance
from the vernal equinox eastward along the celestial
equator to the circle used in measuring its declination (see illustration).
Astronomers measure right ascension in time units
(hours, minutes, and seconds), corresponding to the time required
for the celestial sphere to rotate through this angle.

29. Figure 2-14 The Ecliptic Plane and the Ecliptic
(a) The ecliptic plane is the plane
in which the Earth moves around the Sun. (b) As seen from Earth, the Sun
appears to move around the celestial sphere along a circular path called
the ecliptic. The Earth takes a year to complete one orbit around the Sun,
so as seen by us the Sun takes a year to make a complete trip around the
ecliptic.

30. Figure 2-15 The Ecliptic, Equinoxes, and Solstices
This illustration of the celestial
sphere is similar to Figure 2-14b, but is drawn with the north celestial
pole at the top and the celestial equator running through the middle. The
ecliptic is inclined to the celestial equator by 231⁄2° because of the tilt of
the Earth’s axis of rotation. It intersects the celestial equator at two
points, called equinoxes. The northernmost point on the ecliptic is the
summer solstice, and the southernmost point is the winter solstice. The
Sun is shown in its approximate position for August 1.

31. If the Earth’s axis were not tilted,
a day and night would last 365 Earth days.
the effect of seasons would be exaggerated.
there would be no seasons.
the Earth would always keep the same side facing toward the Sun.
The Earth would be completely covered with ice.
Q2.6

32. If the Earth’s axis were not tilted,
a day and night would last 365 Earth days.
the effect of seasons would be exaggerated.
there would be no seasons.
the Earth would always keep the same side facing toward the Sun.
The Earth would be completely covered with ice.
A2.6

35. Figure The Meridian
The meridian is a circle on the celestial sphere that passes
through the observer’s zenith (the point directly overhead) and the north
and south points on the observer’s horizon. The passing of celestial
objects across the meridian can be used to measure time. The upper
meridian is the part above the horizon, and the lower meridian (not
shown) is the part below the horizon.

37. Box 2-2 Sidereal Time (a) A month’s motion of the Earth along its orbit
If you want to observe a particular object in the heavens, the
ideal time to do so is when the object is high in the sky, on
or close to the upper meridian. This minimizes the distorting
effects of the Earth’s atmosphere, which increase as you view
closer to the horizon. For astronomers who study the Sun, this
means making observations at local noon, which is not too different
from noon as determined using mean solar time. For astronomers
who observe planets, stars, or galaxies, however,
the optimum time to observe depends on the particular object
to be studied. The problem is this: Given the location of a
given object on the celestial sphere, when will that object be
on the upper meridian?
To answer this question, astronomers use sidereal time
rather than solar time. It is different from the time on your
wristwatch. In fact, a sidereal clock and an ordinary clock even
tick at different rates, because they are based on different astronomical
objects. Ordinary clocks are related to the position
of the Sun, while sidereal clocks are based on the position of
the vernal equinox, the location from which right ascension is
measured. (See Box 2-1 for a discussion of right ascension.)
Regardless of where the Sun is, midnight sidereal time at
your location is defined to be when the vernal equinox crosses
your upper meridian. (Like solar time, sidereal time depends
on where you are on Earth.) A sidereal day is the time between
two successive upper meridian passages of the vernal equinox.
By contrast, an apparent solar day is the time between two successive
upper meridian crossings of the Sun. The illustration
shows why these two kinds of day are not equal. Because the
Earth orbits the Sun, the Earth must make one complete rotation
plus about 1° to get from one local solar noon to the next.
This extra 1° of rotation corresponds to 4 minutes of time,
which is the amount by which a solar day exceeds a sidereal
day. To be precise:
1 sidereal day 23h 56m 4.091s
where the hours, minutes, and seconds are in mean solar time.

41. The point on the ecliptic (see diagram) where the Sun crosses from the southern to the northern hemisphere is the
summer solstice.
winter solstice.
autumnal equinox.
vernal equinox.
celestial equator.
Q2.10

42. The point on the ecliptic (see diagram) where the Sun crosses from the southern to the northern hemisphere is the
summer solstice.
winter solstice.
autumnal equinox.
vernal equinox.
celestial equator.
A2.10

43. Figure 2-16 The Sun’s Daily Path Across the Sky
This drawing shows the apparent path of the Sun during the course of a day on four different dates. Like Figure 2-10, this drawing is for an observer at 35° north latitude.

44. Figure 2-17 Tropics and Circles
Four important latitudes on Earth are the Arctic
Circle (661⁄2° north latitude), Tropic of Cancer (231⁄2° north latitude),
Tropic of Capricorn (231⁄2° south latitude), and Antarctic Circle (661⁄2°
south latitude). These drawings show the significance of these latitudes
when the Sun is (a) at the winter solstice and (b) at the summer solstice.

45. Figure 2-18 The Midnight Sun
This time-lapse photograph was taken
on July 19, 1985, at 69° north latitude in northeast Alaska.
At this latitude, the Sun is above the horizon continuously
(that is, it is circumpolar) from mid-May to the end of July.
(Doug Plummer/Science Photo Library)

46. Key Ideas
Constellations and the Celestial Sphere: It is convenient to imagine the stars fixed to the celestial sphere with the Earth at its center.
The surface of the celestial sphere is divided into 88 regions called constellations.
Diurnal (Daily) Motion of the Celestial Sphere: The celestial sphere appears to rotate around the Earth once in each 24-hour period. In fact, it is actually the Earth that is rotating.
The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of the Earth out to the celestial sphere.
The positions of objects on the celestial sphere are described by specifying their right ascension (in time units) and declination (in angular measure).

47. Key Ideas
Seasons and the Tilt of the Earth’s Axis: The Earth’s axis of rotation is tilted at an angle of about 231⁄2° from the perpendicular to the plane of the Earth’s orbit.
The seasons are caused by the tilt of the Earth’s axis.
Over the course of a year, the Sun appears to move around the celestial sphere along a path called the ecliptic. The ecliptic is inclined to the celestial equator by about 231⁄2°.
The ecliptic crosses the celestial equator at two points in the sky, the vernal and autumnal equinoxes.
The northernmost point that the Sun reaches on the celestial sphere is the summer solstice, and the southernmost point is the winter solstice.

48. Key Ideas
Because the system of right ascension and declination is tied to the position of the vernal equinox, the date (or epoch) of observation must be specified when giving the position of an object in the sky.

49. Key Ideas
Timekeeping: Astronomers use several different means of keeping time.
Apparent solar time is based on the apparent motion of the Sun across the celestial sphere, which varies over the course of the year.
Mean solar time is based on the motion of an imaginary mean sun along the celestial equator, which produces a uniform mean solar day of 24 hours. Ordinary watches and clocks measure mean solar time.
Sidereal time is based on the apparent motion of the celestial sphere.
The Calendar: The tropical year is the period between two passages of the Sun across the vernal equinox. Leap year corrections are needed because the tropical year is not exactly 365 days. The sidereal year is the actual orbital period of the Earth.