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Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 1
Discovering the Night Sky
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
6.
7.
8.
Is the North Star—Polaris—the brightest star in
the night sky?
What do astronomers define as constellations?
What causes the seasons?
When is Earth closest to the Sun?
How many zodiac constellations are there?
Does the Moon have a dark side that we never
see from Earth?
Is the Moon ever visible during the daytime?
What causes lunar and solar eclipses?
In this chapter you will discover…
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how astronomers organize the night sky to help
them locate objects in it.
that Earth’s spin on its axis causes day and
night.
how the tilt of Earth’s axis of rotation and Earth’s
motion around the Sun combine to create the
seasons.
that the Moon’s orbit around Earth creates the
phases of the Moon and lunar and solar
eclipses.
how the year is defined and how the calendar
was developed.
The Night Sky Without
Light Pollution
… and With
The universe is huge, and the sizes and distances of
objects in the universe vary greatly. Therefore we use
scientific notation, which involves powers of ten notation to
describe numbers much smaller or much greater than 1.
Some common examples of powers of ten:
POWER
DECIMAL
NAME
METRIC PREFIX
103
1000
one thousand
Kilo
109
1,000,000,000
one billion
Giga
10-6
0.000001
one millionth
micro
THE SCALES OF
THE UNIVERSE
The range of objects
we study are from the
extremely small
subatomic particles,
to objects which are
gigantic, such as a
galaxy or the size of
the known universe
itself.
Each division up the
line indicates an
increase in size by
100,000.
What Have Astronomers Discovered in Our Universe?
In order to more easily locate objects in the sky, we divide
the sky into regions named after familiar patterns of stars
called constellations.
Ancient constellations were
imaginary pictures outlined
by familiar patterns of stars.
Modern astronomers divide the sky into
88 official constellations or regions of
space, many of which contain the
ancient star patterns.
Some Common Guides to Finding Constellations
Using the “Big Dipper” as a guide
The “Winter Triangle”
The “Summer Triangle”
Astronomers describe the universe as an imaginary sphere
surrounding the Earth on which all objects in the sky can be
located, called the CELESTIAL SPHERE.
 As viewed from Earth, the
celestial sphere appears to
rotate around two axis points,
the north and south celestial
poles, which are located directly
above the Earth’s poles.
 Between these is the celestial
equator, which divides the
celestial sphere into northern
and southern hemispheres.
 We define the position of an
object on the celestial sphere
using two coordinates, right
ascension and declination.
Cyclic motions of the Sun and stars in our sky are
due to motions of Earth.
1. ROTATION = the spin of Earth on its axis. It takes one
day for Earth to complete one rotation.
2. REVOLUTION = the movement of Earth in orbit around
the sun. It takes one year for Earth to complete one
revolution.
3. PRECESSION = the slow conical (top-like) motion of
Earth’s axis of rotation. It takes 26,000 years for Earth to
complete one cycle of precession.
Angular Distance (example from The Big Dipper)
The angular distance between the two “pointer stars” at
the front of the Big Dipper is about 5°.
Estimating Angles with the Human Hand
Various parts of the adult human hand extended to arm’s
length can be used to estimate angular distances and angular
sizes in the sky.
Circumpolar Star Trails
The stars near the poles of the celestial sphere (shown here)
move in trails that circle the pole and never set. They are
called circumpolar.
The apparent westward motion of the Sun, Moon,
and stars across our sky each day is caused by
Earth’s rotation.
At middle latitudes, we see the Sun, Moon, and many
of the stars first come into view moving upward, rising
at some point along the eastern horizon. Then, they
appear to arc across the sky. Finally, they disappear
somewhere along the western horizon.
We generalize this motion to make statements such as,
“The Sun rises in the east and sets in the west.”
We can see how different stars appear at different times of day by looking at the
position of the Sun against the backdrop of stars. The side of Earth facing the Sun
is experiencing “day,” while the side of Earth turned away from the Sun is
experiencing “night.”
SEPTEMBER
MARCH
Motion of Stars at the Poles
Because Earth rotates around its poles, stars seen from these locations
appear to move in huge, horizontal circles. This is the same effect you
would get by standing up in a room and spinning around; everything
would appear to move in circles around you. At the North Pole, stars
move left to right, while at the South Pole, they move right to left.
Rising and Setting of Stars at the Equator
Standing on the equator, you are perpendicular to the axis
around which Earth rotates. As seen from there, the stars
rise straight up on the eastern horizon and set straight down on
the western horizon.
Rising and Setting of Stars at Middle Northern Latitudes
Unlike the motion of the stars at the poles, the stars at all other
latitudes do change angle above the ground throughout the night.
This time-lapse photograph shows stars setting. The latitude
determines the angle at which the stars rise and set.
Earth also revolves around the Sun, which changes our view of the
stars.
From our perspective, the Sun
appears to move through the
stars along a special path
called the ecliptic.
From an outside view, we see
Earth revolve around the Sun.
We define the plane of Earth’s
orbit as the ecliptic plane.
Where is the Sun?
At noon, the Sun is in the constellation Leo.
In which constellation is it at 6 PM?
A. Leo
B. Taurus (3 constellations right of Leo)
C. Scorpius (3 constellations left of Leo)
D. Ophiuchus
Seasons are caused because Earth’s axis is tilted, and as
Earth revolves around the Sun, different parts of Earth
receive more direct sunlight (summer), whereas other parts
of Earth receive sunlight that is more spread out (winter).
The seasons we experience are linked to the motion of the Sun
along the celestial sphere.
The point of the Sun’s path
farthest north on the celestial
sphere is called the summer
solstice (JUN 21), whereas the
point of the ecliptic farthest
south is called the winter
solstice (DEC 21).
The two points on the ecliptic
where the Sun crosses the
celestial equator are called
equinoxes. During the vernal
equinox (MAR 21), the Sun is
moving north, while during the
autumnal equinox (SEPT 21),
the Sun is moving south.
Remember that the seasonal
names of the equinoxes and
the solstices refer to seasons in
the NORTHERN hemisphere.
The seasons occurring in the
SOUTHERN hemisphere are
exactly opposite.
The Sun’s Daily Path and the Energy It Deposits Here
(a) On the winter solstice―first day
of winter,―the Sun rises farthest
south of east, it is lowest in the
noontime sky, stays up the
shortest time, and its light and
heat are least intense (most
spread out) of any day of the
year in the northern hemisphere
(b) On the vernal equinox―first day
of spring―the Sun rises
precisely in the east and sets
precisely in the west. Its light
and heat have been growing
more intense, as shown by the
brighter oval of light than in (a)
(c) On the summer solstice―first day of summer― the Sun rises farthest north of east of any
day in the year, is highest in the sky at noontime, stays up the longest time, and its light and
heat are most intense of any day in the northern hemisphere.
(d) On the autumnal equinox, the same astronomical conditions exist as on the vernal equinox.
The Midnight Sun
This time-lapse photograph was taken on July 19, 1985, at 69°
north latitude in northeastern Alaska. At that latitude,
the Sun is above the horizon continuously from mid-May
until the end of July.
Where on Earth is this?
Is this location North or South of the Arctic Circle??
A. North
B. South
C. Precisely on the Arctic Circle
D. Cannot be determined
Different parts of the world experience different times of day
as Earth rotates.
TIME ZONES can be used to calculate the time of day in
any given part of the world.
Gravitational forces of the Sun and the Moon pulling on
Earth as it rotates cause Earth to undergo a top-like motion
called precession. Over a period of 26,000 years, Earth’s
rotation axis slowly moves in a circular motion.
Which is NOT a reason
for the seasons
A. Angle of sunlight at high noon
B. Distance of Sun from Earth
C. Number of hours of daylight
D. Tilt of the Earth’s Axis
This precession causes
the position of the North
Celestial Pole to slowly
change over time. Today,
the North Celestial Pole is
near the star Polaris,
which we call the “North
Star.” However, in 3000
BC, Thuban was close to
the North Celestial Pole
and in 14,000 AD, Vega
will be in this location.
Find your Birthday.
Find your Sign.
Is it what you
thought?
http://www.youtube.com/watch?v=HOPznRRiWOg
Another familiar cycle is the lunar cycle. When the Moon orbits
Earth, the amount of the side facing Earth that is lit changes,
creating the Moon’s phases. This phase cycle is called the
synodic period and is 29½ days long.
Test your understanding
What fraction of the Moon is illuminated
by the Sun at any point in time?
A. All of it
B. 50%
C. None
D. Depends on the phase of the Moon
One common
misconception is that
the Moon is only
visible at night.
However, the time of
day in which the
Moon is in our sky
varies depending on
its phase. This
picture clearly
displays the Moon,
visible during the day.
A synodic month is the time it takes for the Moon to orbit
Earth with respect to the Sun and is 29½ days long.
A sidereal month is the time it takes for the Moon to orbit
Earth with respect to the stars and is 27.3 days long.
The two
times are
different
because
Earth moves
in its orbit
around the
Sun as the
Moon moves
in its orbit
around Earth.
Test your Understanding
Which is longer, a sidereal month or
a synodic month?
A. Sidereal
B. Synodic
C. Both are equal
D. Depends on the month
During a new or full moon phase, when the Moon, Sun, and Earth
are aligned, the Moon may enter the shadow of Earth, or the
shadow of the Moon may reach Earth, creating eclipses.
However, these eclipses do not occur during every full or new
moon because the Moon’s orbit is tilted by 5 with respect to the
Earth-Sun (ecliptic) plane. “Ecliptic” has to do with “eclipses.”
Lunar Eclipse: Moon
passes through Earth’s
Shadow
PENUMBRAL = the Moon
appears dimmed.
PARTIAL = part of the
Moon enters Earth’s umbra
and is darkened.
TOTAL = all of the Moon
enters Earth’s umbra and
becomes a reddish color,
only lit from light bent by
Earth’s atmosphere.
During a total lunar eclipse, the Moon moves in and out of the
umbra of Earth’s shadow.
If you are located
where the umbra
of the Moon’s
shadow reaches,
you will see a total
solar eclipse,
during which the
entire disk of the
Sun is covered by
the Moon,
revealing the faint
solar corona
surrounding the
Sun.
Unlike lunar
eclipses, solar
eclipses occur
at specific
places on
Earth,
indicated by
the arrow.
Test your understanding
At what phase of the Moon can
a solar eclipse occur?
A. Full
B. New
C. First or Last Quarter
D. Any; only the Sun is involved in
a solar eclipse
Solar Eclipse – Actual Scale
Eclipse Paths for Total and Annular Eclipses 2001–2020
This map shows the eclipse paths for the 14 total solar and 13
annular eclipses that occur between 2001 and 2020. In each
eclipse, the Moon’s shadow travels along the eclipse path in a
generally eastward direction across Earth’s surface.
Sometimes eclipses
occur when the
Moon is too far away
from Earth to
completely cover the
Sun in our sky.
When this occurs,
the Moon appears in
the center and a thin
ring, or “annulus,” of
light surrounds it.
These are called
annular eclipses.
Test your understanding
About how often does a lunar eclipse occur?
A. Every month
B. Every 6 months
C. Once per year
D. Once in a blue moon
Summary of Key Ideas
Sizes in Astronomy
Astronomy examines objects that range in
size from the parts of an atom (1015 m)
to the size of the observable universe
(1026 m).
 Scientific notation is a convenient
shorthand for writing very large and very
small numbers.
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Patterns of Stars
The surface of the celestial sphere is
divided into 88 unequal areas called
constellations.
 The boundaries of the constellations run
along lines of constant right ascension or
declination.
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Earthly Cycles
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The celestial sphere appears to revolve around
Earth once in each day-night cycle. In fact, it is
the Earth’s rotation that causes this apparent
motion.
The poles and equator of the celestial sphere
are determined by extending the axis of rotation
and the equatorial plane of Earth out onto the
celestial sphere.
Earthly Cycles
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Earth’s axis of rotation is tilted at an angle of
23½° from a line perpendicular to the plane of
Earth’s orbit (the plane of the ecliptic). This tilt
causes the seasons.
Equinoxes and solstices are significant points
along Earth’s orbit that are determined by the
relationship between the Sun’s path on the
celestial sphere (the ecliptic) and the celestial
equator.
Earthly Cycles
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Earth’s axis of rotation slowly changes direction
relative to the stars over thousands of years, a
phenomenon called precession. Precession is
caused by the gravitational pull of the Sun and
Moon on Earth’s equatorial bulge.
The length of the day is based upon Earth’s
rotation rate and the average motion of Earth
around the Sun. These effects combine to
produce the 24-hour day upon which our clocks
are based.
Earthly Cycles
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The phases of the Moon are caused by the
relative positions of Earth, the Moon, and the
Sun. The Moon completes one cycle of phases
in a synodic month, which averages 29½ days.
The Moon completes one orbit around Earth
with respect to the stars in a sidereal month,
which averages 27.3 days.
Eclipses
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The shadow of an object has two parts: the
umbra, where direct light from the source is
completely blocked; and the penumbra, where
the light source is only partially obscured.
A lunar eclipse occurs when the Moon moves
through Earth’s shadow. During a lunar eclipse,
the Sun, Earth, and the Moon are in alignment
with Earth between the Sun and the Moon, and
the Moon is in the plane of the ecliptic.
Eclipses
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A solar eclipse occurs when a strip of Earth
passes through the Moon’s shadow. During a
solar eclipse, the Sun, Earth, and the Moon are
in alignment with the Moon between Earth and
the Sun, and the Moon is in the plane of the
ecliptic.
Depending on the relative positions of the Sun,
Moon, and Earth, lunar eclipses may be
penumbral, partial, or total, and solar eclipses
may be annular, partial, or total.
Key Terms
angle
angular diameter
(angular size)
annular eclipse
arc angle
autumnal equinox
celestial equator
celestial sphere
circumpolar star
constellation
declination
degree
diurnal motion
eclipse path
ecliptic
equinox
gravitation
line of nodes
lunar eclipse
lunar phase
north celestial pole
partial eclipse
penumbra
penumbral eclipse
precession
precession of the
equinoxes
revolution
right ascension
rotation
scientific notation
sidereal month
sidereal period
solar corona
solar day
solar eclipse
south celestial pole
summer solstice
synodic month
terminator
time zone
total eclipse
umbra
vernal equinox
winter solstice
zenith
zodiac
WHAT DID YOU THINK?
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Is the North Star—Polaris—the brightest
star in the night sky?
No. Polaris is a star of medium brightness compared
with other stars visible to the naked eye.
WHAT DID YOU THINK?
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What do astronomers define as constellations?
Astronomers sometimes use the common definition of a
constellation as a pattern of stars. Formally, however, a
constellation is an entire area of the celestial sphere and
all the stars and other objects in it. Viewed from Earth,
the entire sky is covered by 88 different-sized
constellations. If there is any room for confusion,
astronomers refer to the patterns as asterisms.
WHAT DID YOU THINK?

What causes the seasons?
The tilt of Earth’s rotation axis with respect to the ecliptic
causes the seasons. They are not caused by the
changing distance from Earth to the Sun that results
from the shape of Earth’s orbit.
WHAT DID YOU THINK?
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When is Earth closest to the Sun?
On or around January 3 of each year.
WHAT DID YOU THINK?
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How many zodiac constellations are
there?
There are 13 zodiac constellations, the least-known one
being Ophiuchus.
WHAT DID YOU THINK?

Does the Moon have a dark side that we
never see from Earth?
Half of the Moon is always dark. Whenever we see less
than a full Moon, we are seeing part of the Moon’s dark
side. So, the dark side of the Moon is not the same as
the far side of the Moon, which we never see from Earth.
WHAT DID YOU THINK?

Is the Moon ever visible during the
daytime?
The Moon is visible at some time during daylight hours
almost every day of the year. Different phases are
visible during different times of the day.
WHAT DID YOU THINK?

What causes lunar and solar eclipses?
When the Moon is crossing the ecliptic in the full
or new phase, the shadows of Earth or the
Moon, respectively, then fall on the Moon or
Earth. These shadows on the respective
surfaces are eclipses.