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PA RT I
UNIT
6
The Year
UNIT OUTLINE
6.1 Annual Motion of the Sun
6.2 The Ecliptic and the Zodiac
6.3 The Seasons
6.4 The Ecliptic’s Tilt
6.5 Solstices and Equinoxes
6.6 Precession
For people long ago, observations of the heavens had more than just curiosity value.
Because so many astronomical phenomena are cyclic—that is, they repeat at a regular interval—they can serve as timekeepers. The most basic of these cycles is the
rhythm of day and night, as the celestial sphere appears to rotate about the Earth,
as described in Unit 5. This cycle is not completely uniform—days and nights alternately lengthen and shorten over the year. This slow rhythm is tied to a gradual shift
of the Sun’s apparent position relative to the “fixed stars” on the celestial sphere.
The shifting position of the Sun also leads to seasonal changes in the weather and
temperature.
The motion of the Sun against the celestial sphere provides a means for tracking
these changes predictably. We might imagine ancient peoples asking, When is it time
to plant crops? Or move to the next location to ensure a ready supply of water? Or
prepare for winter? Some societies built monumental structures to mark the changing
position of the Sun. An example is the Mayan pyramid at Chichén Itzá. The pyramid is
designed so that on the first day of spring or fall, shadow play creates the appearance of
a serpent slithering down the staircase (Figure 6.1).
We will use much of the terminology for positions in the sky from Unit 5. Review
that Unit if terms like celestial sphere, constellation, zenith, or declination are unfamiliar.
FIGURE 6.1
On the equinoxes, the Sun casts a shadow
that resembles a serpent slithering down
the steps of the Mayan pyramid (left side
in photograph) at Chichén Itzá. The head
of the serpent is depicted in a sculpture at
the base of the stairs.
Serpent
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Unit 6 The Year
43
66.1
.1 ANNUAL MOTION OF THE SUN
A N I M AT I O N
Constellations by season
Ancient Egyptians watched for the first
signs of the “dog star” Sirius rising just
before dawn. They used this to predict
when the Nile would flood, and it also
marked the start of summer. This is
the origin of the phrase “dog days” of
summer.
FIGURE 6.2
The changing appearance of the evening sky
over several months. In early June, the Sun
appears to lie in the constellation Taurus
and the constellation Gemini is visible in the
west just after sunset. A month later the Sun
is in Gemini, and a month after that it is in
Cancer.
As the Earth orbits the Sun, the stars that are visible each night change. The shift is
so slow that it is difficult to appreciate from one night to the next, but in the span
of a month the changes become obvious. Because these movements repeat after
the Earth completes one orbit around the Sun—a year—they are called annual
motions.
If you watch the sky each evening over several months, you will discover that
new constellations appear in the eastern sky and old ones disappear from the
western sky. For example, across most of North America, Europe, and Asia on an
early July evening, the constellation Scorpius will be visible in the southern half
of the sky. However, on December evenings the brilliant constellation Orion, the
hunter, is visible instead.
The realization that different stars are visible at different times of the year was
extremely important to early peoples because it provided a way to predict the
changing of the seasons. A farmer might be tricked into planting too early by
a short spell of warmer-than-normal weather in the late winter, but experience
would teach that each year the stars could reliably predict when spring was arriving. For example, if you live in the Northern Hemisphere and the early evening
sky shows Leo in the south instead of Scorpius or Orion, it will soon be time to
plant. Even in semitropical climates, where seasonal temperature differences are
much smaller, planting with accuracy is also necessary to avoid crop damage due
to annual flooding or dry periods.
The changing of constellations throughout the year is caused by the Earth’s
motion around the Sun. As the Earth revolves (orbits) around the Sun, the Sun’s
glare blocks our view of the part of the celestial sphere that lies in the direction of
the Sun, making the stars that lie beyond the Sun invisible, as Figure 6.2 shows.
For example, in early June, a line from the Earth to the Sun points toward the
constellation Taurus, and its stars are completely lost in the Sun’s glare. In the dusk
after sunset, however, it is possible to see the neighboring constellation, Gemini, just
above the western horizon. A month later, from the Earth’s new position, the Sun
lies in the direction of Gemini, causing this constellation to disappear in the Sun’s
glare. Looking to the west just after sunset, it is possible to see, just barely, the dim
stars of the constellation Cancer above the horizon. A month after that, the Sun is in
Cancer, and the constellation Leo is visible just above the horizon after sunset.
Month by month, the Sun hides one constellation after another. It is like sitting
around a campfire and not being able to see the faces of the people on the far side.
But if we get up and walk around the fire, we can see faces that were previously
hidden. Similarly, the Earth’s motion allows us to see stars previously hidden in the
Sun’s glare.
Evening twilight on June 1
Cancer
Evening twilight on July 1
Evening twilight on August 1
Leo
Gemini
Leo
Cancer
Taurus
Gemini
Cancer
Sun
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Sun
Sun
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Part One The Cosmic Landscape
Gemini
Taurus
Cancer
Aries
Leo
Virgo
Apparent position
of Sun in early August
Earth
Scorpius
Pisces
August
June
Libra
Apparent position
of Sun in early June
Aquarius
Sagittarius
Capricornus
FIGURE 6.3
As the Earth orbits the Sun, the Sun appears to move around the celestial sphere through the background
stars. The Sun’s path is called the ecliptic. The Sun appears to lie in Taurus in June, in Cancer during August,
in Virgo during October, and so forth. Note that the ecliptic is the extension of the Earth’s orbital plane out
to the celestial sphere. (Sizes and distances of objects are not to scale.)
6.2 THE ECLIPTIC AND THE ZODIAC
6.
The name ecliptic comes from the fact
that only when the new or full moon
crosses this line can an eclipse occur. (See
Unit 8.)
Concept Question 1
Can you think of an astronomical
reason why the zodiac may have been
divided into 12 signs rather than 13—
or some other number entirely?
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If we could mark on the celestial sphere the path traced by the Sun as it moves
through the constellations, we would see that it moves around the celestial sphere,
as illustrated in Figure 6.3. Astronomers call the path that the Sun traces across
the celestial sphere the ecliptic. You can see in Figure 6.3 that the ecliptic is the
extension of the Earth’s orbit onto the celestial sphere, just as the celestial equator
is the extension of the Earth’s equator onto the celestial sphere.
The ecliptic passes through a dozen constellations, which are collectively called
the zodiac. The word zodiac comes from Greek roots meaning “animals” (as in
zoology) and “circle” (as in diameter). That is, zodiac refers to a circle of animals,
which is what most of its constellations represent: Aries (ram), Taurus (bull),
Gemini (twins), Cancer (crab), Leo (lion), Virgo (virgin), Libra (scale), Scorpius
(scorpion), Sagittarius (archer), Capricornus (goat), Aquarius (water bearer),
and Pisces (fish). Actually, the ecliptic passes through a thirteenth constellation,
Ophiuchus (serpent holder), during the first half of December (between Scorpius
and Sagittarius); but this constellation was not included in the zodiac, probably
because of some uncertainty in ancient times about the precise path of the Sun and
some vagueness about the boundaries of the constellations.
The names of some of the constellations of the zodiac may have originated in
the seasons when the Sun passed through them. For example, rainy weather in
much of Europe during winter was foretold by the Sun’s appearance in the constellation Aquarius (the water bearer). Likewise, the harvest time was indicated
by the Sun’s appearance in Virgo (the virgin), a constellation often depicted as the
goddess Proserpine, holding a sheaf of grain.
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Unit 6 The Year
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6.3 THE SEASONS
6.
Seasons are not caused by the Earth’s
distance from the Sun.
The constancy of Earth’s tilt is a consequence of the conservation of angular
momentum. (See Unit 20.)
Concept Question 2
When it is summer in the Northern
Hemisphere, what is the season in
the Southern Hemisphere? What
does this demonstrate about possible
causes of the seasons?
Many people believe that we have seasons because the Earth’s distance from the Sun
changes. They assume that summer occurs when we are closest to the Sun and winter
when we are farthest away. It turns out, however, that the Earth is several million
kilometers closer to the Sun in early January, when the Northern Hemisphere is
coldest, than it is in July. Clearly then, seasons must have some other cause.
To see what causes our seasons, we need to look at how our planet is oriented
in space. As the Earth orbits the Sun, our planet also spins or rotates. That spin is
around a line—the rotation axis—which we might imagine running through the
Earth from its North Pole to its South Pole. The Earth’s rotation axis is not perpendicular to its orbit around the Sun. Rather, it is tipped by 23.5° from the vertical, as
shown in Figure 6.4A.
As our planet moves along its orbit, its rotation axis maintains nearly exactly
the same tilt and direction, as illustrated in Figure 6.4B. That is, the Earth behaves
much like a giant gyroscope or spinning top. In fact, every spinning object shows
this tendency to maintain its orientation. This is why you need to put spin on a
frisbee to keep it from flipping over when you throw it, and it is why a quarterback
puts “spin” on a football.
The constancy of our planet’s tilt as we move around the Sun causes sunlight
to fall more directly on the Northern Hemisphere for half of the year and more
directly on the Southern Hemisphere for the other half of the year, as illustrated
in Figure 6.4B. This in turn changes the amount of heat each hemisphere receives
from the Sun.
A surface directly facing the Sun receives the most concentrated sunlight. If
the surface receives the sunlight at an angle, the light is spread out over a larger
area and therefore is less concentrated, as illustrated in Figure 6.5. An astronomer
might express this in terms of the energy received per square meter. A portion
of the Earth directly facing the Sun receives about 1300 watts on every square
FIGURE 6.4
(A) The Earth’s rotation axis is tilted 23.5°
to its orbit around the Sun. (B) The Earth’s
rotation axis keeps nearly the same tilt and
direction as it revolves (orbits) around the
Sun. As a result, sunlight falls more directly
on the Northern Hemisphere during half of
the year and on the Southern Hemisphere
during the other half of the year. (Sizes and
distances are not to scale.)
North Pole
Equator
A
A N I M AT I O N
March 20
North
Pole
Earth’s rotation axis
North
Pole
INTERACTIVE
Seasons
June 21
December 21
September 22
B
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Part One The Cosmic Landscape
FIGURE 6.5
The portion of the Earth’s surface directly
facing the Sun receives more concentrated
light (and thus more heat) than other parts
of the Earth’s surface. The same size “beam”
of sunlight (carrying the same amount of
energy) spreads out over a larger area where
the surface is “tilted.”
North Pole
Summer
A
Equ
ator
Winter
Full beam falls on A.
Sunlight
A
Only portion of beam falls on A.
The “tilted” surface receives
less light and heats less.
On June 21 at latitude 23.5°N, the Sun
is straight overhead at noon—0° from
the zenith. Because the relative angle
of the Earth’s surface depends on the
difference of latitudes, at 40°N the
Sun is 16.5° (= 40° – 23.5°) from the
zenith. On December 21, when the
Sun is overhead at 23.5°S, the
difference in latitudes places the Sun
63.5° from the zenith.
Seasonal differences between the north
and south are not caused by one hemisphere being closer to the Sun than the
other hemisphere.
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meter. Where the surface is tilted at an angle to the Sun’s light, the same 1300
watts are spread out over a larger area on the ground, and each square meter of the
Earth’s surface receives only a fraction as much energy. You take advantage of this
effect instinctively when you warm your hands at a fire by holding your palms flat
toward the fire. You also may have experienced the high temperature of pavement
or a beach around noon, when the Sun is shining most directly on it, whereas the
same surface will be cooler in the late afternoon, even though it is not shaded,
when sunlight strikes it more obliquely.
Because of the 23.5° tilt of the Earth’s axis, when the Earth is at the point of
its orbit where the North Pole is most tipped toward the Sun, the Sun will pass
straight overhead for someone at a latitude of 23.5°N (Figure 6.5). This occurs on
about June 21 each year. Half a year later, the same is true at a latitude of 23.5°S.
At latitudes between these, the Sun shines straight down at noon at other times
of year.
At regions farther north or south, the Sun can never be straight overhead, and
the different angles of the Sun as the Earth orbits it can lead to strong differences
in heating during the year. Figure 6.6 illustrates the difference between summer
and winter at a latitude of 40°N. In late June the Sun gets most nearly overhead.
Six months later the Sun’s light strikes the ground much more obliquely. The same
size bundle of sunlight is spread out over a much larger area in winter, as shown in
the bottom panel of Figure 6.6. The direct sunlight produces the strongest heating,
whereas the large angle in December produces the least. From Figure 6.6 you can
also see that this makes the seasons reversed between the Northern and Southern
Hemispheres; when it is summer in one, it is winter in the other.
An important point here is not to confuse the “directness” of the Sun’s light with
one hemisphere being closer to the Sun. It is true that the Northern Hemisphere
of the Earth is a few thousand kilometers closer to the Sun than the Southern
Hemisphere during the northern summer. However, the effect of this difference in
distance is tiny. Compared to the millions of kilometers of distance to the Sun, this
difference in distance between the two hemispheres does not produce a change of
even one-tenth of a degree in the temperature. By contrast, the differing angle at
which the Sun shines on higher latitudes during the year changes the solar energy
absorbed by the ground by a factor of two or more.
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Unit 6 The Year
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Earth‘s orbit
Earth on
June 21
Earth on
December 21
Sun
North Pole
Large angle between
overhead and Sun
Small angle between
overhead and Sun
Sunlight
Eq
North Pole
Sunlight
ua
tor
Equa
tor
December 21
June 21
Point exactly
overhead
—the Zenith
Sun in summer
—high in sky
Summer
Sun
a
sti
le
or
at
qu
y
sk
in
th ols
pa r s
n’s me
Su um
s
on
le
Ce
y
sk
in e
th tic
pa sols
n’s r
Su inte
w
on
S
Smaller summer area
more heat for a
given piece of ground
warmer
Winter
Sun
Sun in winter
—lower
in sky
tic
E
e
N
Larger winter area
less heat for a given piece of ground
Cooler
The summer and winter beams carry the same amount of energy,
but spread that energy over very different amounts of ground.
FIGURE 6.6
Why the Sun at noon is high in the sky in summer and low in the sky in winter. On June 21, from a latitude of 40°N, the noontime Sun is at an angle
of just 16.5° (= 40° – 23.5°) from the zenith. On December 21, the Sun is at an angle of 63.5° (= 40° + 23.5°) from the zenith.
6.4 THE ECLIPTIC’S TILT
6.
The tilt of the Earth’s rotation axis not only causes heating differences, it makes
the Sun appear to move north and south on the celestial sphere. As discussed in
Section 6.3, on June 21 when the Northern Hemisphere is tipped most toward the
Sun, it passes straight overhead for someone at latitude 23.5°N. This means that
the Sun lies 23.5° north of the celestial equator. In other words its declination is
+23.5° (see Unit 5.5). Likewise, on December 21 the Sun’s declination is –23.5°.
The Sun lies north of the celestial equator for half of the year and south of the
celestial equator for the other half of the year.
Another way of describing the Sun’s annual motion is that the Sun’s path—the
ecliptic—must cross the celestial equator, and therefore the ecliptic must be tilted
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Part One The Cosmic Landscape
Earth‘s position in its orbit
at different times of year
Sun‘s position on celestial
sphere at start of each season
North
Pole
Earth
To Sun
North celestial pole
June 21
Sun on
September 22
—on Cel. Eq.
To Sun
North Pole
September 22
North
Pole
To Sun
December 21
North Pole
Sun on
December 21
23.5º South
of Cel. Eq.
Sun on June 21
23.5º North
of Cel. Eq.
Ecliptic
C
Sun on March 20 elestia
l Equ a t o r
—on Cel. Eq.
North Pole
To Sun
March 20
FIGURE 6.7
As the Earth orbits the Sun, the Sun’s position with respect to the celestial equator changes. The Sun reaches 23.5° north of the celestial equator
on June 21 but 23.5° south of the celestial equator on December 21. The Sun crosses the celestial equator on about March 20 and September 22
each year. The times when the Sun reaches its extremes are known as the solstices; the times when it crosses the celestial equator are the equinoxes. (The dates can sometimes vary because of the extra day inserted in leap years.)
The Sun’s position in the celestial sphere
is shown in star charts by the curving line
of the ecliptic—see the foldout star chart
in the back of the book.
A N I M AT I O N
The Sun’s seasonal motion
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with respect to that line, as the sequence of sketches in Figure 6.7 shows. Because
the Sun’s declination changes very slowly, its rising and setting is similar to that of
any other star at the same declination. And just as the length of time a star spends
above the horizon depends on its declination, the same is true of the Sun.
The seasonal shifts of the Sun also define three kinds of regions on the Earth:
the polar regions, the tropics, and, lying in between these, the temperate latitudes.
The polar regions mark the latitudes where the Sun does not rise during some
portion of the year. This occurs within 23.5° of the poles—north of 66.5°N, the
Arctic Circle, and south of 66.5°S, the Antarctic Circle. The tropics lie between
latitudes 23.5°S and 23.5°N, where the Sun passes directly overhead at some time
during the year. The northern limit of tropical latitudes, 23.5°N, is called the
Tropic of Cancer because the Sun reached its point farthest north when it was in
the constellation Cancer at the time this term was defined. The southern limit is
the called the Tropic of Capricorn after the constellation where the Sun was at its
farthest south position. The Sun is no longer in these constellations at its northern
and southern extremes because of precession (Section 6.5).
The length of time the Sun is above the horizon is another critical heating factor
during different seasons. When the Northern Hemisphere is tilted toward the Sun
(between March 20 and September 22), the North Pole remains in sunlight continuously for six months. When the Sun is 23.5° north of the celestial equator on June
21, it is 66.5° from the north celestial pole. Therefore it becomes circumpolar for
anyone north of latitude 66.5°N. For these arctic latitudes, you can therefore see
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Unit 6 The Year
Concept Question 3
If the shape of the Earth’s orbit were
unaltered but its rotation axis were
shifted so that it had no tilt with
respect to the orbit, how would seasons be affected?
49
the “midnight sun.” Half a year later, on December 21, the Sun is 23.5° south of the
celestial equator, so the Sun does not rise in those northern arctic regions.
At northern temperate latitudes, when the Sun is north of the celestial equator,
there is sunlight for more than 12 hours each day. The days are longer the higher
your latitude and the farther north of the celestial equator the Sun is. The situation reverses during the other half of the year and in the Southern Hemisphere.
(Differences in the length of the day are examined in more detail in Unit 7.2.) Thus,
because of the Earth’s tilted axis, in summer we receive more hours of sunlight and
the Sun’s light strikes the surface more directly.
6.5 SOLSTICES AND EQUINOXES
6.
The tilt of the ecliptic with respect to the celestial equator means that during the year,
the points on the horizon where we see the Sun rise and set are not due east and west,
except when the Sun is crossing the celestial equator. This occurs on two days of
the year called the equinoxes, from the Latin for “equal night,” so named because the
length of the night is approximately equal to the length of the day on those dates. The
vernal equinox occurs near March 20, when the Sun is moving from the Southern
Hemisphere of the celestial sphere into the Northern Hemisphere. Six months later
the autumnal equinox occurs near September 22 as the Sun crosses the celestial
equator on its way south. In the Northern Hemisphere, these dates mark the first
day of spring and fall, respectively, but in the Southern Hemisphere this is reversed.
On every other day of the year the Sun rises either north or south of due east in
a regular, predictable fashion as illustrated in Figure 6.8A. From the vernal equinox to the autumnal equinox (during the Northern Hemisphere spring and summer, and the Southern Hemisphere fall and winter), the Sun rises in the northeast
and sets in the northwest. During the rest of the year, the Sun rises in the southeast
and sets in the southwest.
Ancient peoples all over the world used the northward and southward journeys
of the Sun to track the seasons. They built a variety of structures to detect the
Sunrise direction
June 21
(Summer solstice)
March 20, September 22
(Equinoxes)
December 21
(Winter solstice)
December solstice
Equinox
June solstice
East
South
North
Solar
observatory
West
A
B
FIGURE 6.8
(A) The direction of the rising and setting Sun changes throughout the year. At the equinoxes, the rising and setting points are due east and due
west. The sunrise direction shifts slowly north from March until the summer solstice, after which it shifts back, reaching due east at the autumnal equinox. The sunrise direction continues moving south until the winter solstice, then reverses direction again back to the north. (B) The
oldest known astronomical observatory in the Americas is found in Chankillo, Peru. This ancient observatory marked the shifting position of
sunrise with a series of 13 towers along a ridge built about 2300 years ago.
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Part One The Cosmic Landscape
North
To summer sunrise
B
A
FIGURE 6.9
Stonehenge. (A) Massive stones were erected
by ancient Britons more than 4000 years
ago to mark the changing position of the
Sun. (B) Diagram showing how an observer
in the stone circle would see the rising Sun
framed by a pair of standing stones on the
summer solstice.
The exact dates of the equinoxes and
solstices vary slightly from year to year,
mainly because of differences in the calendar due to leap years, but also because
of slight variations in the Earth’s orbit.
Concept Question 4
During the course of the year, the
sunset (and sunrise) position shifts.
How should the amount of the shift
depend on latitude?
limits of the Sun’s motion. For example, Figure 6.8B shows an ancient observatory in Peru designed to track the Sun’s progress throughout the year. An even
older structure built to mark these journeys is Stonehenge, the ancient stone circle
in England (Figure 6.9A). Although its exact use in ancient times is lost to us, it
appears that it was laid out so that seasonal changes in the Sun’s position could be
observed by noting through which stone arches the Sun rose or set. For example,
when the Sun reaches its farthest point north, an observer standing at the center of
this immense circle of vertical stones would see the rising Sun framed by standing
stones outside the main circle (Figure 6.9B).
The Sun reaches its farthest point north or south on the celestial sphere, 23.5°
from the celestial equator, midway between the equinoxes. At these times of year,
the Sun pauses in its north–south motion and changes direction. Accordingly,
these times are called the solstices, meaning the Sun (sol ) stops its northward or
southward motion and begins to reverse direction. The winter solstice is particularly celebrated by many northern cultures with holidays and festivals, often symbolizing rebirth as this date marks the beginning of longer days.
The solstices occur close to June 21 and December 21. In terms of celestial
coordinates (Unit 5.5), the Sun is at a declination of 0° on the equinoxes, while it is
at plus or minus 23.5° on June 21 and December 21, respectively. The Sun’s position at the moment it crosses the celestial equator on the vernal (March) equinox
is used to define 0 hours (or 0h) for the right ascension system. On June 21 the Sun
moves to a right ascension of about 6 hours, then 12 hours on September 22, then
18 hours on December 21, before returning to 0 hours of right ascension a year
later (see Figure 6.7).
The seasons “officially” begin on the solstices and equinoxes, with northern
spring running from the vernal equinox to the solstice in June. Even though the
longest day is on the first day of summer, the hottest period of the year occurs
roughly six weeks later, as shown for four cities in Table 6.1. The delay, known as
the lag of the seasons, results from the oceans and land being slow to warm up in
summer. Similarly, there is about a six-week lag after the shortest day of the year
until the coldest period of the year.
TABLE 6.1 Monthly Average Temperatures in Four Cities (in Degrees Celsius)
City
Latitude
Buenos Aires
Boston
Rome
Sydney
34°S
42°N
42°N
34°S
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Jan.
Feb.
Mar.
April
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
23.5
−2.2
7.1
22.1
22.7
−1.6
8.2
22.1
20.6
2.5
10.5
21.0
16.7
8.2
13.7
18.4
13.3
14.1
17.8
15.3
10.4
19.4
21.7
12.9
10.0
22.5
24.4
12.0
11.1
21.5
24.1
13.2
13.2
17.3
20.9
15.3
16.0
11.5
16.5
17.7
19.3
5.5
11.7
19.5
22.0
0.0
8.3
21.2
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Unit 6 The Year
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6.6 PRECESSION
6.
Precession is caused primarily by the
Moon’s gravitational pull trying to
“straighten out” the direction of the
Earth’s spin.
You may be amused to learn that horoscopes listed in newspapers are also
incorrect by about 1 month. If you
thought your “sign” was Cancer, then you
are probably “a Gemini”; if Gemini, then
Taurus; etc.
FIGURE 6.10
Precession makes the Earth’s rotation axis
swing around, slowly tracing out a circle in
the sky, somewhat like a spinning top.
Spinning top
If you watch a spinning top, you will see that it “wobbles,” often more extremely as
it slows down. That it wobbles is another way of saying that its rotation axis slowly
shifts direction (Figure 6.8). The spinning Earth wobbles too, in a motion called
precession. Precession occurs very slowly for the Earth. A single “wobble” takes
about 26,000 years, but it has both interesting and important consequences.
Currently the Earth’s North Pole points very close to the star Polaris. But this
is only temporary. When the Egyptian pyramids were built 4000 years ago, the
“North Star” was Thuban (meaning “the star”) in the constellation Draco (Figure
6.10; also see Looking Up #1 at the front of the book). In the future the axis will
continue shifting direction past Polaris, and it will not point close to any bright
stars for thousands of years. In about 7000 years the south celestial pole will be
very close to a star slightly brighter than Polaris in the constellation Vela. In that
future time there will be a “South Star.” In 12,000 years the rotation axis will have
shifted so that the north celestial pole points fairly close to the bright star Vega.
Then we will have a new, much brighter “North Star.”
The changing direction of the Earth’s pole does not alter the Earth’s orbit, so
the ecliptic and the constellations of the zodiac remain the same. However, it does
change which constellation the Sun is in on the equinoxes and solstices. Several
thousand years ago the Sun was in Cancer on the first day of summer, giving
us the name “Tropic of Cancer” for the northernmost latitude where the Sun is
ever directly overhead. Today it is in Gemini on the first day of summer. Because
astronomers base the zero point of right ascension on the Sun’s position at the
vernal equinox (Section 6.5), celestial coordinates change a little bit every year.
Precession also slowly alters Earth’s climate. At this time we are closest to the Sun
during the northern winter. In about 13,000 years we will be farthest from the Sun
during the northern winter. This will make seasons in the Northern Hemisphere
more severe at that time. Precession is suspected to be one of the components that
affect long-term changes in climate, which may have triggered past ice ages.
Deneb
Spinning and
precessing top
CYGNUS
AD 8000
CEPHEUS
Alderamin
Toward
Vega
Earth’s rotation axis slowly
precesses to new direction.
Toward
Polaris
LYRA
Vega
AD 14000
Eltanin
North Pole in
A.D. 14,000
0
North Pole
now
URSA
MINOR
Polaris
Today
Rastaban
DRACO
Kocab
HERCULES
Thuban
4000 BC
sch12133_ch06.indd 51
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Confirming Pages
52
Part One The Cosmic Landscape
KEY POINTS
• The Sun appears to shift position among the stars during the
course of the year as the Earth orbits it.
• The Sun’s path on the celestial sphere is called the ecliptic, and the
12 constellations it moves through are known as the zodiac.
• The Earth’s spin axis is tilted 23.5° relative to its orbit around the
Sun, so the ecliptic is tilted relative to the celestial equator.
• Because of this tilt, the Northern Hemisphere receives sunlight at a
more direct angle and for a longer period of time each day during
half of the year. In the other six months the opposite is true.
• The changing amount of solar heating causes the seasons, and they
are opposite in the Southern Hemisphere.
• The changing direction of sunrise and sunset throughout the year
has been observed by peoples back to ancient times.
• The direction of the spin of the Earth’s axis has been found to change
very slowly over a 26,000-year period, an effect called precession.
KEY TERMS
Antarctic Circle, 48
rotation axis, 45
Arctic Circle, 48
solstice, 50
autumnal equinox, 49
Tropic of Cancer, 48
ecliptic, 44
Tropic of Capricorn, 48
equinox, 49
vernal equinox, 49
precession, 51
year, 43
revolve, 43
zodiac, 44
rotate, 45
CONCEPT QUESTIONS
Concept Questions on the following topics are located in the margins.
They invite thinking and discussion beyond the text.
1. Why the zodiac is divided into 12 parts. (p. 44)
2. Differences in northern and southern seasons. (p. 45)
3. Seasons if the Earth’s axis were not tilted. (p. 49)
4. The positions of sunset at different latitudes. (p. 50)
REVIEW QUESTIONS
5. What is the ecliptic? What is the zodiac?
6. What causes the seasons?
7. When it is winter in Australia, what season is it in the United
States?
8. Where is the Sun located on the celestial sphere during the
equinoxes and solstices?
9. Why is the summer solstice not the hottest day of the year?
10. How does the Sun’s position on the horizon at sunset change
through the course of the year?
sch12133_ch06.indd 52
11. What effect does precession of the Earth’s rotation axis have on
the Sun’s location in the zodiac?
QUANTITATIVE PROBLEMS
12. Suppose the Earth’s axis were tilted by 10° instead of 23.5°.
Where would the tropics and arctic regions be? How would
seasons be different?
13. Suppose the Earth’s axis were tilted by 50° instead of 23.5°.
Where would the tropics and arctic regions be? How would
seasons be different?
14. Suppose the Earth’s axis were tilted by 90° instead of 23.5°.
Where would the tropics and arctic regions be? How would the
seasons be different?
15. Describe the motion you would see on the solstices and the
equinoxes if you were observing the Sun from the Arctic Circle,
at a latitude of 66.5°N.
16. If you wished to observe a star with a right ascension of 12h,
what would be the best time of year to observe it? What would
be the best time to observe a star with a right ascension of 6h?
(Also see Unit 5.5.)
TEST YOURSELF
17. From what location on Earth will the Sun always rise due east
and set due west?
a. The North Pole
d. A latitude of 23.5°N
b. The South Pole
e. Nowhere
c. The equator
18. On what day(s) of the year are nights longest at the equator?
a. They are the same length throughout the year there.
b. The solstices
c. The equinoxes
d. Around June 21
e. Around December 21
19. During winter in either hemisphere the temperature is lower
because the Sun
a. stops moving.
b. is farthest south.
c. doesn’t rise as high in the sky.
d. has a lower temperature.
e. is farther away due to the Earth’s eccentric orbit.
20. For someone in the Southern Hemisphere, which of the following is correct?
a. The Sun rises in the west.
b. The Sun rises in the southeast on December 21.
c. Summer occurs when the Sun is rising lowest in the sky.
d. The Sun is in the opposite sign of the zodiac than for an
observer in the Northern Hemisphere.
e. All of the above.
21. What is the slow shift of the position of the celestial poles?
a. solstice
d. equinox
b. ecliptic
e. year
c. precession
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