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Astronomy
Rotation is the spinning of Earth on its axis, which causes day and night. It takes Earth
24 hours (one day) to complete one rotation.
Revolution is Earth's motkm around the sun; it takes Earth 365.25 days (one year) to
complete one revolution around the sun.
Earth's axis is tilted 23.5 0 to the plane of its orbit. The northern end of Earth's axis (the
North Pole) points toward the North Star. The seasons are caused by the tilt of Earth's
axis as it revolves around the sun. This tilt causes the sun's vertical rays to strike Earth
.
farthest north on June 21and farthest south on December 21.
Our solar system consists of the sun, the nine planets, satellites, asteroids, meteoroids,
and comets. The sun is at the center; its gravity keeps all other members of the solar
system in orbits around the sun.
The universe contains trillions of stars mostdustered together in galaxies. Yet most of'
the universe is considered to be empty space. Our sun is an average star, one of 800
billion stars dustered in a spiral galaxy called the Milky Way.
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Earlh's Rotation
People once believed that Earth stood still while the sun, moon, stars, and
planets revolved around Earth each day. This seems reasonable, since we
do not feel Earth moving, and the sun does appear to move across the sky
during the day while the moon and stars appear to move at night. However,
today we know that these apparent motions of the sun, moon, and stars are
actually caused by Earth's rotation.
Earth spins like a top. This spinning motion is called rotation. Earth
rotates from west to east, or, put another way, if we could look down at the
North Pole from space, we would see Earth spinning in a counterclockwise
direction.
Extending through Earth between the North and South Poles is an
imaginary rod, or axis of rotation, on which Earth spins (see Figure 8-1).
A basketball spinning on a fingertip gives a good idea of how Earth spins
on its axis. The line from the fingertip through the basketball to the top of
the ball is the axis of rotation.
Earth's rotation produces several effects:
1. Earth's rotation causes the daily change from day to night. At any
given time, half of Earth is in daylight, facing the sun, while half is
in darkness, facing away from the sun. This is shown in Figure 82. Every day, all places on Earth, except the areas near the poles,
experience this change from daylight to darkness. (Areas within the
Arctic and Antarctic circles experience several weeks of continuous
daylight or darkness at certain times of the year.)
Figure 8-1. Rotation of earth: (a) Earth rotates from west to east
around its axis. (b) Viewed from above the North Pole (NP), Earth
rotates in a counterclockwise direction.
Axis of rotation ""'-- .
_ ""'-- North Pole
West
East
South Pole
(a)
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(b)
Arctic Circle
• •
..
. ..•
..
Sun's rays
.. ..•
•
Darkness
(night)
Axis
Figure 8-2. Earth's rotation cause the change from day to night.
2. The speed of Earth's rotation causes the length of one day to be
about 24 hours. This is the amount of time Earth takes to rotate
once on its axis.
3. The apparent daily motions of the sun, moon, planets, and stars
across the sky are also caused by Earth's rotation. These objects appear to rise in the eastern sky and set in the western sky because
Earth rotates from west to east.
Daily Time
The time of day at any location on Earth is based on Earth's rotation and
the position of the sun in the sky. Earth completes one rotation each day.
That is, every location on Earth rotates 360 around Earth's axis each day.
There are 24 hours in a day. If we divide 360 by 24 hours, we find that
Earth rotates IS per hour. Table 8-1 and Figure 8-3 on page 170 show how
the time of day and the location of the sun in the sky are related to Earth's
rotation.
Earth's longitude, imaginary lines that run north and south, is measured in degrees east, and west, and is based on the rotation of Earth. Every
ISO-change in longitude represents a difference of one hour of Earth time.
0
0
0
Table 8-'. Rotation Positions on Earth
Line
Rotation
Angle
Time
A-NP
Start
12:00 noon
B-NP
e-NP
D-NP
E-NP
45
0
90
0
0
180
270
0
Location
of Sun
High in the sky
3:00 P.M.
In the western sky
6:00 P.M.
Near western horizon
12:00 midnight
6:00 A.M.
Opposite side of Earth
Near eastern horizon
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Figure 8-3. The time of day and the sun's location in the sky are
related to Earth's rotation. (That is, it is 6 A.M. everywhere on line NP-E.)
Earth's Revolution
Earth moves around the sun in a motion called revolution. The path Earth
travels around the sun is called an orbit. Earth's orbit is not perfectly circular, it is actually slightly oval in shape (Figure 8-4).
Earth's revolution has two major effects. First, the time Earth takes to revolve once around the sun defines the length of a year. During that time, Earth
rotates on its axis 365-a times, so there are 365{ days in a year. For convenience, the calendar year is 365 days long, and an extra day is added every
fourth year (called a leap year) to make up for each leftover { day.
Second, Earth's revolution around the sun, combined with the tilt of
Earth's axis, causes the changing seasons on the planet. Earth's axis of rotation is not perpendicular (at a right angle) to the plane of its orbit; rather,
it is tilted 23.5° (see Figure 8-5).
No matter where Earth is in its orbit, its axis is always tilted in the
same direction in space, pointing toward the North Star. While all the other
stars seem to move across the night sky, the North Star remains motionless
because Earth's axis points toward it.
Figure 8-4. Earth's orbit around the sun is oval, not a perfect circle.
Earth is furthest
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152 million km
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from the sun about 0,:>----'.::.=....:.:..::.:.:.:c::.:..:....::::..:..:.-.-----'~
July 4
;-//II\'~'
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Reviewing Intermediate-Level Science
Earth is closest
to the sun about
January 3
Perpendicular:
to plane of orbit!
To North Star
~
1 - - - - Tilt of Earth's axis
~-->,---
Earth's axis
Plane of Earth's
orbit around sun
~--'--'
Direction of rotation
Figure 8-5. Earth's axis of rotation is tilted 23V2°.
Yearly Seasonal Time
The length of a year is based on Earth's revolution. Earth revolves completely around the sun once each year. That is, Earth revolves 360° during
a 12-month period of time. If we divide 360° by 12 months, we find that
Earth revolves about 30° per month. Table 8-2 and Figure 8-6 on page 172
show how the date and the season on Earth are related to Earth's revolution around the sun.
Seasonal Changes
Earth's orbit takes it closest to the sun in early January and farthest from
the sun in early July (refer to Figure 8-4). This means it is not Earth's changing distance from the sun that causes the changing seasons. The cause is
the tilt of Earth's axis as the planet revolves around the sun. Because the
axis always points in the same direction while Earth orbits the sun, the
Northern Hemisphere is tilted toward the sun for half the year and away
Table 8-2. Revolution Positions of Earth
Point
Revolution
Angle
Date
Seasonal Information
A
Start
December 21
Winter begins
B
30°
January 21
Winter
C
90°
March 21
Spring begins
D
180°
June 21
Summer begins
E
270°
September 23
Autumn begins
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171
March 21 Earth's orbit
C
June 21 D
A December 21
E
September 23
Figure 8-6. Earth revolves 360 0 around the sun in a 12-month period.
from the sun the other half of the year (see Figure 8-7). This causes changes
in the number of hours of daylight each day and in the angle at which the
sun's rays strike Earth.
The sun's vertical rays strike Earth when the sun is directly overhead.
The vertical rays are the strongest and hottest rays, and are therefore significant in causing the seasons on Earth.
On June 21, the Northern Hemisphere is tilted toward the sun, and the
sun's vertical rays reach 23.5 0 north of the equator, the Tropic of Cancer (see
Figure 8-7. The seasons are caused by the tilt of Earth's axis and
Earth's revolution around the sun.
First day of spring March 21
N
I
Equator
Northern hemisphere
tilted away from sun
Northern .
WInter
Equator
Vertical rays strike
northern hemisphere
Vertical rays strike
southern hemisphere
tvOrth
erns
Northern hemisphere
tilted toward sun
Equator
lJrn rner
I
/
I
S
First day of fall September 23
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Sun's rays
June 21
Vertical rays
Figure 8-8. On June 21, the sun's vertical
rays strike the Tropic of Cancer.
Summer
Figure 8-8). The Northern Hemisphere is heated more effectively and begins
summer. On the first day of summer in the Northern Hemisphere, the sun
follows its highest path across the sky (see Figure 8-9). The area within the
Arctic Circle has 24 hours of daylight, and as you travel south the number of
daylight hours decreases until you reach the Antarctic Circle. Everywhere
within the Antarctic Circle has 24 hours of darkness (night).
On December 21 the Northern Hemisphere is tilted away from the
sun, and the sun's vertical rays reach 23.5 0 south of the equator, the Tropic
of Capricorn (see Figure 8-10 on page 174). It is the first day of winter in the
Northern Hemisphere, the sun follows its lowest path across the sky (see
again Figure 8-9). The area within the Arctic Circle has 24 hours of darkness,
and as you travel south the number of daylight hours increases until you
reach the Antarctic Circle. Everywhere within the Antarctic Circle has 24
hours of daylight.
Figure 8-9. The sun's apparent path across the sky changes with
the seasons.
Winter sun is lower in
the sky; rays strike
Earth at a slant and
days are shorter,
producing less heat
Summer
noon sun
,., liil .
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Point directly
overhead
(zenith)
i
f
\t -------•------------<--
Winter
/'"
noon sun /
"
lii
' \\
~
".111 \'\.
\
\
I
\
\\
/
I
/I
\I
:
South
_
' .. ,
'U·~/
.~
.~
Summer sun is higher in
the sky; rays strike
Earth more directly and
days are longer,
producing more heat
I
u n
East
Sun's path
in summer
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Sun's rays
Dec. 21
Vertical rays
Figure 8-10. On December 27, the
sun's vertical rays strike the Tropic of
Capricorn.
Winter
In the Southern Hemisphere, the situation is reversed. During summer
in the Northern Hemisphere, the Southern Hemisphere experiences winter.
During winter in the Northern Hemisphere, the Southern Hemisphere has
its summer.
On March 21 and September 23 neither hemisphere is really tilted toward the sun. The sun's vertical rays strike the equator, and all areas of Earth
have equal periods of daylight and darkness (see Figure 8-11). Table 8-3 summarizes information about seasonal dates in the Northern Hemisphere.
During any season, the sun is highest in the sky each day at noon. In
the continental United States, the noon sun approaches but never reaches
the point directly overhead, it is always in the southern half of the sky (see
again Figure 8-9). As a result, shadows at noon always point to the north.
Proof of Earth's Revolution
Proof that Earth revolves around the sun comes from observations of stars.
Stars in the night sky form patterns that have reminded people of animals
Figure 8-11. On March 24 and September
23, the sun's vertical rays strike the equator.
March 21
or Sept. 23
Sun's rays
Vertical rays
S.P.
Spring and Fall
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