Download Earth Science Chap.2 Sect. 2

EARTH SCIENCE CHAP.2 SECT. 2
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Objectives:
 Describe Earth’s revolution and rotation
 Tell Why the season’s change
 Explain how the sun is used as a basis
for measuring time
EARTH MOVEMENT
The earth travels around the sun at
approximately 106,000 km/hr (65,720 mph)
 Revolution – the movement of the earth around
the sun – 365.24 days (1 year)
 Rotation – spin of the earth on its axis – one
rotation = approx. 24 hours (1 day)
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THE ROTATING EARTH
Effects of rotation- day & night
 Earth rotates from west to east – making sun
appear to rise in the east in the morning
 At any given time it is daytime on the
hemisphere facing the sun and nighttime on
the hemisphere facing away from the sun
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THE REVOLVING EARTH
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Earth’s orbit around the sun is elliptical, oval shaped
– earth not always at same distance from sun
Perihelion – closest point of earth from sun,
147,000,000 km (91,140,000 miles) away, occurs on
January 3
Aphelion – farthest point of earth from sun,
152,000,000 km (94,240,000 miles) away, occurs on
July 4)
EARTH’S TILT
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Earth’s orbit lies in a plane
Earth’s axis is tilted at 23.5 degrees from the
perpendicular to the plane of the orbit
Earth’s axis always points toward the North Star
(Polaris)
Sometimes the axis is pointed toward the sun
and other times away from the sun
When axis is pointed toward the sun, the
Northern Hemisphere has longer days
When pointed away from sun, Southern
Hemisphere has longer days
THE SUN’S RAYS
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Sun’s rays are nearly parallel to one another when
they strike the earth
Because of the earth’s curvature, the rays strike the
earth at different angles
The amount of solar heat an area receives depends
on the angle the sun’s rays strike that area of the
earth.
SUN’S RAYS
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When the sun is directly overhead the rays
strike the earth at a 90º angle
The closer the rays strike to 90º, the more
concentrated they are and the more heat they
produce
This angle of the rays changes as the earth
travels through its orbit. When the North Pole is
pointed toward the sun, the sun’s rays strike the
Northern Hemisphere at a higher angle and the
opposite (Southern Hemisphere) when pointing
away from sun.
THE SEASONS
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Changes in the angle at which the sun’s rays strike
the earth’s surface and changes in the amount of
daylight causes the seasons
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Ex: When the North Pole is tilted away from the sun, the
angle of the sun’s rays are falling on the Northern
Hemisphere is lower producing fewer hours of daylight. The
weak rays and the short hours of daylight produce the cool
winter season in the Northern Hemisphere
SUMMER SOLSTICE
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Solstice: means “sun stop”- refers to the fact
that the sun follows its highest path across the
sky that day.
Summer solstice: the North Pole tilts toward the
sun, the sun’s rays strike the earth at a 90º
angle along the Tropic of Cancer. Occurs on
June 21st or 22nd each year.
The farther north of the equator you are, the
longer the period of daylight you have.
North of the Arctic circle you have 24 hours of
weak daylight and south of the Antarctic Circle
you have 24 hours of darkness
WINTER SOLSTICE
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Winter Solstice: When the North Pole tilts away from
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the sun. The sun’s rays strike the earth at a 90º angle
along the Tropic of Capricorn. This occurs on December
21st or 22nd each year and marks the beginning of
winter in the Northern Hemisphere.
The northern Hemisphere has the fewest daylight hours
and the sun follows its lowest path across the sky.
Places north of the Arctic Circle have 24 hours of
darkness and south of the Antarctic Circle 24 hours of
daylight.
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AUTUMNAL EQUINOX
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Equinox: means “equal night”and refers to the
fact that the hours of daylight and darkness are
equal everywhere on earth on that day. This is
due to the fact that the North Pole tilts neither
towards or away from the sun.
Autumnal Equinox: Marks the beginning of the
fall season in the Northern Hemisphere. The
sun’s rays strike the earth at 90º along the
equator. Occurs on September 22nd or 23rd each
year.
VERNAL EQUINOX
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Vernal equinox: The sun’s rays strike the earth at a
90º angle at the equator on March 21st or 22nd each
year. This marks the beginning of the spring in the
Northern Hemisphere.
The hours of daylight and darkness are the same all
over the earth on this day.
The North Pole tilts neither towards or away from the
sun
PRECESSION
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As the earth rotates about its axis, the direction in which
the axis points slowly changes in relation to distant
stars. The axis has not always pointed toward the North
Star and will not in the distant future.
Precession: a circular motion of the earth’s axis which
causes the change relative to distant stars. Caused by
forces acting on a spinning body. In the earth’s case, it
is the gravitational pull exerted by the moon, sun, and
other planets. Causes the earth’s axis to move slowly in
a circle.
This circular cycle completes itself every 26,000 years.
TIME ZONES
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12:00 noon is defined as the time when the sun
is highest in the sky. Because of the apparent
movement of the sun from east to west, the sun
appears highest over different locations at
different times.
To avoid problems created by different local
times, the earth is divided into 24 standard time
zones
In each zone, noon is set as the time when the
sun is highest over the center of that zone.
TIME ZONES
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Because the earth is nearly spherical, it
represents a circumference of 360º
Dividing 360º by 24 hours needed for one
rotation, you find the earth rotates at a rate of
15º each hour. Each standard time zone covers
15º
Each time zone is one hour earlier than the time
zone to its east
There are 24 standard time zones and 24 hours
in a day.
INTERNATIONAL DATE LINE
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International Date Line: a line running from
north to south through the Pacific Ocean where
the date changes from one day to the next.
 Ex:
when it is 8:00 A.M. Friday west of the
International Date Line, it is 8:00 A.M. Thursday
east of the line.
 The line is drawn as to not cut between islands or
countries so they are all on the same date
DAYLIGHT SAVING TIME
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Daylight Saving Time: under this system, clocks
are set one hour ahead of standard time in April,
which provides and additional hour of daylight
during the evening. In October, the clocks are set
back one hour to standard time.
Starting this year, 2007, daylight saving time
began the second Sunday in March and reverts
back to standard time the first Sunday in
November – only in the U.S. – (by command of
George W. Bush, Pres. United States)(presumably
to save energy)
BIOLOGICAL CLOCKS
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Our early ancestors lived in harmony with the cycles of
the sun, awakening with the sun’s first rays and
ending their work-day at sunset.
With artificial light, humans have become less
dependent on solar cycles
Research reveals that there is still a link between
humans and solar cycles
CIRCADIAN RHYTHMS
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Circadian Rhythms: many body processes
occurring in a 24 hour cycles – internal clocks
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Ex; sleeping & waking, daily changes in body
temperature, hormone secretions, heart rate, and
blood pressure.Even moods, coordination, and
memory have their own circadian rhythms
Jet lag: combination of exhaustion, irritability,
and insomnia that travelers suffer after a
long flight across several time zones. Due to
the internal clock getting out of sync with the
sun’s cycle.