Earth

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Chapter 25

• Earths Motions

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Rotation

• The spin of the Earth on its axis (rate of
  15º/hr)
• Earth is tilted at an angle of 23.5º from
  perpendicular (90º) . In other words, the tilt
  of the Earths axis is 23.5º off of the 90º it
  should be if it had a perpendicular tilt.
• The Earths axis is always tilted in the same
  direction throughout its orbit (parallelism)

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Remember, the ecliptic is the plane of the path
that the Earth takes around the Sun
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Evidence of Movement

• Foucault Pendulum- Changes its direction of swing
  over time
• Coriolis Effect- The deflection of air and
  water to their right in the northern hemisphere
• Sorry, Bart Simpson was incorrect. Water DOES
  deflect counterclockwise in the Northern
  Hemisphere and clockwise in the Southern
  Hemisphere, HOWEVER in toilets, the direction is
  caused by the force of the pump and direction it
  pumps the water in, not the CORIOLIS force!
• In fact, YOU can determine the direction that
  water goes down the tub drain if you turn your
  hand in the water around the drain and spin the
  water either CW or CCW as it is draining out of
  the tub!!! Try it!!!!!

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Object moves to ITS LEFT
Object moves to ITS RIGHT
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Solar Time

• The measurement of time based on the apparent
  movement of the sun through the sky
• Solar Noon is the time of day when the sun is at
  its highest altitude above the horizon.
  (Wherever the Sun Peaks in the sky)
• Problem - solar noon does not occur at the same
  time for everyone (solar noon differs by approx.
  1 minute for every 12 miles east/west of a
  specific location)

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• To make up for this problem, time zones were
  established (each 15º longitude wide 1 hour of
  rotation)
• Time zones begin at the Prime Meridian
• Every 15 degrees EAST is 1 hour, every 15
  degrees WEST is 1 hour
• Clock time is the average solar time

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Revolution

• Movement of the Earth around the sun in its
  elliptical orbit
• One complete revolution is 365.25 days (1 year)
• In conjunction with AXIAL TILT, this causes a
  change in the seasons
• If there was no tilt of the axis no seasonal
  changes
• Evidence of movement Change in visible
  constellations throughout the year

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Solstices and Equinoxes

• SOLSTICE extreme differences in daylight vs.
  darkness. The following values are applicable to
  the NORTHERN HEMISPHERE ONLY. REVERSE THESE
  VALUES FOR THE SOUTHERN HEMISPHERE
• SUMMER 6/21/05 16 hours daylight/8 hrs
  darkness. The Sun is directly above the Tropic
  of Cancer. (23.5º N)
• WINTER 12/21/05 8 hours daylight/16 hrs
  darkness. The Sun is directly above the Tropic
  of Capricorn. (23.5º S)
• EQUINOX equal distances from the sun (equal
  daylight and darkness)
• Autumn (Autumnal) 9/23/05, Spring (Vernal)
  (Verde/Vern means green) 3/20/05 12 hours
  daylight and 12 hours of darkness FOR THE ENTIRE
  EARTH NORTHERN AND SOUTHERN HEMISPHERES. The
  Sun is directly above the Equator.

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Nighttime on the Artic Circle during the Summer.
The sun never sets! It only gets as low as the
horizon and as high as it looks about an hour
before sunset!!!
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The Suns Apparent Path Across the Sky

• Because of the tilt of the Earth and the
  Revolution of the Earth around the Sun, the Sun
  appears to rise and set each day in a slightly
  different place on the horizon.
• Also, Solar Noon (the highest point the Sun
  reaches in the sky) also varies slightly each
  day.
• These are predictable apparent motions and are
  easiest to pick out on the Solstices and
  Equinoxes.

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Suns Apparent Path from a Viewpoint in NYS
December 21
June 21
March 21 September 21
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Solar Noon at The Equator
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Causes of Seasons

1. Earths revolution around the sun
2. Tilt of the Earth on its axis
3. Parallelism of the axis
4. More daylight hours more insolation warmer
   time of year
5. Less daylight hours less insolation cooler
   time of year
6. Suns rays are more direct (perpendicular) to the
   Earths Surface in the Northern Hemisphere during
   our summer

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Causes of Seasons

• 6 explains why even though the Sun shines for 24
  hours a day for nearly 6 months a year at the
  North and South Poles, the temperature never gets
  above freezing!!!!
• If the Suns rays dont hit the Earth at a
  perpendicular or near perpendicular angle, they
  arent all that powerful and warming!!!!

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Direct solar rays
Less direct solar rays
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