Proof of Earth's Shape and Size - Plainedge Schools

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					 What shape best describes earth?
n Describe   the evidence that supports your
  answer.
n Oceans are flat
n Airplanes all fly level on earth
n Ancient Egyptians: The sky was a tent
  canopy stretched between mountains at
  the four corners of the Earth.
n Inca: Called their land Tehuantinsuyu:
  "The Four Corners of the Earth"
HOW DID THEY PROVE THAT
 THE EARTH IS ROUND?
Ships appear to sink gradually
        below horizon
                            Polaris

Polaris
North Star
n   This is the star that
    lies in space
    practically over the
    geographic North                  Pole
    of the earth.


n   If you stood at the
    North Pole, Polaris
    would be almost
    directly overhead.
         Polaris – North Star
n Ifyou can spot Polaris in the sky, you can
  always tell which way is north. Because of
  this, Polaris was the most important star
  for navigating at sea.
Why do observations of Polaris help
 determine the Earth’s shape?????
n The North Star appears lower and lower in
 the sky as you travel toward the equator
 because of earth’s spherical shape, where
 the North Star is just visible at the
 horizon. The latitude of the equator is 0°.
Polaris Changes in Altitude – Polaris
  is a fixed point above the North
                 Pole.


          90º
     Polaris – Fixed Point above the
                North Pole

n   Because of this, in the Northern Hemisphere, the
    altitude (angle measured in degrees above the
    horizon) of Polaris tells observer his latitude
    position.
n   If observer’s latitude changes in the Northern
    Hemisphere, the altitude of Polaris will exactly
    match observer’s latitude.

         ALT OF POLARIS = LAT OF OBSERVER
 Locating Polaris – our latitude is
41°N , Polaris’ altitude will be 41°
            Lunar Eclipse
n Onlya sphere can cast a shadow that
 appears round. During a lunar eclipse, the
 earth casts its shadow on the moon during
 the full moon phase.
Summary: How did they prove
   the Earth to be round?
n   Ships appear to sink GRADUALLY below the horizon as
    they travel away from observer

n   Polaris (North Star) changes altitude (angle measured in
    degrees above earth’s surface) directly with your latitude


    ALT OF POLARIS = LAT OF OBSERVER

n   Lunar Eclipses - Earth’s shadow on the moon

n   Satellite Imagery – absolute proof
     Is the Earth Perfectly round?
n The   Earth is NOT a perfect sphere
n It is flattened at the poles and bulges at
  the equator
n Earth is slightly out of round or OBLATE.
    What proof is there that we are
           slightly oblate?
n   Gravity measurements. Gravity is the force of
    attraction between any 2 objects.
    Increase mass of objects = increase gravity
    Decrease distances = increase gravity

n   If Earth were a perfect sphere, it would be
    expected to exert an equal force on objects at
    equal distances from the center of earth.
       Weight!!! – measure of
        gravitational force
n We  are further from the center of the
  Earth at the equator – gravity is less
n This means we weigh LESS!!!
n We are closer at the poles…
n We weigh MORE!!!!
       The least amount of gravity is
      farther from the center of Earth




High Mountains farther from the center of earth – gravity is
less
        The Oblate Sphere
n The   difference is small
n We can not see it with the naked eye
n It still appears like a sphere
n Be careful on multiple choice
   Why is the earth not perfectly
              round?
n Earth’s   rotation causes…

                Bulging at equator
             Flattening at the poles
             How big is Earth?
n   Having established the shape, what is the size?

Question:

n   How do you measure something really big?

n   Two centuries before the birth of Christ …
    (-200) Eratosthenes devised a way to determine
    the size of earth.
          NOT CHRISTOPHER COLUMBUS!!!!!
            Eratosthenes
n Based his calculation on observations of
 the sun on June 21, the Summer Solstice
 in Alexandria and Syene – 2 cities in
 Egypt.

n Onthis date, the sun was straight
 overhead at noon in Syene, but not quite
 overhead at Alexandria
Syene   Alexandria
Obelisk in Alexandria, Egypt
n Theobelisk is called
 “Cleopatra’s Needle”
 and it was moved to
 Central Park in 1881.
           Erathosthenes
n Made assumptions that…
     a. The earth is round
     b. The sun’s rays are parallel

Using simple geometry he set up an
 equation that determined the
 circumference of the earth.
       Erathosthenes’ Result
n 240,000 stadia (about 39,250 km) is quite
 close to modern measurements.

n Calculate the percent deviation (Equations
 are located on what page of ESRT?) if the
 actual circumference is 40,070 kms.
n   Eratosthenes' work was lost, except for a
    description of his method in an obscure source

n   140 AD: Ptolemy got an answer smaller by 28%
    using a sound method with questionable data.

n   Both results known by King Ferdinand’s experts.
    Columbus argued for the smaller value for the
    distance from Spain to India but he was turned
    down because they thought the distance too far.

n   In the end Queen Isabella overruled the experts,
    and the rest is history.

				
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