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Latitude

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					Latitude
       Latitude and Longitude
• To locate a position on the earth, map
  makers created a coordinate system with
  imaginary lines of latitude and longitude
  drawn across the earth
                 Latitude
• Lines of latitude are drawn parallel to each
  other with the equator as 0 degrees latitude
  and the poles as 90 degrees north or south
• The location of the   North pole
equator and the north
and south poles are equator
defined by the rotation
of the earth
                       South pole
                 Latitude
• Latitude denotes how far a location is north
  or south of the equator
                                 90
                           60

                         30
                         0
                          30

                           60
                                 90
                   Latitude
• Finding latitude is fairly simple. All one
  needs is:
  – The angle of the sun’s rays with respect to a
    vertical pole at solar noon
  – The sun’s declination -
     the angular distance from
     the equator
                   Latitude
Example 1:
     If at solar noon, the sun makes a 34° angle with a
     vertical stick, and the sun is shining directly down on
     the equator (latitude 0°), then your latitude is:

     34° + 0° = 34° N
                   Latitude
Example 2:
     If at solar noon, the sun makes a 25° angle with a
     vertical stick, and the sun is shining directly down on
     the latitude of 12° N, then your latitude is:

     25° + 12° = 37° N
                   Latitude
Example 3:
     If at solar noon, the sun makes a 47° angle with a
     vertical stick, and the sun is shining directly down on
     the latitude of 6° S, then your latitude is:
     47° - 6° = 41° N
More than 2,200 years ago a
man named Eratosthenes in the
Egyptian city of Alexandria
used the same concept to
calculate the circumference of
the earth.
It had been known for some
time already that the world
was round -- or spherical
Eratosthenes had the idea that by
comparing the angle that the sun
shines down at in two different
locations, he could calculate the
earth’s circumference
Eratosthenes imagined the earth
to be like a sliced grapefruit.         Inner angle   Arc length


If he knew the inner angle of one
grapefruit section . . .

. . . and the arc length along the
outside surface of that section . . .

 . . . he could calculate the full
 circumference of the
 grapefruit without actually
 measuring all the way around.
Eratosthenes compared the
angle that the noon-day sun
makes with a vertical stick
between two cities in Egypt --
Alexandria and a city in
southern Egypt named Syene.

He found that the difference
between the two angles was
7.2 degrees.

This is 1/50 of a whole
circle.
Eratosthenes borrowed the king’s
bematists (trained walkers who
measured distances on foot) to
pace off the distance between
Alexandria and Syene

He found the two cities to be
about 790 kilometers away
from each other.
Since the distance between
these to cities represented 1/50
of the circumference of the
earth, he multiplied the
Alexandria to Syene distance
by 50 and calculated the full
circumference of the earth --
39,503 km.
When the circumference of
the earth was recalculated this
century, Eratosthenes’
calculation was found to only
be off by a few hundred
kilometers.
A more detailed account of
Eratosthenes and his famous
calculation can be found in
this wonderful book:
“The Librarian Who
Measured the Earth”
By Kathryn Lasky

				
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posted:9/18/2012
language:English
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