Transcript Dimensions of the Earthx

Dimensions of the Earth
The Circumference of the Earth
• Eratosthenes (200 BC) determined the earth’s
circumference to be 39,000 km.
• Axis of rotation of the earth
• Added leap day to calendar
• May have calculated distance to sun
• Made first map of the world
How he did it
• He knew that at local noon on the
summer solstice in Syene, the Sun was
directly overhead.
• The shadow of someone looking down a
deep well at that time in Syene blocked
the reflection of the Sun on the water.
• He measured the Sun's angle of
elevation at noon on the same day in
Alexandria.
• The method of measurement was to
make a scale drawing of that triangle
which included a right angle between a
vertical rod and its shadow.
• This turned out to be 1/50th of a circle.
• Taking the Earth as spherical, and
knowing both the distance and direction
of Syene, he concluded that the Earth's
circumference was fifty times that
distance.
Other Earth Dimensions
• Earth is an oblate spheroid, close to a perfect
sphere
• Slightly flattened at the poles, bulges at
equator
• Irregularities (mountains, ocean basins) very
small compared to diameter.
• Relief maps exaggerate sizes of features
Earth Dimensions
Evidence of Earth’s Shape
• Space photos
• Ships heading for horizon appear
to sink
• Altitude of Polaris (equals
latitude)
• Rounded shadow during lunar
eclipse
Earth’s Structure
• The solid Earth is divided
into several parts.
• Crust is thin (10 km)
under oceans, thick (65
km) beneath land.
• Mantle is 2850 km thick
• Outer core is 2100 km
thick
• Inner core is 1400 km
thick
Earth’s Surface
• The earth’s surface is divided into four
structures: lithosphere (geosphere),
hydrosphere, and atmosphere, and biosphere.
Determining Positions on Earth
• A coordinate system of imaginary lines has been
developed to divide the Earth’s surface into a
grid.
• Longitude-Latitude system with equator and
Prime Meridian as reference lines.
Latitude
• Latitude is the distance, in degrees, north or
south of the equator.
• The equator is 0° latitude, the North Pole is
90°N latitude, the South Pole is 90°S latitude
Latitude of a person
• Draw a line from the
equator to center of the
earth
• Draw a second line from
the center of the earth to
the person, place or thing..
• The angle between the
two lines is the latitude.
Slide
Mountain
• If you were
standing on
top of Slide
Mountain, you
would be
almost exactly
on the 42°N
parallel…
49th Parallel
Measuring Latitude
• At night in the Northern Hemisphere, latitude
can be determined by measuring the altitude
of Polaris.
Longitude
• Longitude is the distance, in
degrees, measured east or
west of the Prime Meridian
(0°)
• The reference line for
longitude is the Prime
Meridian, which passes
through Greenwich, England.
• Meridians are imaginary
semicircles on Earth’s surface
which converge at the poles.
The International Date Line
• Travelling east or west of
the Prime Meridian,
longitude increases until
it reaches a maximum of
180°
• The International Date
Line roughly follows the
180° meridian
• Travelling west from the Prime Meridian, you
indicate that by adding a “W”.
• Travelling east, an “E”
Measuring Longitude
• Very accurate clocks
(chronometers) are used,
which keep the time at the
Prime Meridian.
• This time is compared to
the solar time at any other
meridian – most easily
determined at solar noon,
when the sun is highest in
the sky.
Figuring out longitude
• It takes 24 hours to
rotate 360°, or 15° per
hour
• So in the Western
Hemisphere, local time
is earlier than
Greenwich time
• In the Eastern
Hemisphere, local time
is later than Greenwich.
• If it is 12:00 noon where you live and the chronometer
reads 5:00 pm Greenwich time, what is your longitude?
• Since there is a 5 hour difference in time (5 hrs behind)
you are 15° x 5 or 75° away
• Since you are behind Greenwich, you are to the west,
or 75°W
• 15° corresponds to 5 time zones
Subdivisions of Longitude and Latitude
• Degrees are broken down into minutes (60
minutes per degree) and seconds
Longitude and Latitude in New York State