Transcript Earth Dimensions - Harpursville Middle School

Earth Dimensions
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 The
Earth is an oblate spheroid.
 That
is a sphere with slightly flatten
poles and a slight bulging at the
equatorial region.
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 This
means the polar
circumference (40,008 km) is 68
km smaller than the equatorial
circumference (40,076 km).
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A. Evidence for the Earth
shape comes from several
sources.
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1. Early sailors noticed that ships
seem to "disappear" as they sail out
of port over the horizon.
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 However
it is the altitude of Polaris
that provides the earliest proof.
 Polaris
(the North Star) is directly
overhead of the North Pole and has
an altitude of 90⁰.
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 At
the equator, the altitude of
Polaris is 0⁰ (it is located on the
horizon).
 As you head north the altitude
of Polaris increases, and is in
fact equal to the latitude.
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That the altitude
of Polaris and an
observers latitude
are the same.
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Finding Polaris
Finding Polaris can be done using the
"pointer" stars in the Big Dipper (Ursa
Major).
Facing Polaris, points you North,
measuring the angle to Polaris tells you
your latitude in the Northern
Hemisphere
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2. Photographs from space
confirm earth's oblate shape.
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3. Careful measurements of a
known mass shows that
the mass' weight is heavier
at the poles than at the
equator.
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B. How to determine the size
of Earth?

Modern methods
 Photographs from
space
 Measurements of
Polaris (North
Star)
 Non-modern
methods
 Eratosthenes
method
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Eratosthenes

Greek mathematician

Lived approximately 2 x 103 years ago in
Egypt

Used geometry to calculate the circumference
of Earth
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Assumptions Eratosthenes
made

1. All rays of the sun approach Earth in
a parallel manner
Rays of light
Earth
sun
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Assumptions Eratosthenes
made (cont.)

2. Earth is a perfect sphere

He was almost correct
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Geometry that Eratosthenes
used
Alternate interior angles made by parallel
lines of the sun’s rays are equal
sun
a
sun rays
b
angle a = angle b
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sun
= Alexandria
= Syene
Sun’s rays
Distance between
Alexandria and
Syene was 925 km
Shadow
angles = 7.2°
Earth’s center
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How to find circumference?
Use the information from the experiment to
set up a proportion (2 equal ratios)
Distance (km)
Shadow Angle
Circumference =
360°
of Earth (km)
925 km
cross
7.2°
Circumference =
multiply
360°
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of Earth (km)
to solve
925 km x 360° = circumference x 7.2°
925 km x 360°
=
circumference
7.2°
46, 250 km = circumference of Earth
( very close to the actual measurement)
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C. The Outer Parts of the
Earth.
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Hydrosphere – the portion of
the earth that contains earth's
water.
 This
includes the (lower)
atmosphere, surface, and upper layer
of the lithosphere.
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2. Atmosphere – the sphere
of gases that surround the
earth.
The Atmosphere is layered, or divided
into four zones based on temperature.
The lowest, the troposphere contains
most of the gases that we need to
survive and our weather.
The other three layers are the
Stratosphere, Mesosphere, and
Thermosphere.
See Reference Table
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3. Lithosphere (geosphere)–
the outer most layer of the
earth.
Approximately 100 km thick, the
lithosphere is the portion of the crust
and mantle that contain the plates which
move around forming earth's features.
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Using a compass

A compass will not show True North
(geographic north), but rather Magnetic
North, because earth's magnetic field is
tilted at 12⁰ angle to the Earth's axis of
rotation
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