Earth
Science
Aim: What is the size and shape
of Earth?
Objectives:
Describe the size and shape of Earth
 Explain the evidence that determines the
shape of Earth.
 Understand Polaris’ apparent change in
altitude at different latitudes

Vocabulary:
Oblate Sphere or Spheroid
 Axis
 Gravity
 Horizon
 Polaris
 Altitude
 Circumpolar Stars
 Star Trails

1. How do we know the
Earth is round?
There is some evidence…direct
(some we can see) and
indirect (some we infer)
a. Ships appear to sink gradually
below horizon
HORIZON
Ships don’t fall off the “edge”
Who was the first to
circumnavigate the globe?

Ferdinand Magellan….and he did not FALL
OFF the edge.

Christopher Columbus simply tried to find
a shorter route to India by sailing
west….when he ran into the West Indian
Islands.
b. Lunar Eclipse

During a lunar eclipse, the earth casts
a rounded shadow on the moon during
the full moon phase. Only a sphere can
cast a round shadow.
Earth’s shadow on the Moon
during a lunar eclipse
c. Satellite photos from space
Clearly… its round!
d. Polaris
a.k.a North Star

This is the star that
lies in space
practically 90o over
the geographic
North Pole .

If you stood at the
North Pole, Polaris
would be almost
directly overhead.
Polaris
Polaris – North Star is a fixed point in
the night sky above the North Pole.

If you 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.
90º
2. Why do observations of Polaris
help determine the Earth’s shape?

If you travel from the
equator to the North
Pole, Polaris appears
higher and higher in the
sky because of Earth’s
spherical shape

Altitude of Polaris at:
– Equator (0olatitude) = 0o
above the horizon
– North Pole (90oN latitude)
= 90o above the horizon
HHHmmmmm….coincidence?

Altitude of Polaris at:
– Equator (0olatitude) = 0o above the horizon
– North Pole (90oN latitude) = 90o above the
horizon

Question….
– On Long Island, Polaris appears 41o above the
horizon. What is the latitude of Long Island?
Locating Polaris – our latitude is
41°N , Polaris’ altitude will be 41°
Altitude finder,
sometimes called
a sextant.
Map of
NY
Polaris – Altitude above horizon
determines latitude

In the Northern Hemisphere, the altitude (angle
measured in degrees above the horizon) of Polaris
tells observer his latitude position.

If the observer’s latitude changes in the Northern
Hemisphere, the altitude of Polaris will exactly
match observer’s latitude.
ALTITUDE OF POLARIS = LATITUDE OF OBSERVER
Example: What is the latitude of
the observer?
10° North
2. 20° North
3. 40° North
4. 70° North
1.
3. Where is Polaris?
Polaris is the end of
the handle of the
Little Dipper
Constellation, aka
Ursa Minor
 Use the pointer stars
from Ursa Major to
find Polaris.

4. What are
Circumpolar Stars?

The North Star doesn't seem to
move at all. Stars near the North
Star seem to circle around it ,
never setting below the horizon.
These stars are called
circumpolar stars. Because the
Earth's axis points nearly directly
at Polaris, and as we rotate
about the axis, everything seems
to move except for Polaris.
Star Trails
If you leave open a camera's
shutter for a few hours, you
will capture beautiful star
trails showing their circular
paths.
5. Is the Earth perfectly round?
a. The Earth is NOT a
perfect sphere.
b. It is flattened at the
poles and bulges at
the equator
c. Earth is an OBLATE
SPHEROID
6. Why is the Earth not
perfectly round?
a.
The Earth rotates
about its axis at 1000 mph
at the equator.
b.
The force of the rapid
spinning causes…
**Bulging at equator
**Flattening at the poles
c. The diameter and
circumference from pole
to pole is LESS than
around the equator.
Polar
diameter=12,715 km
 Equatorial
Diameter=12,756km

ESRT…..Scavenger Hunt

Find the table in the ESRT that tells you
the Equatorial diameter of planet Earth.
7. What proof is there that Earth
is slightly oblate?
a.
Gravity measurements. Gravity is the force of
attraction between any 2 objects.
b.
Two factors affect the gravitational attraction between
objects
- Mass :
Increase mass of objects (heavier) = increase gravity
- Distance:
Decrease distances (closer) = increase gravity
c.
If Earth were a perfect sphere, it would be expected to
exert an equal force on objects at any point on Earth ….
BUT
There is less force of gravity
farther from the center of Earth
Where would you weigh more?
a.
b.
South pole or Florida?
Bottom of the Gulf of Mexico or Mt.
Everest?
Because ________________________
_______________________________.
Exit ticket: 3 – 2 - 1
On your index card: List
3 facts about the shape of Earth
2 pieces of evidence that explains Earth’s
shape
1 way you can use this info in your life
How did Eratosthenes
measure the Earth?

Two centuries before the birth of Christ …
(-200) Eratosthenes devised a way to determine the size
of earth.
NOT CHRISTOPHER COLUMBUS!!!!!

Based his calculation on observations of the sun on June
21, the Summer Solstice in Alexandria and Syene – 2
cities in Egypt.

On this date, the sun was straight overhead at noon in
Syene, but not quite overhead at Alexandria
Syene
Alexandria
Obelisk in Alexandria, Egypt
Erathosthenes

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

240,000 stadia (about 39,250 km) is quite
close to modern measurements.

Calculate the percent deviation (Equations
are located on what page of ESRT?) if the
actual circumference is 40,070 kms.

http://www.youth.net/eratosthenes/welco
me.html