Do Now:
Write three pieces of evidence
that told us that the earth was
round.
Aim: How do we measure the
shape and size of the earth?
Topic: Unit 1
Main Idea: Eratosthenes’s Method
1) What is the true shape of
the earth?
The earth is a
slightly flattened
sphere called an
“oblate spheroid”.
2) How do we know about the
shape of the earth?
 Observations of the shape of the horizon,
how ships disappear and lunar eclipses.
3) What did Eratosthenes discover
about the earth?
 He was able to calculate the
circumference (distance
around) the earth with great
accuracy.
4) What happens to the angle of the sun
as you travel north or south of the
equator?
 The height of the sun decreases as you move away
from the equator.
5) What formula did he use?
 Angle of the sun = distance
circumference
360o
or
Circumference= (360 )(distance)
angle
6) Eratosthenes calculated the circumference of the
earth to be 24,662 km while it is actually 24,901 mi.
What was his percent deviation?
 24,901 mi – 24,662 mi X 100
24,901 mi
 239mi
X 100
24,901 mi
 0.96% !!!
Summary:
A new planet is discovered and measurements using
Eratosthenes’s method are as follows: distance (S) =
2 km angle (L) = 9 degrees. Calculate the
circumference of the new planet.
 (360o)(2 km)
(9o)

.
720o km
9o
 80 km