Measuring the Earth's circumfurance, using the Sun

1. The Story of
Eratosthenes
The Circumference of
the Earth

2. Eratosthenes was
born in Cyrene (in
modern day Libya),
but worked and
died in Alexandria,
capital of Ptolemaic
Egypt.

3. Eratosthenes
studied at
Alexandria and for
some years in
Athens. In 236 BC
he was appointed as
librarian of the
Alexandrian library.

4. He made several
important
contributions to
mathematics and
science, and was a
good friend to
Archimedes.

5. In 195 BC he
became blind and a
year later he
supposedly starved
himself to death.

6. He is credited with
having calculated
the Earth's
circumference
around 240 BC,
using trigonometry
and knowledge of
the angle of
elevation of the Sun
at noon in
Alexandria and
Syene.

8. Syene
Eratosthenes knew that on the summer
solstice at solar noon in the town of Syene on
the Tropic of Cancer, the sun would appear at
the zenith, directly overhead.

9. Syene
As the legend goes, Eratosthenes was visiting
Syene on the Summer Solstice and went to get
a drink from the town well.

10. Syene
While looking down into the well, Eratosthenes
observed that the sun was perfectly reflected in
the water at the bottom of well.

11. Syene
A perfect reflection of the sun in the water at
the bottom could only happen if the Sun was
directly over the well, 90° to the surface.

12. Syene
A perfect reflection of the sun in the water at
the bottom could only happen if the Sun was
directly over the well, 90° to the surface.
90°

13. Press on the following link for
an interesting animation of
the experiment
• Video
https://www.youtube.com/watch?v=3
5UQVcY0_qw

14. Equinox
Nicosia
Sunlight
On the equinox
day the sun rays
are vertical on
the equinox but
not in any other
parts of the
world, like
Nicosia for
example.

15. Equinox
Nicosia
Sunlight
By using google
earth, we know
exactly that the
distance of our
school from the
equinox is
3906km
35°07'39.8"N
33°18'19.3"E

16. Equinox
Nicosia

17. At 12:35 exactly we will measure the angle created by a pillar
in our school’s yard.

18. So our measured angle will represent 3906km
on the Earth’s circumference
3906km
Same angle

19. Eratosthenes also knew that a complete
circle had 360°.
360°

20. Our
angle
This makes a ratio and we can calculate the
earth’s circumference
360°
3906km
???

21. Our
angle
Earth’s Circumference:
360°
3906km
???
360x3906
Our angle