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1. Classroom rules
• Raise your hand if you
have something to say.
• Listen carefully when
someone is talking.
• Ask permission to leave
the room.
• Respect everyone's

2. HOW THE GREEKS KNEW THAT
THE EARTH IS SPHERICAL

3. • Around 500 B.C., most Greeks believed that the Earth
was round,not flat. It was Pythagoras and his pupil's who
were first to propose a spherical Earth.
• In 500 to 430 B.C., Anaxagoras further supported
Phythagoras proposal through his observations of the
shadows that the Earth cast on the Moon during a lunar
eclipse, the earth's shadow was reflected on the Moon's
surface. The shadow reflected was circular.

4. • Around 340 B.C., Aristotle listed several
arguments for a spherical Earth which
included the positions of the North star, the
shape of the Moon and the Sun, and the
disappearance of the ships when they sail
over the horizon.

5. • North Star
The North Star was believed to be at a fixed
position in the sky. However, when the Greeks
traveled to places nearer the equator, like Egypt,
they noticed that the North Star is closer to the
horizon.
• The Shape of the Sun and the Moon
Aristotle argued that if the Moon and the Sun were
both spherical, then perhaps, the Earth was also
spherical.

6. • Disappearing Ships
If the Earth was flat, then a ship traveling away from an
observer should become smaller and smaller until it
disappeared. However, the Greeks observed that the
ship became smaller and then its hull disappeared first
before the sail as if it was being enveloped by the
water until it completely disappeared.

7. The Size of the Spherical Earth
•Ancient scholars tried to provide proof of a spherical Earth
and its circumference through calculations.
•It was Eratosthenes who gave the most accurate size during
their time.
• While he was working at the Library of Alexandria in
Northern Egypt, he received correspondence from Syene in
Southern Egypt which stated that a vertical object did not cast
any shadow at noontime during the summer solstice.

8. •But this was not the case in Alexandria
where, at noon time during the summer
solstice, a vertical object still casts a shadow.
•These observations could only mean that
the Sun, during this time in Alexandria, was
not directly overhead.

10. 80%
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•Eratosthenes then determined the angle the Sun
made with the vertical direction by measuring the
shadow that a vertical stick cast.
•He found out that in Alexandria, the Sun makes an
angle of 7.2° from the vertical while 0° in Syene.
•To explain the difference, he hypothesized that the
light rays coming from the sun are parallel, and the
Earth is curved.

11. •From his measurements, he computed the
circumference of the Earth to be
approximately 250,000 stadia (a stadium is a
unit of measurement used to describe the
size of a typical stadium at the time), about
40,000 kilometers.