Ancient Greeks: Earth's Shape and Astronomy

• Ancient Greeks contributions were made across many fields including philosophy, math and science.

Ptolemic System

• Claims that the planets moved in a complicated system of circles.
• This model is also known as the Ptolemic System.

Earth's Shape

• Earth is an oblate spheroid.
• It has a bulging equator and squeezed poles.
• The semi-minor axis is 6,356,752m.
• The semi-major axis is 6,378,137m.

North Star

• It was believed to be in a fixed position in the sky.
• When the Greeks traveled to places nearer the equator, like Egypt, they noticed it was closer to the horizon.

Aristotle

• A student of Plato who was considered one of the greatest philosophers of his time.
• His earth-centered view dominated for almost 2,000 years.

Greeks Believed the Earth was Round

• As early as 500 B.C., the Greeks believed the Earth's shape was round, not flat.
• Pythagoras first proposed the spherical Earth.

Additional Evidence of a Spherical Earth

• The position of the North Star.
• The shape of the Moon and the Sun.
• The observation of ships disappearing over the horizon.

Anaxagoras

• Supported the spherical Earth idea of Pythagoras in 500 to 430 B.C.
• Observed that the shadow Earth cast on the moon during a lunar eclipse was circular.

The size of the Spherical Earth

• Ancient scholars tried to provide proof of a spherical Earth and its circumference through calculations.
• Eratosthenes gave the most accurate size during their time.

Eratosthenes measured the Earth's circumference

• He used a stick, knowledge of the distance from Alexandria to Syene, and geometry.
• He determined the angle the Sun made with the vertical direction by measuring the shadow that a vertical stick cast.
• In Alexandria, the Sun makes an angle of 7.2° from the vertical, while it is 0° in Syene.
• Eratosthenes hypothesized that the light rays coming from the sun are parallel and the Earth is curved.
• He computed the circumference to be approximately 250,000 stadia, or about 40,000 kilometers.
• The angle of a shadow cast at noon on the summer solstice in Alexandria as about 7.2 degrees (1/50 of a complete circle).
• The distance from Alexandria to Syene is 500 miles.

Greek Astronomers and their Contributions

• Anaxagoras explained the causes of the phases of the moon.
• Eudoxus proposed a system of fixed spheres where the Sun, Moon, Planets and Stars were attached.
• These attachmed bodies carried the heavenly bodies while they revolved around the stationary Earth.
• Aristarchus proposed that the Sun was the center of the Universe, therefore, Heliocentric.
• Hipparchus made a compilation of 850 stars and developed a method for predicting lunar eclipses.
• Claudius Ptolemy proposed the geocentric model (Ptolemic system) where the Sun, Moon and other planets move in circular orbits around the Earth.
• Ptolemy further explained that planets orbited on small circles called epicycles, revolving around large circles called deferents.
• This was done to justify his earth-centered model using retrograde motion.