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• The Greeks are very much noted for their major contributions in different fields. They were not only great philosophers, they were great scientists and mathematicians as well.
• It was in Greece that the Golden Age of early astronomy was centered.
• The early Greeks had a geocentric view of the earth. For them, it was the center of the universe; hence, a motionless sphere. The sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn orbited the Earth.
• The Greeks also believed that stars traveled daily around the earth. However, they all stayed in a transparent, hollow sphere located beyond the planets. They called this sphere as the celestial sphere.
• Geocentric

  The belief that the Earth is at the center of the universe
• Celestial sphere

  The imaginary sphere on which the stars appear to be fixed

  
• Pythagoras and his pupils were first to propose a spherical Earth.
• Anaxagoras further supported Pythagoras' proposal through his observations of the shadows that the Earth cast on the Moon during a lunar eclipse.

  
• 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.
• North Star

  Believed to be at a fixed position in the sky, but Greeks noticed it was closer to the horizon when traveling nearer the equator

  
• 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

  
• 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.
• How Eratosthenes measured the circumference of the Earth

  1. Received correspondence from Syene stating a vertical object did not cast any shadow at noontime during the summer solstice, but in Alexandria a vertical object still cast a shadow

  2. Determined the angle the Sun made with the vertical direction by measuring the shadow of a vertical stick

  3. Hypothesized that the light rays coming from the sun are parallel, and the Earth is curved

  4. Computed the circumference of the Earth to be approximately 250 000 stadia (about 40 000 kilometers)

  
• Anaxagoras

  Able to explain what causes the phases of the moon - the moon shone only by reflected sunlight, and since it is a sphere, only half of it is illuminated at a time

  
• Eudoxus

  Proposed a system of fixed spheres, with the Sun, moon, five known planets and stars attached to these spheres which revolved around the stationary Earth

  
• Aristotle

  Believed the earth is spherical in shape since it always casts a curved shadow when it eclipses the moon, and that the earth was the center of the universe with the planets and stars as concentric, crystalline spheres centered on the earth

  
• Aristarchus

  The very first Greek to profess the heliocentric view, considering the sun as the center of the universe, and he made an attempt to calculate the distance of the sun and the moon using geometric principles

  
• Eratosthenes

  Made the first successful attempt to determine the size of the earth by applying geometric principles, observing the angles of the noonday sun in two Egyptian cities and assuming they were in the same longitude

  
• Hipparchus

  Considered the greatest of the early Greek astronomers, he observed and compared the brightness of 850 stars and arranged them into order of brightness or magnitude, developed a method for predicting the times of lunar eclipses, and measured the length of the year to within minutes of the modern value

  
• Claudius Ptolemy

  Developed a geocentric model (the Ptolemic Model) that claimed the planets moved in a complicated system of circles around the Earth

  
• According to the Ptolemic Model, the sun, the moon, and the other planets move in circular orbits around the earth, but if observed night after night, these planets move slightly eastward among the stars, and at a certain point, the planet appears to stop then moves in the opposite direction for some time before resuming its eastward motion (retrograde motion)</b>

  
• To justify his earth-centered model using retrograde motion, Ptolemy further explained that the planets orbited on small circles, called epicycles, revolving around large circles called deferents

  
• Aristotle studied and used scientific methods to prove the earth is spherical, including observations of the position of the North Star, the shape of the shadow cast during eclipses, and the disappearance of ships over the horizon

  
• From a distance, he could see the tops of their sails before he saw the rest of the ship
• Aristotle deduced

  This was because of the curvature of the earth
• Aristotle observed the shadow cast during eclipses
• The Earth casts a circular shadow on the moon during a lunar eclipse
• The different positions of the North Star depend on the location of the observer

  
• The Moon and the Sun are both spherical

  
• A sailing ship becomes smaller and then its hull disappears first before the sail as if it is being enveloped by the water until it completely disappears
• The angle of the sun with the vertical direction at noon time during a summer solstice varies from place to place

  
• Greek Philosopher: 'What will you tell him about his findings on ancient history?'