Ancient Greek Models of the Universe

The Greeks and their Contributions to Understanding the Universe

• The Greeks were a remarkable civilization that flourished around 2,500 years ago, known for their achievements in various fields, including philosophy, warfare, literature, arts, and science.
• They lived in mainland Greece, the Greek islands, and colonies scattered around the Mediterranean Sea.
• The Greeks deduced that the Earth is spherical based on observations of lunar eclipses, which produce a circular shadow on the Earth's surface.
• They measured the diameter of the Earth and noted that the stars appear differently as they travel north and south.

Eratosthenes and his Measurement of the Earth's Circumference

• Eratosthenes (276 BC - 194 BC) was a Greek mathematician who measured the circumference of the Earth.
• He observed that the shadow cast by the sun in Alexandria was 7.2° (one fiftieth of a circle) at the same time as the sun was directly overhead in Syene (modern-day Aswan in Egypt).
• Assuming the sun's rays fall parallel to each other on the Earth's surface, Eratosthenes calculated the distance between Syene and Alexandria to be 5000 stadia (approximately 800 km).
• He then estimated the Earth's circumference to be 50 x 5000 stadia, which is approximately 40,000 km.

Significance of the Spherical Shape of Earth

• The Greeks believed in a spherical Earth located at the center of the sphere of heavens, which appealed to their sense of symmetry.
• In the 4th century BC, Greek philosophers developed the geocentric model, which proposed that the center of the Universe lies at the center of a stationary Earth, around which the Sun, Moon, planets, and stars rotate.

Astronomical Events Before the Telescope

• Before the invention of the telescope, humans relied on their senses to understand the universe.
• Ancient civilizations, such as the Babylonians, Assyrians, Egyptians, Chinese, Mayans, and Polynesians, made significant observations about the universe, including the length of the year, the movement of stars, and the behavior of celestial bodies.

Celestial Sphere and Astronomical Events

• The celestial sphere is a fictitious sphere centered on the Earth, used to project the positions of celestial bodies.
• Diurnal motion refers to the apparent daily motion of stars and other celestial bodies across the sky due to the Earth's rotation.
• Annual motion refers to the apparent yearly motion of stars and other celestial bodies across the sky due to the Earth's revolution.
• The zodiac and ecliptic are related to the path the sun takes in the celestial sphere, which is visible from Earth.