Ancient Greek Astronomers: Discoveries and Theories, Study notes of Astronomy

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Lecture 9 -- History I

Ancient Greek astronomers:

-Aristotle

-Eratosthanes

-Aristarchus of Samos

-Hipparchus

-Ptolemy

Ancient Astronomy

• Many cultures throughout the world practiced astronomy.
• They made careful observations of the sky.
• Over a period of time, they would notice the cyclic motions of: - Sun - Moon - planets - celestial sphere (stars)

Stonehenge (completed 1550 BC)

• If you stand in the middle:
  • the directions of sunrise & sunset on the solstices is marked.
  • the directions of extreme moon rise & set are marked.
• The Aubrey holes are believed to be an analog eclipse computer. This famous structure in England was used as an observatory.

Anasazi (ca. A.D. 1000)

• lived in “four corners” area of SW USA
• built structures to mark solstices and equinoxes Pueblo Bonita at Chaco Canyon, NM Sun Dagger at Fajada Butte

Why did they do it?

• archeologists & anthropologists surmise:
  • to keep time
  • for agricultural purposes
  • for religious purposes
• As far as we can tell, none of these ancient cultures tried to build a physical model based on their observations.
• Instead, they created myths to explain the motions of the objects in the sky.

See Inventing the Flat Earth by Russell

Last Lecture Question

Assuming you were living in ancient Greece, how would you convince your friend that the Earth is spherical in shape?

Aristotle

Aristotle (~350 BC) is one of the most famous of the Greek philosophers. He developed a very influential philosophy/model of the universe. It included the following ideas: -The four basic elements (Earth, water, air, fire) tend towards their “natural places”, -The Earth is a small spherical body at the center or “bottom” of a series of concentric spheres, -The lower levels are most subject to change, the highest ones are perfect and unchanging.

Sun, Moon, planets, and stars located on concentric spheres with Earth at center or “bottom”.

From a textbook by Apian (1540).

• Eratosthenes inferred that Alexandria

was 7° of latitude north of Syene.

• The distance between the two cities is

7/360 times the Earth’s circumference.

• His result of 42,000 km is very close

to the right number: 40,000 km.

Eratosthanes’ method …

Aristarchus of Samos (~250 BC)

The most original of the Greek astronomers: -Believed Earth moved around Sun, -Derived geometric methods to determine distances to Moon and Sun (though he didn’t follow through with the calculations).