The Greeks and Other Important Discoveries

Ch. 2.2

What the Greeks Gave Us

More than any other ancient people, the Greeks left thorough written records of the heavens.

Anaximander (610-546/545 B.C.) – often called the founder of astronomy.

He theorized that the world and everything in it were derived from the apeiron (unlimited).

He rejected the notion that Earth was suspended, and believed it floated freely in space.

What the Greeks Gave Us

Anaximander believed the earth was the center of the universe.

The stars were fiery jets. The sun was a chariot wheel whose rim was

hollow and filled with fire.

Anaximenes of Miletus (active around 545 B.C.) Theorized that aer (air or vapor) was the most

basic form of matter and formed the life spirit of animals, the soul of humankind, and the divine essence of the gods.

When rarefied, aer turned to fire, and the Sun Moon, and stars were collections of rarified aer set into a great crystal hemisphere that was the sky.

Pythagoras (ca. 580 – ca. 500 B.C.)

The Pythagorean theorem isn’t his only accomplishment.

He also taught that Earth was a globe fixed within a sphere that held the stars.

Anaxagoras (ca. 500-ca. 428 B.C.)

Believed that the Sun was a large, red-hot body

The Moon was much like Earth, complete with mountains and ravines.

He theorized that solar eclipses were caused not by a dragon eating the Sun, but by the passage of the Moon between the Sun and Earth.

Aristotle (384-322 B.C.)

Formulated a picture of the solar system that put Earth at its center with all the other heavenly bodies orbiting around it. GEOCENTRIC model

The orbits were described as perfect circles.

This failed to explain the behavior of the planets, however.

Aristarchus of Samos (ca. 310-230 B.C.)

First to propose that the Earth is not at the center of the universe, but that it orbits the Sun while also rotating.

This early heliocentric theory was rejected for the geocentric theory for millennia.

The Planetes

The Greeks would watch the night sky, and observe some things they couldn’t explain.

Most of the stars moved predictably across the sky, as if fixed to something.

There were five “stars” (planets) that did not follow this regularity.

They “wandered”, thus called planetes.

The retrograde motions of Mars.A composite image created by superimposing images taken on twenty-nine different dates.

“Wanderers” (planets)

There were five “stars” (planets: Mars, Venus, Jupiter,

Saturn) that did not appear to have a fixed position.

They seemed to slow down and then speed up

They moved in a loop, both forward (prograde) and backward (retrograde)

They varied in their brightness

Claudius Ptolemaeus (Ptolemy) A.D. 127-145

Attempted to find a model of the solar system that explained the motions of the planets.

And that was testable by predicting future motion

Developed the geocentric model into an impressively complex system of “deferents” and “epicycles”.

Astronomy Takes a Break

Observations died out during the Middle Ages, but there were a few exciting discoveries.

Wow, ready for this……. Abu ‘Abd Allah Muhammad Ibn Jabir Ibn Sinan Al-battani Al-harrani As-sabi……. Phew

An Arab more commonly known as al-Battani, made important refinements to calculations of the length of the year and the seasons.

He also demonstrated that the Sun’s apogee (the farthest point from Earth) is variable.

Astronomy Takes a Break

Al-Sûfi (903-986) meaningfully discussed the comparative brilliance of the stars.

Other Arab astronomers contributed star maps and catalogues.

Many of the star names are in use today (such as Betelgeuse, Aldebaran, and Algol).