Classical Greek Astronomy

Mr. CattAstronomy

Quick History

The first traces of settlement date to around 3,000 B.C,

Several major civilizations rose and fell over the centuries Minoans: 2700-1450 B.C., mainly on Crete Mycenaeans: 1550-1050 B.C, mainland Greece Classical Greece: 776-323 B.C. Macedonians: 323-88 B.C. Romans/Byzantines: 88 B.C.-1453 A.D.

Problems

The biggest problem of studying Greek astronomy is the lack of the original works

This is due to both the destruction of libraries, such as at Alexandria, and the early Christian Church's attitudes toward Greco-Roman culture

Most writings have been completely lost and are known only through writings of later authors

Since the original works no longer exist, only the general ideas, not the specific details are known

The Beginning

The earliest astronomical references in Greece were found in the works of Hesiod and Homer, from nearly 3,000 years ago.

These pieces give practical advice for navigation and farming

“When Orion and Sirius are come to the middle of the sky and the rosy fingered dawn confronts Arcturus, cut off all your grapes and bring them home with you”-Hesiod, 700 B.C.

The First Scientists

The Greeks were notable in that they were the first people that attempted to explain the world in terms of natural phenomenon, leaving out gods and myths

Obviously, many of the following theories are wrong and may seem funny today, but they were closer to explaining the world than the stories of gods used by all previous cultures

The key innovation is the Greeks' attempt to explain the world rationally

The Astronomers of Miletus

Thales

Lived 624-547 B.C. One of the legendary Seven Sages, the first

known Greek to explain the world naturally Originally a merchant who gathered knowledge

on his business travels, especially to Egypt Learned to calculate distances/heights by using

geometry, which would later factor into astronomy

Was said to have predicted a solar eclipse

Anaxamander

Lived 611-547 B.C. Said to have been a student of Thales Often called “The Father of Cosmology” Gave the first physical explanations of the

celestial realm The earth was a cylinder floating freely in space The sun, moon, and stars were fire filed wheels

with holes that allowed their light to escape

Anaximenes

Lived 585-526 B.C. Also gave mechanical explanations of the

universe Believed that the stars were like nails, fixed to

the interior of a crystalline vault surrounding the earth.

This idea would eventually grow into the celestial sphere, which would dominate astronomical thought for almost 2000 years

Pythagoras

Pythagoras

Lived 582-500 B.C. Best known for his

theorem on right triangles and his 5 perfect solids

Left no writings of his own, so it's hard to determine what ideas were his and what ideas were those of his followers,

Astronomical Innovations

Believed that the Celestial bodies revolved in circles around a hidden “central fire,” which was rendered invisible by a “counter Earth”

Believed that the Earth and all other celestial bodies were spheres

These two ideas were quickly accepted Also believed that all of the celestial motion

makes a lot of noise, but that we can't recognize it because we have no silence for comparison

The Pythagoras Model

Eudoxus

Universe Model Updated

Lived 408-355 B.C. Expanded greatly on the idea of the crystalline

sphere by using many spheres to explain the motions of celestial objects

The stars' motion was explained by the rotation of a single sphere

The sun needed two spheres, one rotates Westward once a day to account for rising/setting and the other rotates Eastward to account for motion through the Zodiac

Universe Model Updated

Planets were even more complicated in this model, each needing 4 spheres

Spheres 3 and 4 rotate in the planet's synodic period in opposite directions and slightly tilted to account for the cyclical figure eight motion

Sphere 2 rotates Eastward to explain motion through the Zodiac. This, combined with spheres 3/4, accounts for retrograde motion

Sphere 1 rotates Westward once a day What this model fails to account for is the

changing brightness of planets

The Eudoxus Model

Aristotle

Proofs of Earlier Ideas

Lived 384-322 B.C. Offered proofs of earlier ideas Earth is a sphere:

All falling objects move to the center of the Earth, which is straight down. If the Earth were a cube, this wouldn't always be true.

The Earth casts a circular shadow on the moon during an eclipse

Some stars are visible in Greece that aren't in Egypt Other celestial objects are spheres:

The shapes of the shadow on the moon in an eclipse can only be true with a spherical moon

Long Lasting Ideas

Aristotle had the best reasoned argument for a geocentric solar system

Taking the earlier idea of Empedocles that the universe is made of earth, air, fire, and water, Aristotle reasoned that since all of the heavy materials are in the Earth, it is heavy and therefore must be stationary

This reasoning helped cement the idea of an Earth-centered solar system, which would dominate for almost 2000 years

Argument for Spherical Earth

Spherical Earth

Flat Earth

Aristarchus

Astronomy and Geometry

Lived 310-230 B.C. Used geometry to calculate the distance to the

moon and the ratio of the distances between the earth and sun and earth and moon

Used sound geometry, but wrong data for the distances, which resulted in estimates that weren't even close

Once the size of the Earth was determined (later) new calculations were made that were remarkably accurate

Heliocentricism

Aristarchus was one of the few astronomers to support the idea of a sun centered solar system

Aristarchus calculated the sun to be 7 times bigger than the earth

To Aristarchus, it made no sense to place the much smaller Earth at the center

To account for the motion of the sky, Aristarchus reasoned that the motion was only apparent, caused by the Earth spinning on its axis once a day

Objections

The heliocentric model was largely rejected, the only other astronomer known to support it was Seleucus of Selucia

One objection was that, if the Earth moved, there should be a wind caused by its motion

A second was that anything dropped should land to the West of where it was dropped from as Earth rushed by during the fall

A third was that if the Earth orbited the sun, two stars should appear different distances relative to each other in different seasons (Parallax)

Parallax is the apparent shifting of nearby objects with respect to distant ones as the position of the observer changes.

Aristotle argued that the absence of parallax for the stars in the sky implied that the Earth must be at the center of the solar system.

– This is a valid scientific argument.

Objections (Parallax)

Objection (Parallax)

Eratosthenes

Earth Measured

Lived 276-195 B.C. Used geometry to calculate the circumference

of the Earth The drive to do this came from reports that the

Sun cast no shadows at noon in Syene, located in Southern Egypt, on the first day of summer

At the same time on the same day in Alexandria, located in Northern Egypt, the Sun cast shadows

The Method

Knowing the sun was directly 90 degrees overhead in Syene, Eratosthenes measured the sun's height in Alexandria, which was 82.8 degrees, meaning that the latitudes were 7.2 degrees, 1/50th of a circle in difference

He then had the distance between the two cities measured, which was done by men specially trained in pacing out the distance.

The end result came within a few percent of the Earth's actual circumference

Therefore the Earth’s circumference was about 360/7 50 times the distance between the two cities.

Knowing this distance he was able to find the Earth’s diameter.

His calculation was very close to the correct value.

Linear distance between Syene and Alexandria: ~ 574 miles

Earth Radius ~ 4,597 miles (~ 14 % too large) – better than any previous radius estimate. (Actual radius is 3,963 miles)

Measuring the Earth

Hipparchus

Antiquity's Greatest Astronomer

Lived 190-120 B.C. Is responsible for recording the first variable

star, which drove him to make a catalog of the sky, noting stars' position and brightness.

We can thank Hipparchus for magnitudes When charting positions, he noticed that all

stars were about 2 degrees off from older reference materials he was using

With this observation, Hipparchus discovered the Precession of the Equinoxes

Precession Pictured

Claudius Ptolemy

Last Great Classical Astronomer

90-168 A.D. Worked in Alexandria, Egypt His greatest accomplishment was the perfection

of the geocentric system The earlier model of Eudoxus failed to account

for the changing brightness of the planets, the fact that different planets retrograde differently, and was very complex.

Ptolemy's system was far simpler and worked even better for explaining planets' motion

Ptolemy's Universe

Geocentric with the Earth at the center, planets and sun orbit.

Each of the planets, in addition to orbiting the Earth, moves in circles, called epicycles, on their own orbits, which explains both retrograde motion and planets' changing brightness

Since the system wasn't totally accurate for predicting planetary motion, Ptolemy modified the model, moving the Earth slightly out of center

This system goes unchallenged for 1500 years.

Ptolemy's Model

An epicycle is the circular orbit of a planet, the center of which revolves around the Earth in another circle.

In Motion

The Marriage of Aristotle and Christianity

In the 13th century St. Thomas Aquinas blended the natural philosophy of Aristotle and Ptolemy’s work with Christian beliefs.

A central, unmoving Earth fit perfectly with Christian thinking and a literal interpretation of the Bible.

People during the Middle Ages placed a great reliance on authority, especially authorities of the past.