Early Ideas of the Universe

“Though much of modern astronomy deals with explaining the Universe, through History astronomy

has dealt with such practical things as keeping time, marking the arrival of seasons, and predicting

eclipses of the Sun and Moon.” Pasachoff

Very, Very, Very, Early Astronomy

! Few surviving documents ! Astronomical alignments of

buildings and structures! Archeology + Astronomy! Stonehenge

! Dates back 5000 years ! 3000 B.C.! Summer Solstice – Sun

rises over the heelstone! Newgrange & Knowth! Mayan Astronomy! Native American Astronomy

Stonehenge on the Salisbury Plain in Southern England

A Mayan building faces sunset

The Perfect Shape

! “Spherical” — ancient Greeks! “Because it is the perfect shape” — Pythagorus! “Look at the shape of the shadow on the Moon

during an eclipse! ” — Aristotle ! “When you go south, you can see new stars

near the southern horizon that you couldn’t see from farther north.” — Aristotle

Greek Astronomy! Celestial Sphere! Pythagoras (about 560-480 B.C.)! Aristotle (384 - 322 B.C.)

! 55-sphere universe! Dominat for 1800 years

! Eratosthenes (276 - 195 B.C.)! Earth’s circumference

! Ptolemy (140 A.D.)! Epicycle

Early Models of Universe

! Universe is centered on Earth—geocentric! Moon, Venus, Sun, Mars, … are mounted on

separate nested, concentric spheres surrounding the central Earth.

! The spheres rotate slowly to produce observed motions.

! Different rates of rotation account for relative shifts.

! Different axes of rotation account for different paths.

The Pythagoreans! About five centuries B.C., the school founded by the Greek

mathematician, philosopher, and astronomer Pythagorasflourished in Greece.

! The Pythagoreans believed that the universe could be understood in terms of whole numbers.

! This belief stemmed from observations in music, mathematics and astronomy; vibrating strings produce harmonious tones when the ratios of the their lengths are whole numbers.

! The world could be understood through mathematics?! A new and central concept in the development of

mathematics and science.

The Pythagoreans Universe

! The Earth is perfectly spherical and heavenly bodies, also perfect spheres, moved like the Earth, around a central fire (not the sun) invisible to human eyes.

! Ten objects circled the central fire which included a counter-earth assumed to be there to account for eclipses, and because they believed the number 10 to be particularly sacred.

! This is the first coherent system in which celestial bodies move in circles, an idea that was to survive for two thousand years!

Aristotle’s Universe

Aristotle created a complex system containing 55 spheres which explained and predicted many of the observed motions of the stars and planets.

More on Aristotle•Aristotle asserts that the world did not come into being at one point, but that it has existed, unchanged, for all eternity.

•He further believed that the universe did have a center (the Earth) and its material part had an edge, which ended in the fixed star sphere.

•Beyond the sphere of the stars the universe continued into the spiritual realm where material things cannot be.

This is in conflict with the Biblical description of creation, and an enormous amount of effort was spent by the medieval

philosophers in trying to reconcile these views.

How we know the earth is round?

Eratosthenes

! Eratosthenes of Alexandria (276–195 BC)

He Measured the Circumference of the Earth With a Stick - 2,200 Years Ago!

! On the summer solstice, the noontime Sun shines straight down the wells at Syene…

! But pillars in Alexandria cast shadows at that time?! Measured the angle the Sun is south of zenith (Geometry

with pillar height and shadow length)! Angle ≈ one-fiftieth of full circle

The Size of the Earth

The Size of Earth – continued

! Therefore, Alexandria to Syene = one-fiftieth of all the way around the Earth!

! Alexandria to Syene is about 500 miles! Around the Earth

= 50 x 500 miles = 25,000 miles! Modern value: 24,906 miles! Not bad!!!

Aristarchus of Samos

! The Size of the Moon! (75 years before Eratosthenes)

! Compared the size of Earth’s shadow disk to Moon disk

! Moon diameter = 35% of Earth diameter! Actual value ≈ 27%! Again, not bad!

Is Earth at the Center of Universe?

! ... Aristarchus’ idea too revolutionary! contradicts our senses and Aristotle’s physics! 2000 years pass before accepted

! The Sun is so much bigger, it must be at the center?

! Earth must be moving around the Sun?! “But, then, the stars will appear to shift against

each other,” said his critics.! Both were correct. Evidence supporting

Aristarchus —star shifts due to parallax—was too small to measure.

General Themes: Models of Universe

! Universe is centered on Earth—geocentric! Moon, Venus, Sun, Mars, … are mounted on

separate nested, concentric spheres surrounding the central Earth

! The spheres rotate slowly to produce observed motions

! Different rates of rotation account for relative shifts

! Different axes of rotation account for different paths

Models of Universe – continued

! Retrograde motion of planets…problem?! Model can be modified by putting a

planet on two independently moving spheres.

! Differences between what models predicted and what was seen in the sky persisted.

! Need better models!

Retrograde Motion

Claudius Ptolemy

! Ptolemy of Alexandria, circa 140 AD! Built on ideas of Hipparchus, circa 150 BC! Still Earth Centered! Better accounts for retrograde motion! Ptolemy’s aetherial spheres

! Circle is perfect shape! Heavens are perfect! Music of the spheres! Seventh Heaven (Moon, Mercury, Venus, Sun,

Mars, Jupiter and Saturn and the stars)

Ptolemy’s Model! Planets move on small circles

! epicycles! Epicycles move on big circles

! deferents! Deferents are centered on a point near but

not on Earth! Adjusting sizes, rotation rates, etc. of these

circles gave reasonable agreement between model and reality.

Epicycles

Ptolemy and the Church

! Supported by the Church during the Dark Ages, Ptolemy’s model helped prevent any significant advance in astronomy for nearly a millennium!!!

Geocentric Theories…Until 1500s !!!

"The Dark Ages of Europe…not until 12 century AD that Greek books become known in Europe again.

"Arab world preserves Greek astronomy"Ptolemy great work Almagest (Greatest)

"Greek ideas integrated into Christian theology"Thomas Aquinas’s Summa Theologica (1260)

"Dante’s Inferno…circles of HELL"The rediscovery of Ptolemy’s Almagest in the 12th century stimulated interest in observational astronomy

And then…The Sun Centered Universe

"Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus. The world had scarcely become known as round and complete in itself when it was asked to waive the tremendous privilege of being the center of the universe.” Goethe

A Polish Catholic Cleric Names Nicholas Copernicus Postulates a Sun centered universe!

The End

Off to Copernicus…