We saw last time how our ‘keeping’ of time began in the ... · Solar System – the “heliocentric model”, introduced as early as 290 BC by the Greek astronomer Aristarchus

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We saw last time how our ‘keeping’ of time began in the skies –

from the daily whirl of the stars around us, to the motion of the

all-important Sun through the heavens as the year goes by,…

…punctuated by the ever-

changing, yet ever-returning

Moon.

The ease with which these

cyclic phenomena could be

observed – and their utility

in managing time – enabled

the growth and spread of

astronomy-based calendars

very early in human

cultures throughout the

ancient world.

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These calendars imprinted – literally – a cycle on the otherwise

unknown and uncertain future. Their ties to commonly visible

phenomena helped them spread widely, allowing for the

expansion of agriculture, trade, and communication between

widely separated groups across the ancient world.

Perhaps most significantly, increasingly accurate

timekeeping stimulated the growth of religion and science, as

humans moved from simply noting the time, to asking why

the heavens moved in cycles at all – though their answers

might seem odd by modern standards!

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Evidence suggests

these patterns were first

placed in the context of

a physical model of the

universe by the early

Greek culture, most

notably Aristotle (~350

BC). By 200 AD, this

had been refined by the

mathematician Ptolemy

into what would later

be called the

“geocentric model” of

the universe.

This model (also referred

to as the “Ptolemaic

model”) was built upon

the foundation of

Aristotelian physics –

four elements, with the

heavy and base Earth

resting firmly at its

center while the spheres

of the stars, planets, and

Sun (made of some

heavenly 5th element)

whirled around us.

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As Greek culture spread

over the centuries, this

model was widely

accepted. It was easy to

understand, and the

apparent central focus of

the heavens on the Earth

reinforced the importance

of humanity. The planets

and stars seemed clearly

attentive to us, monitoring

and shaping the destinies

of the central and unique

Earth.

Do you recognize any of these

ancient figures? All of them?

Why is that?

These wheels of time and

their repeating cycles

played a significant role in

many historical cultures,

reinforcing myths of

recurrence and rebirth in

the cosmos, as well as the

reliability of established

order – “the King is dead,

long live the King!” – and

the futility of change.

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“Unto the place from whence the rivers came, thither

shall they return again… The thing that hath been, it

is that which shall be; and that which is done is that

which shall be done…”

- Ecclesiastes 1:5-9

“It is indifferent to me where I am to begin, for there

shall I return again.”

- Parmenides, ca. 500BC

However, while the Earth-centered model of Ptolemy and

Aristotle seemed to explain so much, so simply, it was notoriously

clumsy in explaining the phenomenon of retrograde motion, a

complicated ‘loop’ that planets appear to make in the sky.

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Jupiter and Saturn – seen here dancing back-and-forth amongst the stars

over a roughly one-year period between June 2000 and May 2001.

This non-uniformity in the

apparent motion of planets

was modeled in the

geocentric system through

the addition of “epicycles”

to the orbit of each planet –

so that planets orbited in a

funny spiraling sort of way

around the Earth. Epicycles showing Retrograde Motion

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A ‘simpler’ model of the

Solar System – the

“heliocentric model”,

introduced as early as

290 BC by the Greek

astronomer Aristarchus –

explained this retrograde

motion, along with all of

the other apparent

motions of the stars and

planets, by imagining the

Earth itself in orbit

around the Sun.

To preview/remind you –

this is really how it works!

But such a world view

demanded that the

Earth move through

space – which certainly

doesn’t feel like it’s

happening, and would

suggest some bizarre

possibilities! Ptolemy

himself considered the

absurdity of the Earth’s

motion to be “obvious”.

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Nevertheless, by the middle

of the 16th century the

Ptolemaic model – and

indeed, the entire

Aristotelian world view –

was in crisis. Spurred by

increasingly accurate

observations of the planets,

Renaissance astronomers

had been forced to add

more and more epicycles to

the model in order to match

what the planets and stars

appeared to be doing. A clock in Strasbourg, Germany, showing

one of the great technological advancements

of the 15th Century – the minute hand!

In the early 1500’s, Nicholaus

Copernicus, a Polish cleric

and astronomer, began to look

for a way to return simplicity

and uniformity to the heavens.

In 1543, he published De

Revolutionibus Orbium

Caelestium (“Concerning the

Revolutions of the Heavenly

Spheres”), and in it revived

and put math to Aristarchus’

ancient idea of a Sun-centered

solar system.

Nicolaus Copernicus, 1473-1543

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But Copernicus’s model was

no better than the Ptolemaic

model at predicting the

motions of the planets,

required a moving Earth, and

soon met strong resistance

from the Catholic church at

the time. Partly because of

this, the new “heliocentric”

model of Copernicus was not

widely accepted.

Illustration of Tycho Brahe’s

observatory at Uraniborg.

However, Copernicus’s work

had convinced many young

intellectuals, including

Galileo Galilei. As a young

scholar he had begun

overturning Aristotelian

physics, showing that objects

in motion, instead of coming

naturally to rest, tend to stay

in motion – which suggested

that plants and animals would

not necessarily fall off of a

moving Earth. Galileo Galilei (1564-1642)

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But Galileo’s most

significant

contributions

began in 1609

when he turned a

new invention –

the “telescope” –

up to the skies and

found mountains

and valleys on the

supposedly perfect,

made-of-magic-5th-

element Moon…

…as well as four tiny worlds

orbiting around the planet Jupiter,

which proved that the Earth was not

the center of all orbits in the sky!

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These observations by Galileo, along with work by many

others, soon completely overturned the old universe of Aristotle

and Ptolemy. By 1650 most of the informed intellectuals of the

western world had shifted from believing the geocentric model

to accepting the motion of the Earth through space.

And that’s despite the fact that those motions are very fast and

very, very real – from the rotation of the Earth on its axis…

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…to its revolution around the Sun at over 100,000 km/hr!

In the 20th century this

would be expanded

even further – the

whole solar system

moves under the

gravitational influence

of the of our entire

galaxy. This collective

force drives the Solar

System in a “Galactic

orbit” at speeds near

1,000,000 km/hr!

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And there’s more – our entire galaxy is being gravitationally

pulled towards other members of our Local Group,

such as M31 – the Andromeda Galaxy…

…at speeds of almost

300,000 km/hr!

Think for a moment about

what this really means – even

in the quietest moments, you

and every single thing around

you is in a frenzy of motion at

over 300 miles per second.

Do you sense this motion?

Intuitively, we feel we ‘move

on’ in time, but that the spaces

around us – our homes, our

classrooms – do not move.

But this is untrue!

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In fact we and everything around us are all zooming

along through the great stream of our realities, never

experiencing the same time OR space twice – one cannot

move through time without moving through space.

We’ll return to this connection between space and time later when

we explore general relativity – for now, I would note what this

change in our view of the Earth’s motion did to our view of time.

For many scientists, philosophers, and artists our constant motion

into ever-new locations in the universe made it clear that old ideas

of Earth’s constancy and the soothing cycle of time were false.

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Many bold new ideas emerged in the 300 years following

Galileo’s work, including modern views on geology and

biology, but also the general conceit and wide-spread

acceptance of progressivism – the idea that we can shape the

future in a positive way and avoid repeating events of the past.

Far more than ever in

human history, we had

adopted the directional

arrow as our dominant

metaphor for time – a

historical development

that persists to this day.

While retro chic is

always fun*, it’s what’s

new that really drives

our modern cultures!

*Okay – “always fun” at Halloween parties!