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The Early Star Gazers
Dawn of Man
50,000 – 100,000 years ago
First Homo-Sapiens appear
Brains changed very little
Hunters – more in tune with nature’s cycles
Learned night followed day
Moon waxes and wanes consistently
Appearance of certain stars marked events
Winter –Sun appeared low in the day sky
Sought safety & warmth of caves
Needed to wait three full moons
Spring – Sun appears higher in the day sky
Different stars appear
Temperature increases
Learned weather/seasonal cycle repeats
Sky –Stars/Sun seamed to circle the Earth
Perfect untouchable sky dome home of the gods?
The gods controlled their lives
Became angry – thunder/storms
Try to predict/avoid the god’s angry outbursts
Only by carefully watching the heavens above could early man
hope to understand the will of the gods and make efforts to please
them.
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The First Star Hunters
About 6000 years ago – 4000 BC
Early Babylonians carefully charted the heavens
Seeked the will and wisdom of the gods
Most powerful deity was the sun
Provider of light, Regulated the seasons, Gave life
Lesser god was the moon, changed shape
Other lights moved across the night sky – wanderers
Five planets visible
Mercury, Venus, Mars, Jupiter & Saturn
Posse’s special powers, loops controlled by
powerful gods
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Ancient sky watchers projected their heroes and demons into
the stars. Stargazers learned the planets only moved through
certain constellations
 Called the zodiac or houses
 The position of the heavenly orbs (planets, sun and moon)
foretold the future
 Astrologer-priests began to interpret the will of the
celestial gods
 Believed that the sky-gods controlled the world
 Predict the future using “Cosmic Connections”
Real - reinforced beliefs
Tides, growing seasons, weather
Non-real - not learned to completely interpret signs
Rebellions, war, sickness and death
Dawn of Civilization
3000 BC – Cycles of the heavens well known
Civilization along the Nile River –Egypt
Primarily farmers
Needed a way to predict when the Nile would flood
Sky watchers noticed that the Nile flooded when the star
Sirius rose in the East at dawn
Agricultural season begins
Used the celestial cycle to develop a 365¼-day calendar
Divided each day into 24 hours
Measured time using sundials and water clocks
Believed the stars were lamps carried by the deities
Sailboats carried the sun and moon on the celestial river
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Ancient Greeks
Babylonians – Thought gods controlled the motions of the planets
Egyptians – Thought the celestial river, Ur-nes controlled the sky
Greeks wanted to find natural causes for the celestial objects.
Thales ~ 600BC
First true astronomer – built theoretical models
Earth was a circular disk floating on water
Land born from the oceans not created by powerful gods
Predicted the eclipse of 585 B.C. – Soldiers stopped fighting
Pythagoras ~ 550 BC
Nature described by mathematics
Earth is a sphere
Planets moved independently
Planets orbit about perfect circles fit by mathematical laws
Aristotle ~ 350 BC
The universe is governed by physical laws
Promoted the Geocentric view of the universe
The moon circled the Earth as evidenced by its phases
The sun is further then the moon as evidenced by eclipses
Lunar eclipses indicate that the Earth is round
Earth was a sphere
– objects fall down
– sky appears different at other points
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Aristarchus (of Samos) ~ 280 BC
Argued for the Heliocentric view of the universe
Earth rotated on its axis in 24 hrs.
Estimate of how much further the sun is then the moon
Estimate of the relative sizes of the Earth, Sun and moon
 Earth is four times the size of the moon
 Sun is twenty times further then the moon
 Sun larger so the Earth should orbit the sun
Stars appear not to move because they are so far away
Could not explain why
 the planets moved as they did
 the planets brightness changed
Eratoshenes ~ 200 BC
Calculated the size of the Earth
Circumference
Estimated = 42000 Km
Actual = 40000 Km
Hipparchus ~ 150 BC
First to catalog stars
First to estimate brightness
Used over 200 yrs of data to measure 26,000 yr precession
Argued the Earth revolved around the sun
but could not detect parallax
Since Mercury & Venus are always observed near the sun then
they should be closer to the sun
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Alexandria
Spectacular city of marble buildings and statues
Founded by Alexander to encourage the pursuit of knowledge
Quickly became the center of commerce, culture and learning
Great library in the center
Contained ten large research halls, fountains, gardens, a
zoo and an observatory, half a million research volumes
Scholars from around the world came to explore all scientific
fields of study; physics, literature, medicine, math…
Ptolemy ~ 150 AD
Aristotle’s defender
Earth at center of universe
The heavens turned in perfect circles
Accounted for the retrograde motion of the planets
Developed the idea of epicycles and deferent
Wrote the 13 volume Almagest
Used 100’s of yrs of data stored in Alexandria
Model accepted by the church and used till the 16th century
Church declared any other belief was an unforgivable and
punishable sin
Catholic church becomes the powerful entity
Research from Alexandria being used for weapons, superstition,
and the amusement of kings
Mobs of angry citizens destroy the “Great Library”
Hypatia – A woman (property), last of the great thinkers (physics,
math, astronomy) was carried off to be skinned & burned alive
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The Dark Ages
Church gains total power
Dictates its doctrine to the masses
Learning and discovery halted
Metals could be changed in to gold
Troublemakers were excommunicated or beheaded
A Reawakening
1475 AD – The printing press is invented – exposes public to ideas
Magnetic compass – allows explorers to venture to far away places
Copernicus ~ 1500 AD
Developed the heliocentric model of the universe
Stars were globes of fire tremendously far away
Further a planet from the sun the longer it took to go around
Mercury – closest - three months
Saturn – furthest – thirty years
Occum’s Razor – To simple to be wrong
To go against the teachings of the church was dangerous
Used epicycles to account for small errors
Wrote unpublished book but word of his theories spread
“Jehovah ordered the sun, not the Earth, to stand still”
Book published the day he died
1616 –Church placed his book on the list of forbidden books
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Tycho Brahe ~ 1550
Believed that the Earth is at the center of the universe
Known for very precise measurements – one arc minute
Greatest “naked eye” observer
In 1572 observed a Super Nova – no parallax found (over night)
Can’t be a star – heavens unalterable – must be near Earth
In 1577 observed a comet – no parallax found
Observed other stars – no parallax found
Concluded Copernicus was wrong - Earth did not move
Danish King built him the “Sky Castle”
Showed that comets were outside the atmosphere
Proved that the heavens were not pristine and unchanging
The Heavenly bodies were thought to move in circles because circles
were considered to be the most perfect and harmonious shape. If a perfect
God lives in heaven with the stars and planets, then their motions must
also be perfect.
Johannes Kepler ~ 1600 AD
Eccentric – believed heavily in astrology
Mathematician and Hypochondriac
Search for a model that agreed with Tycho’s observations
Kepler’s three laws
1. Planets orbit in ellipses with the sun at one foci
2. Equal areas in equal time periods
3. Harmonic Law – p2 = a3
If p in years and a in AU
Since Mercury and Venus are always observed near the sun they must be
closer to the sun. The other planets, which could be seen at midnight,
must be further away from the sun then the Earth.
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Galileo ~ 1600 AD
First to point a telescope skyward (3X) then (30X)
Profound discoveries
1. Milky Way had many more stars in it
2. Jupiter, now a small round disk, had four orbiting moons
3. Venus had phases
4. Sun had sunspots
5. Moon covered with craters and mountains
These discoveries proved that Copernicus was right.
The Earth was not the center of all things
Even with a telescope of thirty power the stars were still just
points of light
Galileo published “The Celestial Messenger”
Galileo goes to Rome to make peace with the Church
A disastrous trip
Ordered to stop defending the “false” doctrine
Galileo writes another book discussing the theories of
Copernicus and Ptolemy in Italian (not Latin)
Church
Galileo ridicules the Church
Ordered all unsold copies sent to Rome
Printer says none left
Brings charges against Galileo - spreading “false doctrines”
1632 – Galileo stands trial for heresy
Must kneel before the court and retract his beliefs
Then placed under house arrest for life
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Newton ~ 1675
Born in the year of Galileo’s death
Wanted to understand why Kepler’s laws worked
Space is pervaded by an invisible force – Gravity
Gravity
 Seems to originate in the center of all bodies
 A central force
 Proportional to the mass of an object
 Inversely proportional to the distance
Worked with prisms and light
Invented a 35X Newtonian telescope
Free from the chromatic aberrations of the refractor
Formulated the three laws of motion
Newton’s Three Laws
1. Law of inertia
2. F = ma (a = F/m)
3. Principle of Action and reaction
Newton’s description of gravity accounts for Kepler’s laws and
explains the motions of the planets
Gm m
F 1 2
r2
The mass of an object is a measure of the amount matter in the
object.
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