ASTA01 at UTSC – Lecture 7
Chapter 3
The Origin of Modern Astronomy
-Ancient astronomy
- The Copernican revolution
- De Brahe and Kepler
- Galileo
- Newton
- Later developments
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•  Have you ever looked up at the sky and
marvelled at the multitude of tiny specks of
light shining down on you?
•  Have you tried to pick out patterns in the
night sky?
•  Have you stared at the full moon or the
sliver of a crescent moon on a dark night?
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•  Then you are following in the footsteps of
our ancient ancestors who gazed up at the
sky thousands of years ago and took the
first steps along the long road that has led
to our current knowledge of the universe.
•  As you read on, you will learn how the science
of astronomy grew out of careful observations
and gradual development of the scientific
method over thousands of years.
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•  Every culture on Earth has engaged in and
contributed to the development of
astronomy, making it a truly global effort.
•  Ancient astronomy has shaped not only our
scientific thinking but also our culture, art,
language, religions, and traditions.
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A Brief History of Ancient Astronomy
•  The origins of astronomy lay in the
curiosity of our ancestors, who were
excellent at observing and recording the
world around them and recognizing
patterns in what they observed.
•  These skills were critical for survival in a
hostile environment, but although they did not
know it, careful observation and pattern
recognition were also the first steps in what
we now call the modern scientific method.
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A Brief History of Ancient Astronomy
•  A marvelous example of ancient
observations and recordkeeping can be
found in Lascaux, France, where
Paleolithic cave paintings dating back to
15 000 BCE were discovered in 1940.
•  More than 900 images of animals show
incredible attention to detail and anatomical
precision.
•  There are also numerous images of geometric
figures.
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A Brief History of Ancient Astronomy
Cave in Lascaux, France, with a picture of Pleiades(?)
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A Brief History of Ancient Astronomy
•  Some recent scientific studies indicate that
the paintings may have been a star map or
astronomical calendar, but there is no
conclusive proof that this theory is correct.
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A Brief History of Ancient Astronomy
•  More convincing evidence of early
astronomical observations was found in
Africa, where a carved bone that is 8500
years old shows pictographs of the
crescent Moon.
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A Brief History of Ancient Astronomy
•  Ancient people of central Africa could
predict seasons from the orientation of the
horns of the crescent Moon each month
(once again a result of careful observation
and pattern recognition).
•  This allowed them to determine when to plant
seeds and grow their crops.
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A Brief History of Ancient Astronomy
•  Other ancient cultures also used
astronomical observations for
timekeeping.
•  The motions of both the Sun and the Moon
played an important role in the marking of
time, with the Sun used for predicting
seasonal changes and the lunar phases
defining monthly changes.
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A Brief History of Ancient Astronomy
•  Some of the most careful observers and
detailed record-keepers were the
Babylonians.
•  Babylon is now a city in Iraq, in
Mesopotamia (region between the two
rivers: Tigris and Euphrates)
•  The first female ‘astronomer’ recorded in
history was named in a Babylonian tablet
from 2354 BCE. En Hedu’Anna was
priestess of the Moon Goddess.
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A Brief History of Ancient Astronomy
•  Babylonian priests like En Hedu’Anna in
Mesopotamia recorded the detailed
motions of the visible planets on
thousands of tablets more than 4000 years
ago.
•  In 763 BCE, a solar eclipse was also
observed and recorded by the Babylonians.
•  Our division of circle into 360=60*60 units,
and 60’ in one degree, as well as 60” in 1’,
derives from the sexagesimal counting system
(base 60).
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A Brief History of Ancient Astronomy
•  These ancient astronomers have left their mark on
modern astronomy.
•  Our constellations of the zodiac are based on patterns
identified by the Babylonians. Some names are
Summerian
(pre-Babylonian)
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3 stars rising position on a Babylonian disk.
The Chaldean
(Babylonian)
priests-astrologers
knew the 18-year
Saros, the sun-moon
cycle.
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Venus tablets of Ammisaduqa (Babylon)
Tablet 63 of
Enuma Anu Enlili
(“at the times of [gods] Anu &
Enlil”)
The 1st significant astronomical
text in history, dating to
1582-1562 BC, life of the 1st
dynasty king Ammi-Saduqa
Records rising and setting times
of Venus over the period of 21
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years.
A Brief History of Ancient Astronomy
•  Around 4000 BCE, the Egyptians used
their observations of the periodic rising of
the star know to us as Sirius to mark the
first day of a 365-day calendar.
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Egyptian astronomy: Sirius = Isis, Sun = Ra
•  "Her majesty Isis shines into the temple on
New Years Day, and she mingles her light
with that of her father Ra on the Horizon."
Denderah temple
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A Brief History of Ancient Astronomy
•  Sirius, the brightest star in the sky, which
they identified with the fertility goddess
Isis, first appeared in the predawn sky
each year just as the Nile began its lifegiving floods, on the 35th day of the year.
•  (It disappeared on the western sky after
sunset 35 days prior to New Year)
•  While the rising of the Nile varied year to year,
Sirius appeared with perfect regularity.
•  Once again, this helped them predict planting
season and prepare for the floods, or inundations.
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A Brief History of Ancient Astronomy
•  The Egyptians were also the first to divide
the night and day into 12 hours, using the
rising of bright stars (later called decans)
during summer nights.
•  Our modern 24-hour clock owes its origins to
the ancient Egyptians.
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A Brief History of Ancient Astronomy
•  Pyramids were aligned towards the pole star,
which, because of the precession of the
equinoxes, was at that time Thuban, a faint
star in the constellation of Draco
•  Temple of Amun-Re at Karnak, taking into
account the change over time of the obliquity
of the ecliptic, was aligned on the rising of the
midwinter sun
•  Pyramids were sort-of launchers of souls of
dead pharaohs into the afterworld through the
tunnel at the celestial pole – corridors/shafts
where oriented toward Thuban (serpent in
Arabic).
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A Brief History of Ancient Astronomy
•  A 4000-year-old stone circle in Scotland
marks the rise and set of the Moon during
a phenomenon called the lunar standstill,
which occurs every 18.6 years due to the
precession of the Moon’s orbital plane.
•  During this time, the most northerly and most
southerly rising and setting of the Moon occur
every month.
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A Brief History of Ancient Astronomy
•  It is amazing that the builders of the stone
circles in Scotland, as well as astronomers
in other ancient civilizations, appeared to
have been aware of this long-term cycle,
which required careful observations over
many years.
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A Brief History of Ancient Astronomy
•  Stonehenge, Scotland, ca. 2400 BCE
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A Brief History of Ancient Astronomy
•  At the famous monument at Stonehenge,
England, completed around 1550 BCE,
giant boulders are aligned to mark the
rising and setting points of the Sun at the
solstices.
•  The site may have served as an observatory
as well as a sacred ceremonial ground.
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A Brief History of Ancient Astronomy
•  The Mayans also built massive ancient
structures, such as the Templo Mayor, to
mark seasonal events.
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A Brief History of Ancient Astronomy
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A Brief History of Ancient Astronomy
•  In 1400 BCE, an exciting event was
observed and recorded in China – the
sudden brightening and dimming of a
“guest star.”
•  Although they did not know it at the time, the
Chinese had made the earliest known record
of a supernova explosion.
•  The Chinese also recorded solar and lunar
eclipses continuously from the fifth century BCE!
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A Brief History of Ancient Astronomy
•  In 1054 CE another powerful supernova
explosion was recorded by the Chinese.
•  This spectacular event also may have been
depicted in a famous Anasazi rock painting on
the other side of the world, in New Mexico,
USA.
•  The supernova explosion was powerful enough to
be brighter than Venus and visible during the day
for 23 days!
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A Brief History of Ancient Astronomy
•  Today, modern telescopes have been used to
capture stunning images of the Crab nebula,
which is the remnant of the supernova explosion
of 1054 CE.
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A Brief History of Ancient Astronomy
•  The observed connection between the
planting seasons and the position of
celestial objects led to the development of
religions centred on the Sun, the Moon,
and other celestial objects that were
personified as deities.
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A Brief History of Ancient Astronomy
•  Various ceremonies were designed to
please celestial beings, and prayers and
rituals to mark important seasonal
changes were performed.
•  Many festivals and ceremonies still celebrated
in various parts of the world today have their
roots in these ancient practices.
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A Brief History of Ancient Astronomy
•  Priests became important members of
society thanks to their ability to predict
celestial events, which was viewed as a
connection to heavenly deities.
•  The association of celestial objects with one
or more gods led to the idea that these gods
could affect individual human lives.
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A Brief History of Ancient Astronomy
•  This was the birth of astrology – the
search for influences on human lives
based on the positions of planets and
stars in the sky.
•  However, numerous scientific tests have
shown that astrological predictions are no
more accurate than we should expect from
pure chance.
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ASTROLOGY
•  If you were take only one thing from this
lecture, it would be that we are dealing in
ASTA01 with astronomy not astrology.
•  Astrology is guessing the best time to do important
things (electoral astrology), or a persons character or
future (natal astrology), or finding an answer to a
question (mundane a.) from the positions and patterns
of planets in the sky. It is non-scientific and unproven,
but popular, partly because horoscopes and
predictions sell well. In the past, it was used in place of
today’s medicine and meteorology as well.
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A Brief History of Ancient Astronomy
•  The days of the week were named for the Sun, the
Moon, and the five planets visible to the naked eye.
•
•
•
•
•
•
•
Mone’s dag -> Monday
Tyr’s dag  Tuesday
Odin’s dag  Wedneday
Thor’s dag  Thursday
Freya dag  Friday
Saturn’s day  Saturday
Sun’s day  Sunday
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A Brief History of Ancient Astronomy
•  Astronomical observations benefited and
impacted ancient societies in many ways,
including timekeeping, efficient agricultural
practices, navigation, and the
development of religious and ceremonial
practices.
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A Brief History of Ancient Astronomy
•  Thus far astronomy had only involved
making observations, recognizing basic
patterns, and making rudimentary
predictions.
•  A crucial part of the scientific method –
building and testing models and hypotheses –
was missing.
•  This was about to change in Greece.
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The Geocentric Model of the Universe
•  The ancient Greeks were
interested in building models of
nature based on reasoning and
observation.
•  Thales of Miletus, an influential
scientist and mathematician,
assumed that the world was
understandable and attempted to
create models to explain major
events in the universe.
•  It was said that he correctly
predicted a solar eclipse in 585
BCE.
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Early materialist philosophers: Leucippus and Democritus
Some of the earliest recorded physics
was very far-sighted &
essentially correct!
Democritus predicted: evolution (formation/decay), role of
disks, and diversity of “worlds”=planets.
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Antique theory #1: plurality of worlds
Kosmos: unique or multiple (infinite in number?)
Greek atomists Leucippus and Democritus considered the world
built of the same (`solar abundance') atomic matter that forms
the Earth, subject to constant motion through vacuum, collision,
and coalescence (accretion).
Ancient atomists wrote about what we now call the solar nebula:
The worlds come into being as follows: many bodies of all
sorts and shapes move from the infinite into a great void;
they come together there and produce a single whirl, in
which, colliding with one another and revolving in all manner
of ways, they begin to separate like to like.
Leucippus (480-420(?) B.C.), after Diogenes Laertios (3rd c. A.D.)
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This following predictions make anticipate planets around pulsars
and binary stars; evolutionary aspect stressed; hot planets.
In some worlds there is no Sun and Moon, in others they
are larger than in our world, and in others more numerous.
In some parts there are more worlds, in others fewer (...);
in some parts they are arising, in others failing. There are
some worlds devoid of living creatures or plants or any
moisture.
Democritus (ca. 460-370 B.C.), after Hyppolytus (3rd cent. A.D.)
Plurality and diversity of planetary systems reaffirmed:
There are infinite worlds both like and unlike this world of
ours. For the atoms being infinite in number, as was
already proven, (...) there nowhere exists an obstacle to
the infinite number od worlds.
Epicurus (341-270 B.C.)
Similar writings by Lucretius (ca. 99-55 B.C.).
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Antique theory #2: a unique terrestrial system
The atomist system was eclipsed by a cohesive system of
Aristotle, (384-322 B.C.), a student of Plato and tutor of
Alexander the Great. Aristotle was not very interested in
extrasolar planetary systems or their formation, or other
unobservable things. But (unfortunately) he was extremely
influential after 1.5*103 yrs. His world was geocentric,
unchanging and unique.
The four elements moved each to their 'natural place' with
respect to the center of the world. The existence of many such
centers was unthinkable:
There cannot be more worlds than one.
Aristotle
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The Geocentric Model of the Universe
•  In 500 BCE, Pythagoras suggested that
the Earth was a sphere and not flat, as
had been previously assumed.
•  His model was primarily based on the widely
held belief that the sphere is an object of
geometrical perfection.
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The Geocentric Model of the Universe
•  Based on this spherical model,
Eratosthenes was able to calculate the
Earth’s circumference by observing the
position of the Sun at noon in two different
cities on the first day of summer.
•  Incredibly, his estimate was accurate to within
a few percent of the currently known value.
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The Geocentric Model of the Universe
•  The two great authorities of Greek
astronomy were the brilliant philosopher
Aristotle and a later follower of Aristotle’s
principles, Claudius Ptolemy (pronounced
TAHL-eh-mee; the ‘P’ is silent).
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The Geocentric Model of the Universe
•  Ancient Greek philosophers and
astronomers accepted without question
that heavenly objects must move on
circular paths at constant speeds, and that
Earth is motionless at the centre of the
universe.
•  This geocentric (Earth at the centre) model
was championed by Aristotle.
•  Although a few ancient writers mentioned the
possibility that Earth might move, most of them did
so in order to point out how that idea is “obviously”
wrong.
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The Geocentric Model of the Universe
•  As viewed by you from Earth, the planets
seem to follow complicated paths in the
sky, including episodes of “backward”
motion that are difficult to explain in terms
of motion on circular paths at constant
speeds.
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The Geocentric Model of the Universe
•  You can see how Ptolemy created an
elaborate geometrical and mathematical
model to explain details of the observed
motions of the planets while assuming
Earth is motionless at the centre of the
universe.
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The Geocentric Model of the Universe
•  Aristotle lived in Greece from 384 to
322 BCE.
•  He believed as a first principle that the
heavens were perfect.
•  Because the sphere and circle were considered
the only perfect geometrical figures, Aristotle also
believed that all motion in the perfect heavens
must be caused by the rotation of spheres carrying
objects around in uniform circular motion.
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The Geocentric Model of the Universe
•  Aristotle’s writings became so famous that
he was known throughout the Middle Ages
as “The Philosopher,” and the geocentric
universe of nested spheres that he
devised dominated astronomy.
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The Geocentric Model of the Universe
•  Claudius Ptolemy, a mathematician who
lived roughly 500 years after Aristotle,
believed in the basic ideas of Aristotle’s
universe but was interested in practical
rather than philosophical questions.
•  For Ptolemy, first principles took second place
to accuracy.
•  He set about making an accurate mathematical
description of the motions of the planets.
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The Geocentric Model of the Universe
•  Ptolemy weakened the first principles of
Aristotle by moving Earth a little off-centre
in the model and inventing a way to
slightly vary the planets’ speeds.
•  This made his model (published around
140 CE) a better match to the observed
motions.
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The Geocentric Model of the Universe
•  Ptolemy’s model could thus handle the
complicated retrograde motion of the
planets.
•  Aristotle’s universe, as embodied in the
mathematics of Ptolemy’s model, dominated
ancient astronomy for almost 1500 years.
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The Geocentric Model of the Universe
•  One of Aristotle’s students was the military
leader Alexander the Great.
•  Although he used military force to conquer
much of the Middle East all the way to India,
he also promoted science and encouraged
learning.
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The Geocentric Model of the Universe
•  He founded the city of Alexandria in Egypt,
on the delta of Nile river, renowned for its
great library that eventually gathered
500,000 papyrus scrolls (books) and served
as a major centre of knowledge for
hundreds of years.
•  One of the most famous library scholars was a
female astronomer and mathematician named
Hypatia, whom we would today call the director
of the observatory in Alexandria.
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The Geocentric Model of the Universe
•  Hypatia was a well-respected scientist and
teacher, and wrote several books on
algebra, geometry, and astronomy.
•  She may have also edited past work by
Ptolemy.
•  She was eventually killed by religious fanatics
in 415 during a highly turbulent time of
political struggle between the secular Roman
governor Orestes and Christian bishop Cyril.
Shortly afterwards the library of Alexandria
was destroyed.
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The Geocentric Model of the Universe
•  Despite the library’s destruction, Greek
knowledge was not lost, but was
preserved in the Islamic world.
•  Caliph Al-Ma’mun’s House of Wisdom in
Baghdad was a great centre of learning
around 800 CE.
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The Geocentric Model of the Universe
•  Many ancient texts by scientists and
mathematicians from India were also
translated into Arabic during this time.
•  Foremost among these was Aryabhatiya, by the
Indian astronomer and mathematician
Aryabhata.
•  In it, a planetary model was described that
was geocentric but included the possibility of
the Earth spinning on its own axis.
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The Geocentric Model of the Universe
•  Building on ancient texts from Greece,
India, China, and Babylon, Arab scientists
made many advances in mathematics and
astronomy.
•  The Greek geocentric models were expanded
and corrected, and many new astronomical
observations were made.
•  The names of numerous stars today, such as Algol
or Deneb, have their roots in this golden age of
astronomy.
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The Geocentric Model of the Universe
•  Although the Ptolemaic geocentric model
was widely accepted, some philosophers –
such as Abu Rayhan al-Biruni – started
thinking about arguments for a heliocentric
model.
•  It is likely that Arab astronomers were aware
of the heliocentric model proposed by the
Greek philosopher Aristarchus in 270 BCE.
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The Geocentric Model of the Universe
•  Meanwhile, in India, Nilakantha Somayaji
developed a partially heliocentric model
that included elliptical orbits and the
Earth’s rotation.
•  After the fall of Constantinople (now: Istanbul)
in 1453, many scholars travelled to Europe,
carrying with them knowledge that contributed
to the birth of the Renaissance.
•  The stage was set for a revolution in scientific
thought.
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Medieval theories: The pendulum starts swinging
Aristotle's work rediscovered in 13th century, starts
Renaissance in Europe. For 100 years everybody agrees with
him on most issues.
Roger Bacon (1214-1292)
at Oxford cites the argument about the impossibility of vacuum
between the planetary systems. A similar thinking prevailed at
other rising universities, like Paris.
But the Aristotelian insistence on unity and uniqueness begun to
contradict the Christian doctrine of the time.
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The Church mandates(!) the many-worlds view in 13th century
In 1277 Etienne Tempier, the bishop of Paris,
condemned opinions based on 219
statements in Aristotelian writings, among
them "that the First Cause cannot make many
worlds".
The many-worlds opinion was hotly contested at the
universities but prevailed, as was mandated by the
Church under the threat of excommunication.
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