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Astrophysics
Théory 12 chapters Exercises 20 min talk
on a chosen subject Exam oral, 2 questions
Final note 2/3 exam 1/3 talk
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Structure of the course
1. The birth of astronomy 2. The solar system 3.
Basic concepts in astrophysics 4. Astronomical
observations 5. Energy sources 6. Interstellar
matter and stellar birth
7. Stellar evolution 8. Life in the Universe
9. The Milky Way 10. Galaxies 11. The
extragalactic Universe 12. Cosmology
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The birth of astronomy
The Universe in prescientific civilisations
Astronomy, daughter of astrology Astronomy in
antique Greece The heliocentric world
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The Universe in prescientific civilisations
Genesis 1.14 And God said, Let there be lights
in the arch of heaven, for a division between the
day and the night, and let them be for signs, and
for marking the changes of the year, and for days
and for years 1.19 And there was evening and
there was morning, the fourth day.
The biblical world (influenced by Babylon) Flat
earth, floating on waters Firmament supported by
pillars (mountains) The whole inside waters of
heaven
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The Universe in prescientific civilisations - 2
Creation of the world in babylonian mythology The
female monsterTiamat (primordial chaos) is killed
by Marduk (god of thunder) Tiamat is cut in two
pieces one half forms earth and the other forms
heaven
The blood of Tiamats partner gives birth to
men Mens mission serve the gods
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The Universe in prescientific civilisations - 3
Creation of the world in egyptian mythology
(Heliopolis version ) Atum (primordial god) gives
birth to Chu et Tefnu Chu (god of air) and Tefnu
(goddess of humidity) give birth to twins Geb
and Nut


Chu (god of air) separates Nut (goddess of
heaven) from Geb (god of earth), ? birth of the
world as we know it
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The Universe in prescientific civilisations - 4
Magical interpretation of events Example the
daily cycle of the Sun corresponds to the trip of
the god Rê in the heaven, on his million years
boat During night, Rê enters the underworld
where he has to fight against the forces of
darkness victorious, he rises again
The unexplained phenomena are interpreted as
individual actions of deities No natural laws
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Astronomy, daughter of astrology
Heaven/sky, domain of the gods gods live in
heaven mens life is subject to the whims of
gods ? watch the sky to find signs of mens
destiny
All celestial bodies keep the same relative
positions except the sun the moon the 5
wandering stars (planets) ? their positions are
signs
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Astronomy, daughter of astrology- 2
The zodiac The ancients located positions in the
sky with respect to arbitrary stellar groupings
that seem to draw easily recognizable pictures
the constellations
The apparent motion of the Sun and planets takes
place in a zone of the celestial vault named
zodiac That zone has been divided in 12
constellations (of slightly adapted size)
corresponding to the 12 months in a year (one
month one lunar cycle)
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Astronomy, daughter of astrology- 3
Babylonian and greek astrologies For the
babylonians, the position of planets did
influence the destiny ok kings ? it was
important to predict the motion of sun, moon and
planets in order to
know their configuration at the time of kings
birth predict their future positions ? birth
of astronomy The greeks adopt the babylonian
ideas but generalize them to all people
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Astronomy, daughter of astrology- 4
Precession of the equinoxes Planet earth is not
perfectly spherical Solar attraction on the
equatorial bulge causes an oscillation of Earths
rotation axis with a period of 26000 years,
around the perpendicular to the orbital plane
(ecliptic)
The equatorial plane also rotates ? the
intersection between the equatorial plane and the
orbital plane also rotates ? the
constellations of zodiac shift by one
astrological sign every 26000 / 12 2170 years,
which is not taken into account by the astrologers
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Astronomy in ancient Greece
Thales of Miletus The first scientist born ca.
625 BC Q How is the world made?
A The first and basic principle of all things
is water (common element that can be found in
the 3 phases solid, liquid, gas) Imagines the
Earth as a disk floating on waters
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Astronomy in ancient Greece - 2
Anaximander Student of Thales, born around 610
BC Replaces the single element of Thales by the 4
elements water earth
air fire Earth is not floating on
waters but is suspended in pace,
equally distant from all things
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Astronomy in ancient Greece - 3
Plato Born around 430 BC For him, true knowledge
is acquired by reason (eye of the soul) and not
by observation (eye of the body)
Heavenly bodies must be perfect ? they must move
along perfect, immutable orbits The perfect
geometric forms and the sphere and the circle The
circular motion of heavenly bodies being perfect,
it can go on forever
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Astronomy in ancient Greece - 4
Eudoxus Student of Plato, born around 410
BC Imagines Universe as concentric spheres
(Eudoxus spheres) Earth at worlds center Each
sphere rotates with its own speed Only
approximately explains the planetary motions
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Astronomy in ancient Greece - 5
Eratosthenes Alexandria, 3rd Century
BC Determines circumference of Earth June 21st at
noon, the Sun is straight above Syene However, at
Alexandria, its rays make a 7 angle with
vertical
Distance between Alexandria and Syene 5000
stadia ? circumference of Earth
5000 360 / 7 257 000 stadia Historians
thinks that one stadium 157.5 m ?
circumference 40 500 km!
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Astronomy in ancient Greece - 6
Did Eratosthenes prove that Earth is
spherical? Eratosthenes model spherical Earth,
Sun very far away
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Astronomy in ancient Greece - 7
Alternative model flat Earth, nearby Sun d
/ D tg 7 ? D d / tg 7 40 000
stadia 6400 km
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Astronomy in ancient Greece - 8
Hipparcos (2nd Century BC) Determines Earth-Moon
distance Max. duration of lunar eclipse 2.5
h Moon synodic period 708 h 2pD/e 708/2.5
? D/e 45 2? 0.5 1/114 rad (FSun)
e 2?D d (1/45 1/114) D d D 32
d Modern value D 30 d
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Astronomy in ancient Greece - 9
Retrograde motion of planets Just like the Sun
and stars, planets rise East and set West They
seem to move slightly faster than the stars ?
their Eudoxus sphere rotates faster
However, one some occasions, a planet seems to
move more slowly ? moves back with respect to
stars retrograde motion How can it be reconciled
with uniform circular motion?
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Astronomy in ancient Greece - 10
Ptolemy Born in Alexandria around 90 AD Modifies
the Eudoxus system to explain the retrograde
motion
Each planet moves on a circle called epicycle The
centre of the epicycle moves an another circle
called deferent Earth is the center of deferent ?
reproduces the retrograde motion, with one
epicycle and one deferent for each planet
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Astronomy in ancient Greece - 11
Ptolemy (2nd act) The original Ptolemy system
does not reproduce accurately the measurements of
Hipparcos (variation of angular velocity)
? Ptolemy complexifies it to better match the
observations the deferent centre is shifted
with respect to Earth the circular motion is
uniform with respect to a point named equant,
symmetrical to Earth with respect to the deferent
centre
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Astronomy in ancient Greece - 12
Unexplained coincidences The centres of the
Mercury and Venus epicycles are on the Earth-Sun
line For Mars, Jupiter and Saturn radius
of epicycle parallel to the Earth-Sun line ?
tendancy of planets to position with respect to
the Sun
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Astronomy in ancient Greece - 13
The legacy of the Greeks recourse to reason and
not to myths or revealed truths
freedom of thought minor role of observation
they knex Earth is spherical (forgotten later)
might even have suggested Earth moved around the
Sun (Aristarchus of Samos, 3rd Century BC)
the belief into perfection of celestial
phenomena (? circular motions) any progress
of astronomy (and science in general) for more
than 1000 years ? contrasted legacy
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The heliocentric world
Nicolaus Copernicus (1473 1543) Born in Torun
in a wealthy family, studies 10 years in Italy ?
gets in contact with new ideas Back in Pologne,
canon in Frauenburg cathedral
Studies the texts of Ptolemy Builds a small
observatory in a tower Uses the same measurements
of planet positions as Ptolemy Shows that they
can be interpreted in another way
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The heliocentric world - 2
Copernicus world Central Sun Earth and other
celestial bodies (Moon excepted) revolve around
the Sun Circular orbits Simply explains the
retrograde motion Does not account accurately for
the Hipparcos measurements ? reintroduces
epicycles
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The heliocentric world - 3
How to choose between Ptolemy and Copernicus ?
In favour of Ptolemy tradition (mostly
religion) common sense if Earth moved, we
would feel it (but Nicolaus de Cusa (1450)
passenger inside a ship) lack of stellar
parallax
In favour of Copernicus simpler explanation of
retrograde motion decreasing amplitude of the
Mars Jupiter Saturn retrogradations
Ex-aequo similar complexity level similar
accuracy ( 5)
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The heliocentric world - 4
Tycho Brahe (1546 1601) Danish aristocrat,
studies philosophy at university but is mostly
interested in mathematics During an eclips, he is
strongly impressed by the fact that such events
can be predicted ? studies astronomy 1572 a
Nova is observed in the Cassiopeia constellation
Change in the heavens
? contradiction with the ideas
of the Greeks ? attempt to measure its motion
(celestial or atmospheric
phenomenon?) Lack of accuracy ? contradictory
conclusions
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The heliocentric world - 5
Tycho Brahe (2) Builds a 5½ feet sextant ? shows
that the Nova does not move ? established
reputation King Frederick II of Denmark grants
him a large sum of money the Hven island where
he builds an observatory
During 20 years, Tycho holds court at the
palace of Uranienborg carries out measurements
with an accuarcy never achieved before After the
death of Frederic II, Tychos character causes
troubles with the new king ? exiles in Prague in
1597
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The heliocentric world - 6
Johannes Kepler (1571 1630) Exiled in Prague
because of religions wars
Hired as assistant by Tycho Brahe, for analyzing
his planetary positions measurements Believed
that there was some sort of overall scheme in the
Universe Spent a large part of his life
searching for that scheme, which would reveal the
ultimate beauty of nature
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The heliocentric world - 7
First model of Kepler Based on the fact that 6
planets and 5 regular solids were known
Heliocentric The 5 regular solids fit the space
in between the 6 planetary spheres
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The heliocentric world - 8
Keplers laws (1) Analysis of Tycho measurements
gt Kepler reject both geocentrism and orbits based
on circles et discovers 2 empirical laws (1609)
1st law Planets move on elliptical orbits with
one focus of the ellips at the Sun
2nd law The line drawn from the Sun to the
planet sweeps equal areas in equal times
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The heliocentric world - 9
Keplers laws (2) Ten years later, he publishes
his 3rd law The square of the period T of a
planet is proportional to the cube of the semi
major axis a of its orbit
3rd law T2 / a3 Ct Contrary to the
models of the Greeks, the Kepler laws are based
on a careful and detailed analysis of observations
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The heliocentric world - 10
Galileo Galilei (1564 1642) Born in an
unwealthy family of italian minor nobility Reads
about the invention of the telescope, builds one
for himself and turns it toward the heavens
? discovers mountains on the Moon
sunspots phases of Venus 4 satellites of
Jupiter ? challenges for the platonician /
geocentric system
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The heliocentric world - 11
Galileos problems with catholic church Becomes a
strong advocate of the heliocentric system Gifted
writer, in italian ? popularizes that world
model Dialog concerning the two world systems
Salvatio (partisan of Copernicus)
(1630) Simplicio (partisan of the greek
system) Sagredo (the one who seeks truth) Puts
into Simplicios mouth many arguments advanced by
the pope ? Trial Galilée, old and sick, is
force to recant the heretical doctrine that the
earth is moving
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The heliocentric world - 12
Isaac Newton (1642 1727) Born in a relatively
wealthy english family Studies natural philosophy
at Cambridge university
1665 1666 epidemy of pest Newton isolates
himself in Woolsthorpe and invents or
discovers the differential and integral
calculus the theory of colours the theory of
universal gravitation
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The heliocentric world - 13
Philosophiæ Naturalis Principia Mathematica
(1687) Newton shows that the whole mécanics can
be deduced from a few basic principles
the 3 laws of motion
including the fundamental law of mecanics
F m a the law of
universal gravitation
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The birth of astronomy
The Universe in prescientific civilisations
Astronomy, daughter of astrology Astronomy in
antique Greece The heliocentric world
End of chapter