Théory: 12 chapters Exercises: ± 20 min talk on a chosen subject Exam: oral, 2 questions Final...

• Théory: 12 chapters

• Exercises: ± 20 min talk on a chosen subject

• Exam: oral, 2 questions

• Final note: 2/3 exam + 1/3 talk

Astrophysics

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

Structure of the course

7. Stellar evolution

8. Life in the Universe

9. The Milky Way

10. Galaxies

11. The extragalactic Universe

12. Cosmology

• The Universe in prescientific civilisations

• Astronomy, daughter of astrology

• Astronomy in antique Greece

• The heliocentric world

The birth of astronomy

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

The Universe in prescientific civilisations

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 Tiamat’s partner gives birth to men

Men’s mission : serve the gods

The Universe in prescientific civilisations - 2

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


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’


All celestial bodies keep the same relative positions except:

– the sun

– the moon

– the 5 wandering stars (planets)

→ their positions are signs

Astronomy, daughter of astrology

Heaven/sky, domain of the gods

• gods live in heaven

• men’s life is subject to the whims of gods

→ watch the sky to find signs of men’s destiny

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)

Astronomy, daughter of astrology- 2

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 king’s birth

– predict their future positions

→ birth of astronomy

The greeks adopt the babylonian ideas but generalize them to all people


Precession of the equinoxes

Planet earth is not perfectly spherical

Solar attraction on the equatorial bulge causes an oscillation of Earth’s 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


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

Astronomy in ancient Greece

Thales of Miletus

The ‘first scientist’ born ca. 625 BC

Q : How is the world made?

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’

Astronomy in ancient Greece - 2

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


Eudoxus

Student of Plato, born around 410 BC

Imagines Universe as concentric spheres (Eudoxus spheres)

Earth at world’s center

Each sphere rotates with its own speed

Only approximately explains the planetary motions


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!


Did Eratosthenes prove that Earth is spherical?

Eratosthenes model: spherical Earth, Sun very far away

Alexandria

Syene

→ 7°

7°

d


Alternative model: flat Earth, nearby Sun

d / D = tg 7° → D = d / tg 7° ≈ 40 000 stadia ≈ 6400 km

Alexandria Syene

7°

d

7°D


Hipparcos (2nd Century BC)

Determines Earth-Moon distance

Max. duration of lunar eclipse: 2.5 h

Moon synodic period: 708 h

2πD/e = 708/2.5 → D/e = 45

2θ = 0.5° = 1/114 rad (ΦSun)

e + 2θD = d

(1/45 + 1/114) D = d

D = 32 d

Modern value: D = 30 d

θ θ

d

D

e


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?


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

deferent

epicycle


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

deferent

epicycle

equant


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


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


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

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

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

The heliocentric world - 2

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°)


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

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


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, Tycho’s character causes troubles with the new king

→ exiles in Prague in 1597

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


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

Johannes Kepler (1571 – 1630)

Exiled in Prague because of religions wars


Heliocentric

The 5 regular solids fit the space in between the 6 planetary spheres

First model of Kepler

Based on the fact that 6 planets and 5 regular solids were known


1st law:

Planets move on elliptical orbits with one focus of the ellips at the Sun

Kepler’s laws (1)

Analysis of Tycho measurements > Kepler reject both geocentrism and orbits based on circles et discovers 2 empirical laws (1609)

2nd law:

The line drawn from the Sun to the planet sweeps equal areas in equal times

f1 f2


Kepler’s 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

b

a

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


→ discovers:

• mountains on the Moon

• sunspots

• phases of Venus

• 4 satellites of Jupiter

→ challenges for the platonician / geocentric system

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


Galileo’s 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 Simplicio’s mouth many arguments advanced by the pope

→ Trial: Galilée, old and sick, is force to recant the heretical doctrine that the earth is moving


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

Isaac Newton (1642 – 1727)

Born in a relatively wealthy english family

Studies natural philosophy at Cambridge university


• the 3 laws of motion including the fundamental law of mecanics :

F = m a

• the law of universal gravitation

Philosophiæ Naturalis Principia Mathematica (1687)

Newton shows that the whole mécanics can be deduced from a few basic principles:

221

rmm

GF


• The Universe in prescientific civilisations

• Astronomy, daughter of astrology

• Astronomy in antique Greece

• The heliocentric world

The birth of astronomy

End of chapter…

Théory: 12 chapters Exercises: ± 20 min talk on a chosen subject Exam: oral, 2 questions Final...

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