Celestial Sphere

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•   Fiz463e
•   CRN 10419
•   Thursday 9-12.00 / Y5
•   There will be 1 midterm
•   Many quizes
•   Many reading material
•   Follow the website.
    Contents of Astrophysics I
• Basic concepts in astrophysics
• Astronomical distance scale
• Observational instrumentation
• Solar system (Sun, Moon, Planets,
  Comets and asteroids)
• Stellar structure and evolution
 Contents of Astrophsics II
• Compact objects (white dwarfs,
  neutron stars and black holes)
• Magnetohydrodynamics (MHD) and
  plasma physics
• Galactic dynamics
• Cosmology
• Accretion processes around compact
Lecture I-History of Astronomy
• Celestial Sphere=Gökküre=Felek (Çoğ:Eflak)
• Planets (=Gezegen=Seyyare) and Their Retrogade Motion
• Aristotelian World View
• Ptolemian Astronomy
• The Copernican Revolution-The Earth is a
  Celestial Object!
• Galileo-The First (or the second after Gilbert) Scientist
• Universal Law of Gravity
• The Chemistry of Stars-How the
  spectroscopy challenges Aristotle.
Diurnal Motion   Daily Motion
                 of Stars
                 the Earth
Stars move East
to West as the
Earth rotates
West to East.

They cover 15
degrees per hour
which amounts to
360 degrees per
Ancient World View
        • Ancient people have been
          observing the sky and were
          well aware this circular
          trajectory of the stars.
        • Ancient people believed that
          the Earth was at the center of
          the Universe, motionless and
        • For the ancient people the
          circular trajectories of the
          stars were not just an
          apparent motion but was real,
          the stars were indeed rotating
          around the Earth.
Star Trails on the Equator (Kenya)
   Constellations     • Ancient people also
                        noticed that the
Preserve Their Form     constellations (e.g.
                        Ursa Major=büyükayı)
                        preserve their form
                        during diurnal rotation,
                        hence they concluded
                      • The stars are not
                        rotating by their own
                        but they are fixed
                        onto a sphere and the
                        diurnal rotation of the
                        stars is due to the
                        rotation of this
                      • This sphere is called
                        the celestial sphere.
   Constellations                     Constellation=takımyıldız

The stars in the constellations are not physically close but
their projections onto the celestial sphere appear to be close.
Constellations help to locate objects on the sky.
Globular Cluster   These are stars physically
                   close to each other.
• Celestial Sphere
  is still a useful
  concept because
  we only see the
  projections of
  celestial objects
  on such a
  fictitious sphere.
• Measuring the
  distances is a hard
  problem of
  astronomy (see
  Next Weeks

                    Celestial Sphere=Gökküre=Felek (Çoğ:Eflak)
   Seven Objects not Fixed to
      the Celestial Sphere
• For the ancient people the celestial sphere was a
  real object!
• All stars were fixed onto this sphere but there
  were 7 objects moving independent of the celestial
• These are the 5 planets that can be identified by
  the naked eye (Mercury, Venus, Mars, Jupiter,
  Saturn), the Moon, and the Sun.
• These objects are all located along the ecliptic and
  appear to be not fixed on the celestial sphere.
Retrogade Motion of Mars
Understanding the Retrogade Motion
   Modern Science, to a large
  extend, was born out of the
  human desire to explain the
retrogade motion of the planets.
• Planetes means wanderers in Greek.
• Planet = Gezegen = Seyyare
• Notice they all carry the same meaning
  referencing their apparent wandering with
  respect to the “fixed” stars.
• For the ancient people planets were gods
  and they gave their name to each day of the
                         Days of the Week
         Saturday                                   Saturn
         Sunday                                     Sun
         Monday                                     Moon
         Tuesday                                    TiwMars
         Wednesday                                  OdinMercury
         Thursday                                   Tor  Jupiter
         Friday                                     Frie  Venus
Astronomical Names for the Days of the Week, Falk, M.., 1999, J. of the Royal Astron. Soc. of Canada, Vol. 93, p.122
    Aristoteles (M.Ö.384-322)
• The Earth is a sphere at
  the center of a spherical
• The Moon, Merkury, Venus,
  the Sun, Mars, Jupiter,
  Saturn and fixed stars each
  rotate around their own
• These spheres are made of
  crystal (so that they are
  not seen)
    The 7
  Above the
  Below the

7 katlı gök

• Feleğin tekerine çomak sokmak
• Felekten bir gün çalmak
• Feleğin çemberinden (çeperinden?)
• Dominated the philosophy all throughout
  the medieval ages.
• The research at that time simply meant
  finding out what Aristoteles said about
  that research topic. No experiment, no
There can be different models on any
observed phenomena but there is no a priori
authority accepted to be true without
question. The validity of models is checked
by experiment and/or observation whoever
the proposer of the model is. The reference
of truth is expriment. Hence we select
between the models by asking the nature.
     Aristoteles: Objects on Earth and
     Celestial Objects are composed of
            Different Elements
• Objects on Earth (everything below the sphere of
  the Moon) are a mixture of 4 elements: Earth,
  Water, Air and Fire. Such objects are subject to
  change, decay and/or death and are defected.
• Celestial Objects are made from a fifth element
  (Ether). Such objects are defectless/perfect, and
  eternal. They are not subject to any change.
• Aristoteles was a student of Platon and envisaged
  the world of ideas in the sky.
Aristoteles: Objects on the Earth and the
 Celestial Objects Obey Different Laws
• Each element has a natural place determining its natural
  motion: Earth belongs to the Earth. The natural place of Water is arround earth.
   Natural place of Air is above Earth & Water. And Fire is to be above the Air. A
   stone falls down because it belongs to the Earth. Fire tends to rise up because it
   wants to reach the greatest fire (the Sun), the bubbles in water rise up because air
   is to be above water, etc.
• Apart from the natural motions there are forced
  (violent) motions. One has to apply force in order to
  keep objects in motion: The card stops when the horse
• Heavier objects fall more rapidly than the lighter
• Celestial objects eternally follow circular trajectories.
  They do not change their speed during this motion.
• Each celestial object rotates around the Earth.
 Comets According to Aristoteles

• The celestial objects
  are eternal and the
  sky is not subject to
• Hence comets must
  be inside the sphere
  of the Moon, i.e.
  they are
  atmospheric events.
Aristarchus of Samos (310-230 BC)

• Proposed the heliocentric model
• This was not widely accepted because…
• …the parallax can not be measured with
  naked eye.
• …the rotation of the Earth would throw
  us away.
• …the Earth would leave behind the Moon
Summary: For the Ancient People...

 • Objects on the Earth and the Celestial
   Objects have different structures.
 • Celestial objects are perfect while the
   objects on the Earth are defected and are
   subject to decay.
 • Different laws in the sky and on the Earth.
 • The Earth is at the center of the Universe
 • Force is needed to keep objects in motion.
Flat Universe?
                 The Universe
                 of the
                 ancient people
                 very small
                  Though this
                  picture is
                  misleading because
                  humans have been
                  believing that the
                  Earth is spherical
                  since long time.

                  It is a 19th century
                  misconception to
                  think that the
                  ancients believed
                  the Earth was flat.
Hipparchus & Epicycles
• Retrograde motion of planets
  could be explained by a
  combination of circular motions;
• the planet moves in a small
  circle called an epicycle
• the centre of the epicycle
  moves around a larger circle
  called the deferent
• If the planet moves around the
  epicycle faster than the
  epicycle moves around the
  deferent, retrograde motion will
  occur in some regions of the
(Claudius Ptolemaeus-140AD)

• Ptolemy expanded upon Aristotles geocentric
  model to predict the motion of planets accurately.
• Following Hipparchus, he assumed that planets
  moved around circular epicycles. The centres of
  the epicycles moves around the Earth in circular
• He added a number of refinements to the old
  model to obtain better agreement with
  observations. In particular, he offset the centre of
  the deferent from the centre of the Earth!
Ptolemy’s Epicycles
Ptolemaic System
        • Mathematike Syntaxis
          (13 Volume book of
        • Arabic Scientists loved
          the book and named it
          al-kitabu-l-mijisti, i.e.
          "The Great Book"
        • Today this book is
          called “Almagest”
          because of this.
Ptolemaic Model of the
       The Ptolemaic Model

• Ptolemy was able to predict the
  motions of the seven celestial objects
  to great accuracy by introducing more
  and more epicycles (equivalent to
  Fourier analysis)
• Ptolemy’s model had been used nearly
  for 1500 years by the western and
  eastern astronomers.
  Nasir al-Din Tusi (1201–1274)
• Resolved significant
  problems in the
  Ptolemaic system by
  developing the Tusi-
  couple as an
  alternative to the
  physically problematic
  equant introduced by
• A mathematical device in
  which a small circle rotates
  inside a larger circle twice
  the radius of the smaller
• Rotations of the circles
  cause a point on the
  circumference of the
  smaller circle to oscillate
  back and forth along a
  diameter of the larger
  circle.                        Tusi's diagram of the Tusi couple
                                 (Vatican Arabic ms 319, fol. 28v; 13th. c.)
 Size of the
 to the size
   of the

The rest of the
was comparable
to the
size of the
Nicolaus Copernicus (1473-1543)

• “Who in this most
  beautiful of temple
  would put his lamp at
  a better place than
  from where it can
  illuminate them all?.
  Thus the Sun sitting as
  on a Royal throne,
  leads the surrounding
  family of stars!”
         Copernican Model

• The Sun is at the center of the
  Universe (not just the Solar System)
• The orbits are circular (and still
  there are epicycles though they are
  significantly less than the geocentric
• The crystal spheres are still there.
Tycho Brahe (1546-1601)
            • Observed a supernova
              explosion (1572).
            • He thought this was a
              new star.
            • Surprised because he
              thought the celestial
              objects are eternal
              and the sky does not
                  Stella Nova

  • The image in
    this page is
    from Tycho
    Brahe's "Stella

     Tycho Brahe and Comets
• Observed a comet
• Made collaborations with an observer at a
  different location to find that the comet
  was nearly in the same position with
  respect to the background stars for both
  observers (i.e. no parallax).
• Concluded that the comet was at least six
  times farther away than the Moon.
Tycho Brahe and the Crystal Spheres

• Tycho Brahe also understood that the
  comet must have passed through the
• So the crystal spheres of Aristoteles
  can not be real!
    Around the Same Time in
• Takiyuddin founded the Istanbul
  Observatory in Tophane (1577)
• He had similar instruments as with Tycho
  Brahe. Some of the measurements of
  Takiyuddin are even more precise.
• However, Takiyuddin wasn’t able to
  continue his observations as long as Tycho
Galileo (1564-1642)

• The Second Scientist after
• A strong refuter of the
  Aristotelian world view.
            Galilei Supernova
• In 1604 he observed the SN studied by Kepler.
• He thought this was a new star.
• The new star showed no motion accross the sky
  compared with the other stars (i.e. No parallax)
• Gave series of well recieved lectures arguing that
  it must be as far away from the Earth as the
  other stars.
• This refutes the Aristotelian notion of an
  unchanging celestial sphere.
Galileo’s Poem for the New Star

No lower than the other stars it lies
And does not move in other ways around
Than all fixed stars-nor change in sign or size.
All this is proved on the purest reasons ground;
It has no parallax for us on Earth
By reason of the sky’s enormous girth.
  A Celestial Object Defected?

• Directed his
  telescope to the sky
• The Moon is not
  perfect! It has
  craters which are
• Discovered Jupiter’s moons.
• This implies that (independent
   of whether the geoentric or
   heliocentric model is true) not
  every celestial object
  rotates around the Earth.
• This also obviates the
  argument against the
  Copernican sytem that if
  the Earth rotated around
  the Sun then the Earth and
  the Moon would get
  separated from one
Galilei observed the Phases of Venus

• Heliocentric Model: All phases should be visible
• Geocentric Model: Only crescent and new phases
  would be seen
Galilei and the Milky Way

                     As seen with
                     the telescope
                     Milky Way is a
                     myriad of
                     individual stars.
       Aristotelian Response
• Aristotelians refused to accept that what
  was seen through the telescope was real.
  (Some of them even refused to look
  through the telescope saying it is simply a
  nonsense device).
• Galileo himself tested the possibility by
  observing hundreds of objects to see if
  the instrument does anything except
The Sun also is not Perfect
      (Galilei 1613)
Galileo's thought experiments and real
 – falling bodies:
     • according to Aristoteles, heavy bodies (contain more earth element) fall
        faster than lighter bodies
     • observation: fall equally fast if they have same shape and size
     • Galilei: difference in speed of differently shaped falling bodies due to air
 – thought experiment about two falling bodies - “reductio ad absurdum”:
     • consider two bodies, one light (L), one heavy (H)
     • Aristoteles: L falls more slowly than H  L put under H should slow down fall
        of H;  H with L under it should fall more slowly than H alone; but (L + H)
        heavier than H alone  should fall faster than H alone  contradiction.
 – ball rolling on inclined plane:
     • ball rolling down inclined plane speeds up;
     • ball rolling up slows down; rate of slowing down depends on steepness of
        incline: less steep  longer distance travelled; extrapolation to zero slope of
        incline: ball will go on forever
    Achievements of Galileo
•   founder of modern science;
     – new methods introduced by Galilei include:
          • controlled experiments designed to test specific hypotheses
          • idealizations to eliminate any side effects that might obscure main
          • limiting the scope of enquiry - consider only one question at a time;
          • quantitative methods - did careful measurements of the motion of falling
     – from observations and thought experiments, generalizes to two new laws:
     – without external influence (force) acting on it, a body will not change its
        speed or direction of motion; it will stay at rest if it was at rest to begin with.
     – inertia = property of bodies that makes them obey this law, their ability to
        maintain their speed (or stay at rest)
     – if air resistance is negligible, any two objects that are dropped together will
        fall together; speed of falling independent of weight and material.
Three Quotations from Galileo
• In questions of science the
  authority of a thousand is not
  worth the humble reasoning of a
  single individual.
• Instead of philosophical arguments, I
  prefer to discover a single fact.
• What has philosophy got to do with
  measuring anything?

• If the celestial objects are not
  perfect their orbits also may not be
• Then introduce elliptic orbits into the
  Copernican system.
• Kepler’s laws for the motion of
           Newton (1687)
• Gravitational attraction is between all bodies.
• The force that keeps the Moon in orbit is the
  same force that causes the apple to fall
• Objects on the Earth and the objects in the
  sky obey the same laws.

      M m                         11
F  G 2            G  6.67 10          3   2
                                         m /(s kg)
           New Mechanics

   Aristotelian view: forces cause velocity
        (force necessary to maintain uniform motion).
   Newtonian view: forces cause acceleration
                 (force necessary to change motion)
           Bessel (1838)

• Successfully measured the parallax
  of the star 61 Cygni.
• This was considered conclusive
  evidence that the Earth was in
   Objects on the Earth and
 Celestial Objects are made of
     same kind of elements
• Spectroscopy discovered in the 19th century
• Using spectroscopy astronomers understood that
  stars are made of mainly hydrogen and some other
  elements like Carbon and Oxygen that also make
  up the Earth.
• This is the last blow to the Aristotelian World
              Short History
• Tycho Brahe supplied the accurate data about the
  motion of the planets.
• Kepler analysing the data arrived at the Kepler’s
• Newton, trying to explain Kepler’s laws found the
  general laws of motion which are not only valid in
  the solar system but can also be applied to many
  different phenomena.
• This was the physics behind the industrial
  revolution which took the Western civilization
  ahead of the Eastern world.
• The history unfolded differently in the Eastern
    The Fate of Takiyuddin’s
    Observatory in Istanbul
• Takiyuddin (1526-1580) came to Istanbul in
• Convinced the Ottoman King (III.Murat) to
  build an Observatory.
• By observations Takiyuddin was to prepare
  a “zic” in the name of the King.
• With this renewed catalogue whorshipping
  times would be more accurately
Istanbul Observatory (1577)
A comet at November 1577
       Black Death in 1578

• Gossip that all this evil arising
  because they have been observing the
  legs of the angels in the observatory
• Şeyhülislam (Ahmed Şemseddin
  Efendi): Observing the legs of the
  angels from the ground with pipes is a
  grand sin.
  Black Death and the Şeyhülislam
• Şeyhülislam:İhrac-ı Rasad meş'um perde-i esrarı
  felekiyeye küstahane itlak-ı cür'etin vehamet ve
  akibeti meczumdur. Hiçbir mülkde mübaşeret
  olunmadı ki ma'mur iken harap ve bünyan-ı devleti
  zelzelenak-i inkilab olmaya.
• Observation brings illomens. The serious danger in
  attempting to understand the secrets of the
  celestial spheres should be obvious. In no country
  where observation is conducted there has ever
  been a case that the strong state had not been
  destroyed and the structure of the state was not
   The End of the Observatory
• The king was scared and wanted the observatory
  to be broken down.
• This was done by Kılıç Ali Paşa in 1580 just after 3
  years that the observatory started working.
• The second attempt to build a telescope in
  Ottoman Empire was in 1873 afer about 300 years.
• This also was broken down by the softas in 31
  Mart event in 1909.
• Kandilli observatory was built in 1911 by Fatin
      End of the Lecture
• Be sure to read some of the material
  on the web.
• http://www.fizik.itu.edu.tr/eksiy/astr

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