1. History of Astronomy Enka Schools, April 24th, 2007
By Yavuz Ekşi (İTÜ)

2. Briefly… Celestial Sphere=Gökküre=Felek
Planets and Their Retrogade Motion
Aristotelian World View
The Copernican Revolution-The Earth is a Celestial Object!

3. Apparent Daily Motion
of Stars Around
the Earth
Diurnal Motion

4. Diurnal Motion
Stars move East to West as the Earth rotates West to East.
They cover 15 degrees per hour which amounts to 360 degrees per day!

5. Ancient World View
Ancient people believed that the Earth was at the center of the Universe, motionless and non-rotating.
For the ancient people the circular trajectories of the stars were not just an apparent motion.
According to them the stars were indeed rotating around the Earth.

6. Contellations
Ancient people identified patterns of stars called constellations.
This allows one to find stars easy.

8. 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.

9. Globular Cluster
These are stars physically
close to each other.

10. Constellations preserve their form during diurnal motion

11. Celestial Sphere
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 sphere.
This sphere is called the celestial sphere.

12. Measuring the distances is a hard problem of astronomy.
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.
Celestial Sphere=Gökküre=Felek (Çoğ:Eflak)

13. 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 sphere.
These are the 5 planets that can be identified by the naked eye (Mercury, Venus, Mars, Jupiter, Saturn), the Moon, and the Sun.

14. Retrogade Motion of Mars

15. Retrogade motion of Mars according to the heliocentric model

16. Retrogade motion of Mars according to the heliocentric model

17. Modern Science was born out of the human desire to explain the retrogade motion of the planets.

18. 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 week.

19. Days of the Week Saturday Saturn=dies saturni Sunday Sun=dies solis
Monday
Moon=dies lunae
Tuesday
TiwMars=dies martis
Wednesday
OdinMercury=dies Mercurii
Thursday
Tor  Jupiter=dies jovis
Friday
Frie  Venus=dies veneris
Astronomical Names for the Days of the Week, Falk, M.., 1999, J. of the Royal Astron. Soc. of Canada, Vol. 93, p.122

20. Aristoteles (M.Ö )
The Earth is a sphere at the center of a spherical universe
The Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn and fixed stars each rotate around their own spheres.
These spheres are made of crystal (so that they are not seen)

21. The 7 Spheres Above the Earth Below the Celestial Sphere
Seven perfect concentric spheres surrounded the Earth.
On each sphere was embedded one of the seven celestial
objects. The spheres rotated around the Earth at different
speeds
• The motions of the spheres affected each other producing
the strange behaviours like retrogade motion etc.
• Surrounding the seven spheres was another fixed sphere
which contained the fixed stars
• Outside this sphere was the “prime mover” which caused
the spheres to rotate
• Inside the closest sphere (which carried the moon), was
earth, air, fire and water - the four “essences”
• The spheres contained a transparent fifth essence –”the quintessence” or the “ether”

22. Felek Feleğin tekerine çomak sokmak Felekten bir gün çalmak
Feleğin çemberinden (çeperinden?) geçmek.

23. Aristoteles...
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 questioning.
The Aristotelian view (with Ptolemies contributions)
dominated Western Science for almost 2000 years until
Renaissance.

24. Aristotelian Chemistry

25. 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 (Aristotle) ( BC) had very strong influence on European philosophy and science;
everything on Earth made of (mixture of) four elements: earth, water, air, fire
every element has a “natural place”:
earth at center of Earth,
water above earth,
air above water,
fire above air;
celestial bodies (stars, planets, Moon) made from fifth element, “ether”, which also fills space between them; ether is perfect, incorruptible, weightless;
two kinds of motion of things on Earth: “natural” and “violent” motion
natural motion: things tend to move towards their natural place - natural motion happens by itself, needs no push/pull (e.g. stone falls).
violent motion: = motion contrary to natural motion; needs effort (external push or pull)
celestial motion = natural motion of ether; natural motion of bodies made from ether is circular motion, regular and perpetual

26. 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 stops.
Heavier objects fall more rapidly than the lighter objects.
Celestial objects eternally follow circular trajectories. They do not change their speed during this motion.
Each celestial object rotates around the Earth.

27. Comets According to Aristoteles
The celestial objects are eternal and the sky is not subject to change.
Hence comets must be inside the sphere of the Moon, i.e. they are atmospheric events.

28. Aristarchus of Samos (310-230 BC)
Many canturies before Aristoteles, he suggested the helicentric universe model.
His heliocentric model was not commonly accepted at that time because...

29. ...parallax can not be measure with naked eye.

30. The Heliocentric model of Aristarchus was not generally accepted because...
It is hard to accept the stars being so distant that they do not show any parallax.
If the Earth is rotating, it would be very hard for us to walk on it. We would be centrifugally expelled.
Say the Earth is rotating around the Sun, then how would the Moon be carried along with us?

31. 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 (obviously!)
So how could they explain the retrogade motion of Mars?

32. 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 orbit

33. Ptolemy=Batlamyus (Claudius Ptolemaeus-140 AD)
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 deferents.
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!

34. Geocentric model and the retrogade motion of Mars

35. Ptolemaic Model of the Universe

36. The Ptolemaic Model
Ptolemy was able to predict the motions of the seven celestial objects to great accuracy by introducing more and more epicycles.
Ptolemy’s model had been used nearly for 1500 years by the western and eastern astronomers.

37. Size of the Earth Comparable to the size of the Heavens
The rest of the Universe
was comparable to the
size of the Earth.
History of astronomy is the history of the enlarging horizons of the human kind.

38. 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!”

39. 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 change.
When Tycho Brahe was on his way home on November 11, 1572, his attention was attracted by a star in Cassiopeia which was shining at about the brightness of Jupiter and which had not been seen in this place before. Tycho reports (from Burnham's Celestial Handbook):
``On the 11th day of November in the evening after sunset, I was contemplating the stars in a clear sky. I noticed that a new and unusual star, surpassing the other stars in brilliancy, was shining almost directly above my head; and since I had, from boyhood, known all the stars of the heavens perfectly, it was quite evident to me that there had never been any star in that place of the sky, even the smallest, to say nothing of a star so conspicuous and bright as this. I wqs so astonished of this sight that I was not ashamed to doubt the trustworthyness of my own eyes. But when I observed that others, on having the place pointed out to them, could see that there was really a star there, I had no further doubts. A miracle indeed, one that has never been prevoiously seen before our time, in any age since the beginning of the world.'' Tycho was so impressed by this event that he devoted the rest of his professional life to astronomy only. Nevertheless, Tycho had not been the first to discover this "new" star; according to Burnham, it was probably first seen by W. Schuler on November 6, Tycho found it at about as brilliant as Jupiter, and it became soon equal to Venus. For about two weeks the star could be seen in daylight. At the end of November it began to fade and change color, from bright white over yellow and orange to faint reddish light, finally fading away from visibility in March, 1574, having been visible to the naked eye for about 16 months.
The remnant of this supernova had to wait for its discovery until the 1960s, when extremely faint nebulosity was identified on Mt. Palomar photo plates near the position, and gaseous remainders were identified by their radio emission; a stellar remnant has not been found. The radio source has been cataloged as 3C 10, the SNR as G in Dave Green's catalogue of galactic SNRs. The gas shell is now expanding at about 9000 km/s (compare with the Crab Nebula's about 1000 only), and has reached an apparent diameter of 3.7 arc minutes. Tycho's supernova remnant appears to be the more typical representative of these class of objects of the two.

40. Stella Nova
The image in this page is from Tycho Brahe's "Stella Nova“.

41. 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.
In 1577, Danish astronomer Tycho Brahe carefully examined the positions of a comet and the Moon against the star background. Using observations of the comet made at the same time from two different locations, Tycho noted that both observers saw the comet nearly in the same location with respect to the background stars. If the comet was closer than the moon, this would not have been the case. This so-called parallax effect can be demonstrated if you hold up a finger and look at it while closing one eye and then the other. Tycho concluded that the comet was at least six times farther away than the moon.

42. Tycho Brahe and the Crystal Spheres
Tycho Brahe also understood that the comet must have passed through the spheres.
So the crystal spheres of Aristoteles can not be real!

43. Galileo (1564-1642) The Second Scientist after Gilbert
A strong refuter of the Aristotelian world view.
When he was about 17 he discovered that hailstones reached the ground at the same time. According to the Aristotelian notion that heavy objects fall faster, this could only be possible if heavy hailstones were all coming from a higher cloud than the other. He thought that the simpler explanation is that both heavy and light hailstones were produced in the same cloud but that heavy and light objects fall at the same rate.

44. 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 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.
this was the last supernova which was definitely observed in our Milky Way Galaxy. It was discovered on October 9, 1604, when it was already brighter than all stars in the sky, by several persons including Brunowski in Prague (who notified Kepler), Altobelli in Verona, Clavius in Rome, and Capra and Marius in Padua. Kepler first saw it on October 17, and started a systematic study of the phenomenon, inspired by Tycho's work on the supernova of 1572.
Initially as bright as Mars, the supernova brightened up and surpassed Jupiter in brilliance within a few days. According to a study of Baade 1943, the peak brightness was perhaps close to magnitude It was still about as bright as Jupiter when it became invisible in twilight of November. At its reappearance in January 1605, Kepler found it still brighter than Antares, and it remained visible until March, 1606, after a naked-eye visibility of 18 months. From its light curve, it was suspected that this had been a type I supernova.

45. A Celestial Object Defected?
Directed his telescope to the sky (1609).
The Moon is not perfect! It has craters which are defects.

47. Galileo Galilei
I would rather discover a single fact, even a small one, than debate the great issues at length without discovering anything new at all. -Galileo Galilei

48. ...Galileo 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 another.

49. Galilei observed the Phases of Venus
The heliocentric model predicted that all phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun and to face away from the Earth when it was on the Earth-side of the Sun. In contrast, the geocentric model predicted that only crescent and new phases would be seen, since Venus was thought to remain between the Sun and Earth during its orbit around the Earth. Galileo's observations of the phases of Venus proved that it orbited the Sun and lent support to (but did not prove) the heliocentric model: Venus could orbit the Sun but the Sun could be orbiting the Earth with its Venus.
Heliocentric Model: All phases should be visible
Geocentric Model: Only crescent and new phases would be seen

50. The phases of Venus according to the geocentric model
Venus can only be seen just after the sun sets. This means that it must be very close to the Sun.
In order that they always remain close, their spheres must be locked to each other.
According to this model Venus can never be in an opposite position to the Sun and so show the phases that Dalileo observed.

51. Galilei and the Milky Way
As seen with
the telescope
Milky Way is a
myriad of individual stars.
All these discoveries were presented in a little book “starry messenger” in 1610.

52. The Sun also is not Perfect (Galilei 1613)
These had already been observed by another astronomers but Galileo did not know that.

53. Aristotelian Response
Aristotelians refused to accept that what was seen through the telescope was real.
Galileo himself tested the possibility by observing hundreds of objects to see if the instrument does anything except magnify.

54. 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 down.
Objects on the Earth and the objects in the sky obey the same laws.

55. New Mechanics Aristotelian view: forces cause velocity
(force necessary to maintain uniform motion).
Newtonian view: forces cause acceleration (force necessary to change motion)

56. Bessel (1838) Successfully measured the parallax of the star 61 Cygni.
This was considered as the conclusive evidence that the Earth was in motion.

57. Spectroscopy Each element has its own signature.
The light from the stars carry information about the elements in the stars and planets.
Scientists can understand which elements make up the celestial objects by looking at the spectra.

58. 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 was the final stroke to the Aristotelian world view.

59. A century of Astronomy