History of Astronomy

• Enka Schools, April 24th, 2007• By Yavuz Ekşi (İTÜ)• http://www.fizik.itu.edu.tr/eksiy/

Briefly…

• Celestial Sphere=Gökküre=Felek

• Planets and Their Retrogade Motion

• Aristotelian World View• The Copernican Revolution-The

Earth is a Celestial Object!

Diurnal MotionApparent Daily Motion of Stars Around 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 day!

Diurnal Motion

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.

Contellations

• Ancient people identified patterns of stars called constellations.

• This allows one to find stars easy.

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.

Constellations preserve their form during diurnal motion

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.

• 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)

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.

Retrogade Motion of Mars

Retrogade motion of

Mars according to

the heliocentric

model

Retrogade motion of Mars

according to the

heliocentric model

http://faculty.fullerton.edu/cmcconnell/Planets.html

Modern Science was born out of the human desire to

explain the retrogade motion of the planets.

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.

Days of the Week

Saturday Saturn=dies saturni

Sunday Sun=dies solis

Monday Moon=dies lunae

Tuesday TiwMars=dies martis

Wednesday OdinMercury=dies Mercurii

Thursday Tor Jupiter=dies jovis

Friday Frie Venus=dies veneris

http://en.wikipedia.org/wiki/Days_of_the_weekAstronomical 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 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)

The 7 Spheres

Above the Earth

Below the Celestial Sphere

Felek

• Feleğin tekerine çomak sokmak• Felekten bir gün çalmak• Feleğin çemberinden (çeperinden?)

geçmek.

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.

Aristotelian Chemistry

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: 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.

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.

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...

...parallax can not be measure

with naked eye.

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?

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?

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

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!

Geocentric model and the

retrogade motion of Mars

Ptolemaic Model of the Universe

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.

The rest of the Universe was comparable to the size of the Earth.

Size of the Earth

Comparable to the

size of the Heavens

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!”

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.

Stella Nova

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

http://www.texts.dnlb.dk/DeNovaStella/Index.html

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 spheres.

• So the crystal spheres of Aristoteles can not be real!

Galileo (1564-1642)

• The Second Scientist after Gilbert

• 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 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.

A Celestial Object Defected?

• Directed his telescope to the sky (1609).

• The Moon is not perfect! It has craters which are defects.

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

...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.

Galilei observed the Phases of Venus

• Heliocentric Model: All phases should be visible• Geocentric Model: Only crescent and new phases

would be seen

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.

Galilei and the Milky Way

As seen withthe telescopeMilky Way is amyriad of individual stars.

The Sun also is not Perfect (Galilei 1613)

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.

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.

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mMGF

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 as the conclusive evidence that the Earth was in motion.

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.

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.

A century of Astronomy