Unformatted text preview: 12/24/11 Astronom Notes 3 - Histor of Astronom Motions of Planets - His tor of Science and As tronom
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2/6 12/24/11 Astronom Notes 3 - Histor of Astronom ci c la pa h? In pa , beca e he ci cle i a pe fec geome ic hape and he e a he belief ha he k
ho ld be he place of e e nal pe fec ion - onl pe fec , nchanging hing a e p he e, hile he impe fec ,
changing hing a e fo nd on he Ea h. An hing ha changed, like an hing ha g e o e ol ed, had o be
fo nd on he Ea h. The ligh in he k ne e change, o o he ho gh .
An a , hi i ha mo model make and heo i had o o k i h, and in ome ca e , hei li e
depended pon i . The belief o he p omo ion of a
em ha en con a o he e concep co ld mean
pe ec ion o dea h in ome ca e . No onde no one an ed o go again he e concep ! He e e'll look a
ome of he main a chi ec in he hi o of a onom . Some of hei idea e e ong, and ome e e igh ,
b ega dle of ha he did, he con ib ed o he o in ome a o ano he . Bab lonian As tronom - 17th Centur BC
Fi g re 2. A a cie ab e
ab e i a c , ade a Ni
e e h ce
BC, f b e
i ad a, i g f Bab
B ii hM e . iha
ica b e a i . The e
e eh (c e
I a ,b
g ag Bab
) i he
f he a e Ve
ade i he eig f
1000 ea ea ie . I age i f
he The hole o
i h he Bab lonian . The e e among he ea lie
ho eco ded he mo ion of he plane in he k and p od ced de ailed
a cha . The a e he one e ha e o hank fo he a ological ign and ha
hole b ine - ho gh he Eg p ian did al o con ib e ome of he idea . B
a ching he mo ion of he S n and he Moon, he Bab lonian e e al o able o
p edic e en
ch a eclip e . Thei main con ib ion a hei acc a e and long
e m eco ding of he mo ion of objec in he k . The e de ailed eco d o la ed
he Bab lonian and e e la e picked p b o he ci ili a ion . I g e he ge he a a d fo ho o gh eco d
keeping. Aris totle - 4th Centur BC
I age f P a a d A i
e (384 - 322 BC) f
he S a a de a
Seg a a i he Va ica , i he ai i g " Sch
f A he " b Ra hae .
Yo ma ha e hea d of Aris totle , and he a p e m ch a dabble in all
o of a ea of d , no j a onom . In pa beca e of hi ep a ion,
hi model of he co mo had a e
ong infl ence on la e a onome . Hi
infl ence a o g ea ha hi model, o a lea he ba ic concep behind i ,
la ed abo 2000 ea . A i o le did no ac all make a model, b a he
de c ibed one. A i o le p opo ed ha he mo ion in he k co ld be
e plained b ha ing 56 concen ic phe e loca ed abo he Ea h. A
implified e ion of A i o le' model i ho n in Fig e 3. The mo ion of he
phe e o ld p od ce all of he mo ion of he S n, Moon and plane .
Since a o a ing phe e o ld make a ci c la mo ion, ha a oka i h he
a mp ion of ha ing ci c la mo ion and he model a geocen ic, o ha
a oka a ell. The comple mo ion e e e plained b ha ing he phe e
www.uni.edu/morgans/astro/course/Notes/section1/new3.html 3/6 12/24/11 Astronom Notes 3 - Histor of Astronom mo ing one ano he . One phe e o ld b again ano he and mo e i in a ce ain di ec ion, hich o ld hen
mo e ano he phe e in ano he a , and o on. No oo p eci e, b i did e plain he mo ion in a o gh o of
Fi g re 3. A i o le' model of he k , made p of man
concen ic phe e , i h he Ea h in he middle
Ano he a pec of hi model ha a la e ed b o he
a ha b ha ing he Ea h i ing fi ed in he
middle of he ola
em he e o ld be no s te llar
paralla . Pa alla i he appa en hif ing of objec ca ed
b o changing di ec ion of ie . Tha e plana ion a a
clea a m d, o le '
a li le e pe imen . Take o h mb
and hold i o a a m' leng h. Line i p i h ome di an
objec , ie ing i i h j one e e. No
i ch o e e
(b hi I mean clo e one and open he o he ). Wha
happen ? Did o h mb mo e? Didn' o hold i ead ?
Yo p obabl did, b o al o did change he di ec ion f om
hich o e e ie ing o h mb. B ie ing ome hing
f om a diffe en di ec ion (o loca ion), he alignmen of hing
change . Thi hif ing i he pa alla . See Fig e 4 fo ha ' happening. Wha doe hi ha e o do i h he idea
ha he Ea h i i ing fi ed in he cen e of he ola
em? Wha o ld happen if he Ea h did mo e? Ancien
a onome ho gh ha if he Ea h a mo ing, i o ld be like hif ing o e e - a one ime o o ld ee
a nea b a in f on of one g o p of di an a , and hen he Ea h mo ed o a diffe en poin in i o bi , o
o ld ee i in f on of a diffe en g o p of a . Thi i j like he a ha o h mb appea ed o mo e
hen o changed o ie . Ancien a onome co ld no ee an pa alla mo ion of he a ( ella
pa alla ). The ho gh i a d e o he fac ha he Ea h a n' mo ing - no mo ion, no hif in pe pec i e, no
ob e ed ella pa alla . Unfo na el , he e i ano he e plana ion fo he lack of ob e ed pa alla ; can o
hink of i ? The ancien g didn' b
e'll ge o i la e .
Fi g re 4. Pa alla demon a ion. Each ob e e
(A, B, C) ee a dif f e en alignmen of he olf and
he di an cl mp of ee . Thi pe of change in
ho he o ld look ba ed pon he e o a e
loca ed a one of he ea on people did no
belie e he Ea h mo ed ( ince he co ld ee no
change in he ela i e po i ion of a ). Hipparchus - 2nd Centur BC
Hippa ch (c. 190 BC - c. 120 BC) mea ing he
po i ion of he a
Thi i one g mo people fo ge abo , b he ho ld
no be igno ed. People e e ing o fig e o he
mo ion of he plane and he e e ha ing a eall ha d
ime ge ing he plane ' mo ion fig ed o b onl ing
www.uni.edu/morgans/astro/course/Notes/section1/new3.html 4/6 12/24/11 Astronom Notes 3 - Histor of Astronom those dang circles - they just weren't flexible enough to reproduce all the weird planetary motions. Astronomers
had been observing the motions of the planets for centuries and knew that they had really tricky motions to figure
out, like the way that they sometimes go backwards (re trograde ) or just move at a different rate in the sky
relative to the stars. Earlier astronomers came up with a multi- circle system known as the e pic cle and de fe re nt
that caused the objects to move in loops, however models based upon those produced motions that were very
steady and uniform, unlike the observed motions in the sky. Hipparchus improved upon the basic epicycle and
deferent model by introducing what is called the e cce ntric. This is basically saying that the Earth is not at the
center of the deferent, but is a bit off of the center. Think about how motions would look if you are in the middle
of a circular race track. If the cars are moving around at a steady pace, then all of the motions would look
smooth. Now if you were to stand near the edge of the track, the motions would not be steady as you see them cars would zoom past you and then appear to move very slowly when they are the other side of the track. This
sort of "speed up" - "slow down" motion is what we see in planets and by placing the Earth a bit out of the
center, Hipparchus was able to reproduce some of this irregular motion. Remember, the actual speeds of the
planets can not change, but our view of their motion can be influenced by our location. This is shown in Figure 5.
Fi g re 5. The orbit of t he planet is considered eccent ric, since t he cent er of t he orbit and t he Eart h's
locat ion are not t he same place. Since t he obj ect is not mov ing at a unif orm pace as seen f rom t he Eart h,
we would see it changing speed as it went along.
The epicycle and deferent are a way to explain the retrograde motions of the outer planets. The deferent is the
large circle that is also eccentric (Earth not in the middle), and upon this large circle, the epicycle is locatd. The
planet is found on the epicycle. The planet generally moves in one direction (eastward) most of the time mainly
due to the motion of the deferent, except when the epicycle is turned so the planet moves in the opposite
direction (westward or retrograde). The entire arrangement is shown in Figure 6.
Fi g re 6. Hipparchus's earl model, wit h t he t hree main aspect s all shown - eccent ric, def erent and
epic cle. Most of t he t ime t he planet mov es in an east ward direct ion, but on occasion it mov es in t he
opposit e direct ion (west ward).
All of Hipparchus s fixes worked pretty good at explaining the general motions of the planets, though they were
still not as precise at predicting the motions of the planets - more work needed to be done. Claudius Ptolem - 2nd Centur AD
Claudius Pt olem (c. 87 - c. 165) in a woodcut made long af t er his deat h.
The next person in the story was probably better able to test his model than others who came before him. Since
Ptole m lived in Alexandria, Egypt and he was a citizen of the Roman Empire, he was able to do much more
than those before him. One thing that the Romans did when they conquered other people was to take all of the
knowledge (scrolls, records, philosophy, art, etc.) back with them. A large amount of this information was
placed into the great library of Alexandria. Ptolemy had access to hundreds of years of astronomical data,
previous astronomers' theories, and their philosophies, with which he could work. Ptolemy, unlike some earlier
astronomers, was interested in making a very accurate mathematical, working model of the sky. He could work
on his model by testing its results against the years of observational data that were available in the library.
One of the tricky points that Ptolemy tackled was the retrograde motion of the outer planets. He was able to
www.uni.edu/morgans/astro/course/Notes/section1/new3.html 5/6 12/24/11 Astronom Notes 3 - Histor of Astronom combine several previously proposed devices with some of his own into a system that actually worked.
Ptolemy's model was thought to be the best model out there because it produced numbers that were much more
accurate than anyone else's. His model used devices such as the the deferent and epicycle of Hipparchus and his
own idea, the e q an . An equant is sort of like an eccentric, but a bit more off center (this is shown in Figure 7).
The planet doesn't move at a uniform rate as measured from the center, but from a point off center. If you were
standing at the equant, the motion of the planet would be smooth and uniform - no speed ups or slow downs. If
you viewed the motion from any other location, the motion wasn't uniform. The Earth was also off center, so all
of this off- centerness really helped Ptolemy in getting the rate of motion correct. Of course, retrograde motion
was pretty easy to get into his model by using the epicycles and deferents that Hipparchus introduced.
Fi g e 7. Pt olem 's model of t he sk including t he equant . If ou wat ched t he planet 's mot ion f rom t he
locat ion of t he equant , it would mov e at a unif orm (st ead ) rat e. Viewing f rom ot her locat ions, lik e t he
of f cent ered Eart h, would result in a non unif orm mot ion.
Of course, the worst thing about Ptolemy s model was that it worked very good at predicting the motions of
planets - and he could test his model against all those years of observations in the library, so he could fine tune
the parameters in the model. People accepted his model because it worked so well and used all of the basic
assumptions (though he sort of fiddled with some stuff - the Earth wasn't exactly in the middle). The details of
this model and other observations of Ptolemy are included in his classic work, Al mage . This was pretty much
the main textbook of astronomy for many centuries.
Arabic ast ronomers using a v ariet of inst rument s, including ast rolabes at t he observ at or of Taqf adDin at Ist anbul in 1577. Paint ing is f rom t he book Shahinshah-nama (Hist or of t he King of Kings), an
epic poem b 'Ala ad-Din Mansur-Shira i, writ t en in honor of Sult an Murad III (reigned 1574-95).
It should be noted that the work of Ptolemy, Aristotle and all others from ancient times would have been lost if
not for Arabian and Islamic astronomers. Actually, the name of Ptolemy's book is based upon its Arabic name
(which means "the greatest"). While Europe was wallowing in the Dark Ages, astronomers in what is now the
Middle East preserved, translated and adapted many of the works of the ancient Greek www.uni.edu/morgans/astro/course/Notes/section1/new3.html 6/6 ...
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- Fall '09
- Astronomy, Planet, Ion, Al-Andalus, apparent retrograde motion, Deferent and epicycle, mo ion