1

History of Astronomy - Part I

• Astronomy is a science that has truly taken shapeonly in the last couple centuries– Many advances have been made in your lifetime

• However, astronomical concepts and ideas havebeen around since the birth of man

• The next two lectures will lead us through thedevelopment of astronomy from ancient timesthrough the modern age

Ancient Astronomy• Astronomy held a special

significance for ancientcultures for differentreasons– Very important navigation

tool– Mark the seasons– Religious implications

• Many erected structures inorder to track celestialevents

• Ancient records are usedto track supernovae andcomets

Native American

Mayan

Stone Age England

Ancient Greece• The Greeks were one of the first cultures to concentrate and

debate the nature of the universe

• The matter was chiefly philosophical, but did seek to matchthe observed properties of the stars and planets with asuitable model

• The study of the universe as a whole, its structure, and itsevolution is now known as cosmology

• For the Greeks, the universe was rather smaller than today,limited only to the Earth, Moon, Sun, planets and thecelestial sphere

2

The Greek Frame of Mind

• Much of the Greek method of thinking revolvedaround philosophy instead of scientificreasoning

• Greeks valued perfection and therefore anymodel of the universe should involve theperfect shape, the circle

• Greek also had no reason to believe that theEarth was not the center of the universe.Egotistical, yes - but completely reasonable atthe time

• The only 'scientific' data they had available tothem was the motion of the Sun, Moon, andplanets, which were monitored heavily at thetime

Ptolemy ~140 AD

The Motion of the PlanetsRetrograde Motion

• A model of the universewould be very simple exceptfor the fact that the planetsundergo a looping motion intheir orbits

• Remember, in one night, allplanets still rise in the eastand set in the west

• However, if you keep track ofthe planet's position versusthe background stars night tonight, you will see the planet'move'

• The word 'planet' meanswanderer in Greek

Retrograde Motion

Jupiter and Saturn (6/2000 - 5/2001)

Ptolemaic Model

• In order to produce theretrograde motion of the planets,Ptolemy created a model withepicycles

• All the planets orbited the Earthin a perfect circle

• The planet itself made a smallerorbit centered upon the largerorbit around the Earth

• With the right timing, this modelcan reproduce the retrogrademotion seen from Earth

Deferent = larger circular orbit around EarthEpicycle = smaller circular orbit around the deferent

3

Ptolemaic Model• In Ptolemy's complete model,

each planet had its own orbitaround the Earth with its ownepicycle– By changing the period of the

orbit and the epicycle, the modelcould match observationsrelatively well

• The Sun and the Moontraveled around the Earth inperfect circles

• The entire model wascomposed of more than 80circles and was verycomplicated

Simplified Ptolemaic Model

The Ptolemaic Model Survives• Since the Ptolemaic model matched observations sufficiently

and no contrary evidence was produced, it was supportedfor nearly 1,500 years!

• After all, if the Earth was moving, shouldn't we feel it?

• Also, the Greeks were smart enough to realize that if theEarth was orbiting the Sun, it would produce stellar parallax– The Greeks didn't believe it existed because they didn't have

telescopes to observe such small variations in a star's position

• On top of all this, the Dark Ages provided relatively littleadvance in any sciences for Europe

Astronomy in the Dark Ages• While Europe was

suffering through the DarkAges, Islamic nations wereundergoing a golden ageof astronomy

• Much of the knowledge ofthe Greeks was preservedand expanded upon duringthis time

• Many of the proper namesfor stars are relics of thisera– Rigel, Betelgeuse, Vega ...

4

The Copernican Revolution

• At the end of the Dark Ages, a Polishcleric name Copernicus devised a newmodel of the universe where the Earthwas no longer at the center

• The heliocentric (Sun centered) modelplaced the Earth out of its centralposition, yet still maintained many ofthe observations we see

• The beauty in his model was itssimplicity over the Ptolemaic– Occam's Razor

• The simplest solution is the best Nicolaus Copernicus (1473-1543)

The Copernican Model

In the Copernican model, retrograde motion is anapparent effect caused by the Earth 'overtaking' anouter planet in its orbit

The Copernican Revolution

• Despite the fact that the Copernican model was a betterrepresentation of the solar system, it was not widelyaccepted

• While it did provide a much simpler description compared toPtolemy, it did not necessaryly improve the predictive powerof the model

• The religious dogma of the time insisted upon Earth beingthe center of the universe

• Copernicus published his works in Latin, which wasunreadable by the common public

5

Galileo - The Observer

• A century after Copernicus' work,other scientists began to makestrides toward popularizing theheliocentric model

• Galileo was the first to use atelescope to make detailedobservations of the sky

• Though he did not invent thetelescope, he made many workingprototypes and trained them on avariety of celestial bodies

Galileo Galilei (1564-1642)

Galileo's Observations - I• Galileo used his telescopes to make

observations of many heavenlyobjects

• The sketch to the right shows Galileo'sobservations of the moons of Jupiter

• He noticed that the position of thesefour moons changed night to night, asif they were rotating around Jupiter

• These moons now bear his name– The Galilean moons are:

• Io• Europa• Ganymede• Callisto

Galileo's Observations - II• Galileo also noticed that

Venus was not simply apoint of light, but actually adisk

• He watched Venus gothrough complete phases,just like the Moon

• This cycle of phases canonly be satisfied by theheliocentric model, not thegeocentric

The phases of Venus

6

Galileo's Observations - III• Galileo also pointed his

telescope toward the Sun– NEVER DO THIS

• He discovered that the disk ofthe Sun was not perfect andwas occasionally dotted withsmall black spots

• By making daily sketches ofthese spots, he was able todetermine that the Sun itselfwas rotating

Galileo's Conclusion• All of Galileo's observations were pointing towards

a heliocentric view of the universe

• Galileo published his observations andconclusions in multiple works, including somepublished in Italian to appeal to a wider audience

• Galileo was threatened with torture, forced todeny his beliefs in the heliocentric model, andsentenced to house arrest for the rest of his life

• The seeds of the Copernican Revolution hadbeen planted

Acceptance of the Copernican Model

• Despite the efforts ofGalileo and many others,the acceptance of a Sun-centered Solar Systemwas a very slow process

• In 1728, James Bradleydemonstrated theaberration of starlight,proving that the Earth ismoving in its orbit aroundthe Sun

• In 1838, Friedrich Besselwas the first to observestellar parallax

Aberration of starlight