5. Copernican Revolution

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Copernican RevolutionCopernican RevolutionJ.T.II OlivarJ.T.II Olivar

Faculty of Arts and LettersFaculty of Arts and Letters

University of Santo TomasUniversity of Santo Tomas11stst Semester AY 2011-2012Semester AY 2011-2012



Outline of the LectureOutline of the Lecture

The Geocentric UniverseThe Geocentric Universe

The Heliocentric Model of the SolarThe Heliocentric Model of the Solar

SystemSystem The Laws of Planetary MotionThe Laws of Planetary Motion



The Geocentric UniverseThe Geocentric Universe

To the Greeks, the universe was basicallyTo the Greeks, the universe was basically

the solar system.the solar system.

– The Sun, Earth, Moon and the planets knownThe Sun, Earth, Moon and the planets known

at that timeat that time

The Greeks did not consider the Sun, theThe Greeks did not consider the Sun, the

Moon, and the planets to be part of theMoon, and the planets to be part of the

celestial sphere.celestial sphere.



Planets do not behave in as regular andPlanets do not behave in as regular and

predictable a fashion as the Sun, Moon,predictable a fashion as the Sun, Moon,

and stars.and stars.

Unlike the Sun and the Moon, the planetsUnlike the Sun and the Moon, the planets

seem to wander around the celestialseem to wander around the celestial

sphere.sphere.

– Greek wordGreek word planetes planetes , meaning “wanderer” , meaning “wanderer”



Planets never stray far from the plane of Planets never stray far from the plane of

the ecliptic and generally traverse thethe ecliptic and generally traverse the

celestial sphere from west to east, as thecelestial sphere from west to east, as the

Sun does.Sun does.

– They seem to speed up and slow down duringThey seem to speed up and slow down during

their journeys, and at times they even appeartheir journeys, and at times they even appear

to loop back and forth relative to the starsto loop back and forth relative to the stars



Direct or Prograde, andDirect or Prograde, and

RetrogradeRetrograde Motion in the eastward sense is referredMotion in the eastward sense is referred

to as direct, or prograde motion.to as direct, or prograde motion.

The backward (westward) loops areThe backward (westward) loops are

known as retrograde.known as retrograde.



The earliest model of the solar systemThe earliest model of the solar system

followed the teachings of the Greek followed the teachings of the Greek

philosopher Aristotle (384-322 B.C.)philosopher Aristotle (384-322 B.C.)

– Geocentric in nature, meaning that Earth layGeocentric in nature, meaning that Earth lay

at the center of the universe and that all otherat the center of the universe and that all other

bodies moved around it.bodies moved around it.



– In this model, each planet was taken to moveIn this model, each planet was taken to move

uniformly around a small circle, called anuniformly around a small circle, called an

epicycleepicycle, whose center moved uniformly, whose center moved uniformly

around the Earth on a second larger circle,around the Earth on a second larger circle,known as theknown as the deferentdeferent..



Epicycle and DeferentEpicycle and Deferent



The full Ptolemaic model required a seriesThe full Ptolemaic model required a series

of no fewer than 80 distinct circles.of no fewer than 80 distinct circles.

Syntaxis or Almagest provided theSyntaxis or Almagest provided the

intellectual framework for all discussion of intellectual framework for all discussion of

the universe for well over a thousandthe universe for well over a thousand

years.years.

– The geocentric model went largelyThe geocentric model went largely

unchallenged until the sixteenth century A.D.unchallenged until the sixteenth century A.D.



The Heliocentric Model of theThe Heliocentric Model of the

Solar SystemSolar System Nicholas Copernicus rediscoveredNicholas Copernicus rediscovered

Aristarchus’s heliocentric (Sun-centered) Aristarchus’s heliocentric (Sun-centered)

model.model.

– Copernicus asserted that Earth spins on itsCopernicus asserted that Earth spins on its

axis and, like other planets, orbits the Sunaxis and, like other planets, orbits the Sun

– Only the Moon orbits EarthOnly the Moon orbits Earth



Copernican RevolutionCopernican Revolution

The critical realization that Earth is not atThe critical realization that Earth is not at

the center of the universe is now knownthe center of the universe is now known

as the Copernican revolution.as the Copernican revolution.



The Foundations of theThe Foundations of the

Copernican RevolutionCopernican Revolution The following seven points are essentiallyThe following seven points are essentially

Copernicus’s own words.Copernicus’s own words.

1.1. The celestial spheres do not have just oneThe celestial spheres do not have just one

common center.common center.

2.2. The center of Earth is not the center of theThe center of Earth is not the center of the

universe but is instead only the center of universe but is instead only the center of

gravity and of the lunar orbit.gravity and of the lunar orbit.3.3. All the spheres revolve around the Sun. All the spheres revolve around the Sun.



4.4. The ratio of Earth’s distance from the Sun toThe ratio of Earth’s distance from the Sun to

the height of the firmament is so muchthe height of the firmament is so much

smaller than the ratio of Earth’s radius to thesmaller than the ratio of Earth’s radius to the

distance to the Sun that the distance to thedistance to the Sun that the distance to theSun is imperceptible when compared withSun is imperceptible when compared with

the height of the firmament.the height of the firmament.



5.5. The motions appearing in the firmament areThe motions appearing in the firmament are

not its own motions but those of Earth. Earthnot its own motions but those of Earth. Earth

performs a daily rotation around its fixedperforms a daily rotation around its fixed

poles while the firmament remains immobilepoles while the firmament remains immobileas the highest heavens.as the highest heavens.



6.6. The motions of the Sun are not its motionThe motions of the Sun are not its motion

but the motion of Earth.but the motion of Earth.

7.7. What appears to us as retrograde andWhat appears to us as retrograde and

forward motion of the planets is not theirforward motion of the planets is not theirown but that of Earth.own but that of Earth.



Copernicus’s writings on the heliocentricCopernicus’s writings on the heliocentric

universe were placed on the Index of universe were placed on the Index of

Prohibited Books in 1616.Prohibited Books in 1616.

– 73 years after they were first published73 years after they were first published

– Remained there until the end of theRemained there until the end of the

eighteenth centuryeighteenth century



The Laws of Planetary MotionThe Laws of Planetary Motion

Kepler’s simple laws:Kepler’s simple laws:

1.1. The orbital paths of the planets are ellipticalThe orbital paths of the planets are elliptical

(not circular), with the Sun at one focus.(not circular), with the Sun at one focus.

An ellipse is a flattened circle. An ellipse is a flattened circle.



2.2. An imaginary line connecting the Sun to any An imaginary line connecting the Sun to any

planet sweeps out equal areas of the ellipseplanet sweeps out equal areas of the ellipse

in equal intervals of time.in equal intervals of time.



3.3. The square of a planet’s orbital period isThe square of a planet’s orbital period is

proportional to the cube of its semi-majorproportional to the cube of its semi-major

axis.axis.

P P 2 2 (in Earth years) =(in Earth years) = a a 3 3 (in astronomical(in astronomical

units)units)



Newton’s Laws of Motion andNewton’s Laws of Motion and

GravitationGravitation In 1684, Newton had a discussion withIn 1684, Newton had a discussion with

Edmund Halley about the leadingEdmund Halley about the leading

astronomical problem of the day.astronomical problem of the day.

– Why do planets move according to Kepler’sWhy do planets move according to Kepler’s

laws?laws?

Philosophiae Naturalis Principia Philosophiae Naturalis Principia

Mathematica (The Mathematical Principles Mathematica (The Mathematical Principles of Natural Philosophy) of Natural Philosophy)

– Basis for what is known as NewtonianBasis for what is known as Newtonian

mechanicsmechanics



11stst LawLaw

– Every body continues in a state of rest or in aEvery body continues in a state of rest or in a

state of uniform motion in straight line unlessstate of uniform motion in straight line unless

it is compelled to change that state by a forceit is compelled to change that state by a forceacting on it.acting on it.



22ndnd LawLaw

– When a forceWhen a force F F acts on abody of massacts on abody of mass m m , it, it

produces in it an accelerationproduces in it an acceleration a a equal to theequal to the

force divided by the mass.force divided by the mass. – Thus,Thus, a=F/m a=F/m , or, or F=ma F=ma ..



33rdrd LawLaw

– To every action there is an equal and oppositeTo every action there is an equal and opposite

reactionreaction

5. Copernican Revolution

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