The Origin of Modern The Origin of Modern AstronomyAstronomy

Essential QuestionsEssential Questions How did the ancients describe How did the ancients describe

Earth’s place in the universe?Earth’s place in the universe? How did Copernicus revise that How did Copernicus revise that

ancient idea?ancient idea? Why was the Copernican model Why was the Copernican model

gradually accepted?gradually accepted? What were the results of What were the results of

Galileo’s observations?Galileo’s observations?

First PrinciplesFirst Principles Philosophers of the ancient world Philosophers of the ancient world

attempted to deduce the true attempted to deduce the true structure of the universe by structure of the universe by reasoning from reasoning from first principles.first principles.

A first principle was something that A first principle was something that was obviously true.was obviously true.

This is the opposite of science which This is the opposite of science which looks at evidence and then make looks at evidence and then make deductions.deductions.

For example…For example… Plato accepted that Earth Plato accepted that Earth

was the unmoving center was the unmoving center of the universe.of the universe.

Also, the heavens were Also, the heavens were perfect, and since Plato perfect, and since Plato argued that the circle was argued that the circle was the perfect geometric the perfect geometric figure, the heavens must figure, the heavens must move in move in uniform circular uniform circular motion-motion-constant speed in constant speed in a circle.a circle.

Plato 427?-347 BC

AristotleAristotle Plato’s most famous Plato’s most famous

student was Aristotlestudent was Aristotle Often simply known as Often simply known as

“The Philosopher”“The Philosopher” He advocated a He advocated a

geocentric universegeocentric universe.. i.e. the Sun, Moon, and i.e. the Sun, Moon, and

stars were carried around stars were carried around the Earth on rotating the Earth on rotating crystalline spheres.crystalline spheres.

Aristotle 384-322 BC

Claudius PtolemyClaudius Ptolemy Around 140 AD, the Greek Around 140 AD, the Greek

Ptolemy (living in Alexandria, Ptolemy (living in Alexandria, Egypt) gave mathematical Egypt) gave mathematical form to Aristotle’s geocentric form to Aristotle’s geocentric model.model.

His work is called His work is called The The AlmagestAlmagest, from the Arabic , from the Arabic Al Al MagistiMagisti (Greatest). (Greatest).

In it, he addressed the major In it, he addressed the major problem with the geocentric problem with the geocentric model.model.

Ptolemy ~100-187 BC

The biggest problem for The biggest problem for ancient astronomers…ancient astronomers…

Planetary motion was the hardest thing Planetary motion was the hardest thing to explain to the ancients. In fact, the to explain to the ancients. In fact, the word word planetplanet comes from the Greek comes from the Greek “wanderer,” referring to the motion of “wanderer,” referring to the motion of the planets against the constellations.the planets against the constellations.

The problem was planets did not move The problem was planets did not move a constant rate, they sometimes a constant rate, they sometimes stopped and moved in the opposite stopped and moved in the opposite direction before resuming normal direction before resuming normal eastward motion!eastward motion!

Retrograde MotionRetrograde Motion This backward This backward

motion of the motion of the planets is called planets is called Retrograde Motion.Retrograde Motion.

In order to preserve In order to preserve first principles, first principles, Ptolemy had to Ptolemy had to make some drastic make some drastic changes to the changes to the geocentric model.geocentric model.

The Ptolemaic ModelThe Ptolemaic Model In order to preserve uniform circular In order to preserve uniform circular

motion, Ptolemy created a motion, Ptolemy created a mathematical model in which the mathematical model in which the planet followed a small circle called planet followed a small circle called the the epicycleepicycle. .

The epicycle would slide around a The epicycle would slide around a larger circle called the larger circle called the deferentdeferent (what we would call the orbit).(what we would call the orbit).

By adjusting these two, he could By adjusting these two, he could approximate the planet’s retrograde approximate the planet’s retrograde motion.motion.

Ptolemy’s Model cont.Ptolemy’s Model cont. To adjust the speed of the planet, To adjust the speed of the planet,

Ptolemy supposed that the Earth was Ptolemy supposed that the Earth was slightly off center.slightly off center.

The planets moved such that it The planets moved such that it appeared to move at a constant rate appeared to move at a constant rate as seen from a point he called the as seen from a point he called the equantequant..

Further adjustments were made by Further adjustments were made by adding more epicycles producing a adding more epicycles producing a highly complex model.highly complex model.

Grrrr…Grrrr… Ptolemy’s crazy (and untrue) model Ptolemy’s crazy (and untrue) model

actually made decent predictions! actually made decent predictions! Although adjustments were made over Although adjustments were made over

the years, it stayed essentially intact.the years, it stayed essentially intact. Since it “wasn’t broke” no one tried to Since it “wasn’t broke” no one tried to

“fix it.”“fix it.” Ptolemy’s mathematical genius Ptolemy’s mathematical genius

prevented the advance of astronomy prevented the advance of astronomy (and physics) for nearly 1,500 years!(and physics) for nearly 1,500 years!

Finally the Renaissance!Finally the Renaissance! During the Renaissance, During the Renaissance,

a new idea was being a new idea was being batted about—a batted about—a heliocentric universeheliocentric universe..

Nicolaus Copernicus, a Nicolaus Copernicus, a Catholic monk, Catholic monk, mathematician, and mathematician, and astronomer, devised astronomer, devised such a heliocentric such a heliocentric model.model.

Copernicus 1473-1543

The Copernican ModelThe Copernican Model Copernicus and others realized that if Copernicus and others realized that if

the Sun was at the center of the the Sun was at the center of the universe, retrograde motion was very universe, retrograde motion was very easy to explain.easy to explain.

If Earth moves faster along its orbit If Earth moves faster along its orbit than the planets that are farther than the planets that are farther out…out…

Earth periodically overtakes and Earth periodically overtakes and passes these planetspasses these planets

Think of a race car passing another Think of a race car passing another on the inside lane…on the inside lane…

Follow the numbers to see why retrograde motion actually occurs.

Copernican reluctance…Copernican reluctance… Copernicus was reluctant to publish Copernicus was reluctant to publish

his work because it contradicted his work because it contradicted Church teaching.Church teaching.

In addition Martin Luther was In addition Martin Luther was running around leading to the running around leading to the Reformation—Church officials Reformation—Church officials already nervous.already nervous.

Also, he was embarrassed that his Also, he was embarrassed that his model did not yet accurately predict model did not yet accurately predict the positions of the planets.the positions of the planets.

De RevolutionibusDe Revolutionibus

Published in 1543, the same year of Published in 1543, the same year of Copernicus’s death, Copernicus’s death, De De Revolutionibus Revolutionibus was not initially was not initially accepted by many.accepted by many.

It was just so “revolutionary.”It was just so “revolutionary.” Because he kept uniform circular Because he kept uniform circular

motion, the model did not predict the motion, the model did not predict the motion of the planets well.motion of the planets well.

Yet…Yet…

The Copernican RevolutionThe Copernican Revolution The model did offer a simple The model did offer a simple

explanation of retrograde motion explanation of retrograde motion without the use of big epicycles.without the use of big epicycles.

The model was more elegant.The model was more elegant. Venus and Mercury were treated the Venus and Mercury were treated the

same as other planets.same as other planets. The velocity of each planet was related The velocity of each planet was related

to its distance from the Sun.to its distance from the Sun. Eventually, these reasons and Eventually, these reasons and

subsequent research compelled subsequent research compelled acceptance.acceptance.

Further ObservationsFurther Observations Tycho (most refer to Tycho (most refer to

him by his first name) him by his first name) was a Danish was a Danish nobleman, nobleman, mathematician, and mathematician, and astronomer.astronomer.

He built the most He built the most accurate naked-eye accurate naked-eye observatory in the observatory in the world—world—UraniborgUraniborg—in —in Hveen, Denmark.Hveen, Denmark.

Tycho Brahe 1546-1601

At At UraniborgUraniborg Tycho worked to Tycho worked to

accurately measure the accurately measure the positions of stars in order positions of stars in order to attempt to observe to attempt to observe parallaxparallax..

Parallax is the apparent Parallax is the apparent motion of an object caused motion of an object caused by the motion of the by the motion of the observer.observer.

You can observe it by You can observe it by closing one eye at a time closing one eye at a time and covering an object and covering an object with your thumb.with your thumb.

Tycho wanted to believe in a Tycho wanted to believe in a heliocentric universe.heliocentric universe.

If the Earth moved around If the Earth moved around the Sun, Tycho knew that the Sun, Tycho knew that he should observe parallax.he should observe parallax.

He build the most accurate He build the most accurate instruments available to instruments available to measure angles.measure angles.

Alas, the stars are too far Alas, the stars are too far away and he measured no away and he measured no parallax.parallax.

The Tychonic ModelThe Tychonic Model Tycho incorrectly concluded the Tycho incorrectly concluded the

Earth did not move and so created Earth did not move and so created yet another model…yet another model…

In his model, Tycho placed the Earth In his model, Tycho placed the Earth at the center. Other planets orbited at the center. Other planets orbited the Sun as the Sun orbited the Earth.the Sun as the Sun orbited the Earth.

““No worky”, but he still had the best No worky”, but he still had the best measurements available…measurements available…

Enter the great Johannes…Enter the great Johannes…

KeplerKepler Tycho needed help with Tycho needed help with

analysis of his 20 years of analysis of his 20 years of data so he hired another data so he hired another mathematician—Johannes mathematician—Johannes Kepler.Kepler.

Tycho kept much of the Tycho kept much of the data from Kepler because data from Kepler because he knew Kepler was a he knew Kepler was a Copernican and did not Copernican and did not agree with Tycho’s model.agree with Tycho’s model.

However…However…

Johannes Kepler1571-1630

Tycho admired Kepler’s skillTycho admired Kepler’s skill Upon Tycho’s death in 1601, Upon Tycho’s death in 1601,

Kepler inherited control of all Kepler inherited control of all the precious data.the precious data.

Over the next 18 years, Over the next 18 years, Kepler worked on the data Kepler worked on the data and deduced what we today and deduced what we today callcall

Kepler’s Three Laws of Kepler’s Three Laws of Planetary MotionPlanetary Motion

Circles be gone!Circles be gone! The first law of planetary motion The first law of planetary motion

does away with the old idea of does away with the old idea of perfect circles.perfect circles.

““Planets travel in ellipses with the Planets travel in ellipses with the Sun at one focus.”Sun at one focus.”

An An ellipseellipse is a figure drawn around is a figure drawn around two points (two points (focifoci) such that the ) such that the distance from one focus to a point on distance from one focus to a point on the ellipse and back to the other the ellipse and back to the other focus is a constant value.focus is a constant value.

You can make your own You can make your own ellipse…ellipse…

The 2The 2ndnd Law of Planetary Law of Planetary MotionMotion

““A line connecting a planet and the Sun A line connecting a planet and the Sun sweeps over equal areas in equal sweeps over equal areas in equal intervals of time.”intervals of time.”

In other words…In other words… The 2The 2ndnd law indicates that the planet law indicates that the planet

does does notnot travel at a constant speed. travel at a constant speed. At perihelion it moves faster and at At perihelion it moves faster and at

aphelion it moves slower.aphelion it moves slower. A planet’s A planet’s PeriodPeriod is the time it takes is the time it takes

for one complete orbit.for one complete orbit. The period of Mercury is 88 days, for The period of Mercury is 88 days, for

Earth 365 days, and for Mars 687 Earth 365 days, and for Mars 687 days.days.

TT2 2 ~ r~ r33

The 3The 3rdrd Law of Planetary Motion Law of Planetary Motion relates a planet’s period to its relates a planet’s period to its average distance from the Sun.average distance from the Sun.

““A planet’s orbital period squared is A planet’s orbital period squared is proportional to its average distance proportional to its average distance from the Sun cubed.”from the Sun cubed.”

TimeTime22 ~ Radius ~ Radius33

Try a few using time in years and Try a few using time in years and distance in Astronomical Units…distance in Astronomical Units…

TT22yearsyears = r = r33

AUAU

Fill in the missing valuesFill in the missing valuesPlanetPlanet PeriodPeriod Av. DistanceAv. Distance

MercuryMercury 0.24 years0.24 years

VenusVenus 0.72 AU0.72 AU

EarthEarth 1 year1 year 1 AU1 AU

MarsMars 1.5 AU1.5 AU

JupiterJupiter 11.9 years11.9 years

SaturnSaturn 9.5 AU9.5 AU

UranusUranus 84 years84 years

NeptuneNeptune 30 AU30 AU

Kepler’s MasterpieceKepler’s Masterpiece Kepler’s final book, the Kepler’s final book, the

Rudolphine TablesRudolphine Tables (1627), combined (1627), combined heliocentrism with heliocentrism with elliptical orbits.elliptical orbits.

It made the most It made the most accurate planetary accurate planetary position predictions ever.position predictions ever.

Yet, not everyone was Yet, not everyone was convinced.convinced.

Galileo GalileiGalileo Galilei Galileo was an Italian Galileo was an Italian

mathematician and mathematician and natural philosopher.natural philosopher.

He heard about the He heard about the telescope, built his own, telescope, built his own, and used it to examine the and used it to examine the heavens.heavens.

Many of his observations Many of his observations served to discredit the served to discredit the ideas of Aristotle and ideas of Aristotle and Ptolemy.Ptolemy.

Galileo Galilei1564-1642

Sidereus NunciusSidereus Nuncius In this book (The Starry In this book (The Starry

Messenger) Galileo described 3 Messenger) Galileo described 3 observations that contradict observations that contradict Aristotelian ideas:Aristotelian ideas:

1.1. An imperfect MoonAn imperfect Moon

2.2. Milky Way was made of starsMilky Way was made of stars

3.3. Moons of JupiterMoons of Jupiter

Two more observations…Two more observations… A short time after the publication of A short time after the publication of

Sidereus NunciusSidereus Nuncius Galileo made two Galileo made two further observations…further observations…

The Sun had spots and it rotated.The Sun had spots and it rotated. And…this one clenched it…And…this one clenched it… Venus had phases like the Moon! Venus had phases like the Moon!

This was only possible if it orbited This was only possible if it orbited the Sun! (Seeds, page 65)the Sun! (Seeds, page 65)