Ancient to Modern AstronomyAncient to Modern Astronomy

The Astronomers and what The Astronomers and what they didthey did

The AncientsThe Ancients

Early Babylonians Egyptians and Chinese Early Babylonians Egyptians and Chinese made important astronomical observationsmade important astronomical observations

The five closest planets were observed The five closest planets were observed and novas and comets were recordedand novas and comets were recorded

Comets were blamed for disasters and Comets were blamed for disasters and were seen as portents of doomwere seen as portents of doom

The Golden Age of Astronomy – The Golden Age of Astronomy – Greece 600 BC to 150 ADGreece 600 BC to 150 AD

The basics of geometry and trigonometry helped The basics of geometry and trigonometry helped them explain the motions of planets, sun and them explain the motions of planets, sun and moonmoonHowever, they erroneously decided that However, they erroneously decided that everything in the “heavens” revolved around the everything in the “heavens” revolved around the earthearthThat is what appears to happen, but this That is what appears to happen, but this apparent motion is really due to the earth’s apparent motion is really due to the earth’s rotationrotationThey decided that the Earth couldn’t be rotating They decided that the Earth couldn’t be rotating because we feel no motionbecause we feel no motion

Some of the famous GreeksSome of the famous GreeksAristotle knew that the earth was a Aristotle knew that the earth was a sphere because during an eclipse sphere because during an eclipse the earth’s shadow was a circle. the earth’s shadow was a circle. However Aristotle believed in an However Aristotle believed in an earth centered(Geocentric) solar earth centered(Geocentric) solar systemsystemAristarchus was the first to propose Aristarchus was the first to propose a sun-centered solar a sun-centered solar system(Heliocentric), but no one system(Heliocentric), but no one agreed with him. Aristotle’s influence agreed with him. Aristotle’s influence was too strong and lasted over a was too strong and lasted over a thousand years.thousand years.Aristarchus also tried to calculate Aristarchus also tried to calculate sizes of the moon and sun and sizes of the moon and sun and distances to them. The method was distances to them. The method was good, but his measurements were good, but his measurements were off, so his answers were wrong.off, so his answers were wrong.

Aristotle

Aristarchus

Some more!Some more!

Eratosthenes calculated the Eratosthenes calculated the circumference of the earth. circumference of the earth. (We will demonstrate his (We will demonstrate his method.) method.) Eratosthenes LinkEratosthenes Link

Hipparchus observed and Hipparchus observed and recorded over 1,000 stars for recorded over 1,000 stars for his star catalog. These his star catalog. These measurements were later measurements were later recorded by Ptolemyrecorded by Ptolemy

Eratosthenes

Hipparchus

Earth circumference mathEarth circumference mathTwo cities have incoming light at two different anglesTwo cities have incoming light at two different anglesWe can measure the distance between the two ctiesWe can measure the distance between the two ctiesUsing basic geometry (Can geometry students understand the Using basic geometry (Can geometry students understand the method?) he found the angle difference between two citiesmethod?) he found the angle difference between two citiesThen Circumference/distance = 360/angle difference Then Circumference/distance = 360/angle difference (Why 360? Because there are 360 degrees in a circle)(Why 360? Because there are 360 degrees in a circle)

Sample problem: The difference in latitude between city A and city B is 30 degrees.

The distance between cities A and B is 2000 km. What is the circumference of the planet?

Did you get 24,000 km?

Solution:

Circumference = 2000 * (360/30)

WORKSHEET

PtolemyPtolemy

Ptolemy compiled a Ptolemy compiled a 13 volume book 13 volume book which explained the which explained the work of Greek work of Greek AstronomersAstronomers

In the dark ages to In the dark ages to follow, these works follow, these works were preserved by were preserved by Arab scholarsArab scholars

The problemThe problem

The Greeks believed that the planetary orbits The Greeks believed that the planetary orbits were circles and that everything went around the were circles and that everything went around the earth, including the sun and planets, and the earth, including the sun and planets, and the stars. The stars resided on a crystal sphere stars. The stars resided on a crystal sphere through which light shown.through which light shown.Unfortunately for the Greeks, the solar system Unfortunately for the Greeks, the solar system was not that simple. was not that simple. They had trouble reconciling this model with the They had trouble reconciling this model with the observed motion of the planets.observed motion of the planets.So, they tried changing things slightlySo, they tried changing things slightly

The problem with retrograde motionThe problem with retrograde motion

As planets revolve, they As planets revolve, they sometimes appear to go sometimes appear to go backwards in their orbit.backwards in their orbit.This is called retrograde This is called retrograde motion.motion.How can this be How can this be explained?explained?This geocentric model This geocentric model shows one model that shows one model that was triedwas tried

The Ptolemaic SystemThe Ptolemaic System

Ptolemy had Ptolemy had introduced the introduced the concept of epicyclesconcept of epicycles

As a planet orbited As a planet orbited the earth, it moved in the earth, it moved in small circlessmall circles

These were called These were called epicycles Linkepicycles Link

Did it work?Did it work?This made it somewhat better, but it was not This made it somewhat better, but it was not simple. Ptolemy had to devise many of these simple. Ptolemy had to devise many of these to explain the apparent motion of the planets.to explain the apparent motion of the planets.What is actually going on? Look at the What is actually going on? Look at the picture here.picture here.

What is actually happening is that Mars appears to go backwards as we pass it in our orbit, like an inside car passing an outside car on a racetrack.

Watch this link.

But the problem with this explanation is that you need to realize that the solar system is heliocentric, not geocentric.

Problems with modelsProblems with models

This illustrates the danger of models.This illustrates the danger of models.If you too firmly believe in your model, you If you too firmly believe in your model, you tend to make your data fit that model.tend to make your data fit that model.It has been said that scientists, like artists, It has been said that scientists, like artists, fall in love with their models.fall in love with their models.You have to be willing to throw out your You have to be willing to throw out your model if it doesn’t fit the data.model if it doesn’t fit the data.And revise it as new data becomes And revise it as new data becomes available.available.

What’s nextWhat’s next

For over a thousand years, there were very few For over a thousand years, there were very few changeschanges

The middle ages were a time of scientific The middle ages were a time of scientific stagnation, as well as hard times for societystagnation, as well as hard times for society

It is often true that changes in science and It is often true that changes in science and society feed off of each othersociety feed off of each other

Many of the most revolutionary times in science Many of the most revolutionary times in science occur with revolutionary times in society and occur with revolutionary times in society and changing political systemschanging political systems

CopernicusCopernicus

Nicholas Copernicus, a Nicholas Copernicus, a Polish astronomer, was Polish astronomer, was the first great astronomer the first great astronomer in a long time. in a long time. He knew that a He knew that a heliocentric system heliocentric system worked better than a worked better than a geocentric model.geocentric model.Unfortunately, he insisted Unfortunately, he insisted on using circular orbits, on using circular orbits, so he still had to add so he still had to add epicycles to make his epicycles to make his system work.system work.

Copernican ModelCopernican Model

His revolutionary book was His revolutionary book was published as Copernicus published as Copernicus was dying.was dying.He was afraid to publish it He was afraid to publish it because the Catholic because the Catholic Church, very powerful at Church, very powerful at the time, firmly believed in the time, firmly believed in the geocentric model, which the geocentric model, which put man in the center of the put man in the center of the known universe.known universe.

His theory was a threat to church teachings and you were in trouble His theory was a threat to church teachings and you were in trouble with the church if you were accused of being a “Copernican.”with the church if you were accused of being a “Copernican.”

Religion and Science IssuesReligion and Science Issues

Giordano Bruno, who refused to deny the Giordano Bruno, who refused to deny the heliocentric theory of Copernicus, was heliocentric theory of Copernicus, was burned at the stake in 1600burned at the stake in 1600

We will see what happens to Galileo We will see what happens to Galileo shortlyshortly

Tycho BraheTycho BraheThe Danish astronomer Brahe was a The Danish astronomer Brahe was a meticulous observer who made many meticulous observer who made many accurate measurements of the motions accurate measurements of the motions of heavenly objects.of heavenly objects.

Telescopes were not yet invented and Telescopes were not yet invented and he used devices called pointers to he used devices called pointers to accurately line up stars to measure accurately line up stars to measure themthem

Tycho had some interesting personal Tycho had some interesting personal historyhistory

Also, he did not believe in the Also, he did not believe in the heliocentric model due to what he saw heliocentric model due to what he saw as a lack of stellar parallax. as a lack of stellar parallax.

ParallaxParallax

The concept of parallax The concept of parallax shows that as you view shows that as you view an object from two an object from two different positions it will different positions it will appear to shift against the appear to shift against the background.background.

This is actually what This is actually what gives us 3D vision and gives us 3D vision and why we must view things why we must view things straight on to measure straight on to measure them accurately.them accurately.

Brahe stated, accurately, that if the earth were traveling around the sun, we should be able to see this parallax shift of closer stars against the stellar background, at different positions in the earth’s orbit. Since we don’t, we can’t be revolving around the sun.

Stellar ParallaxStellar Parallax

This was very smart of Brahe.This was very smart of Brahe.

But there was one problem with his thinking.But there was one problem with his thinking.

Let’s try the parallax demo to see why.Let’s try the parallax demo to see why.

See the small change with a farther star. And, as we now know, the stars are VERY far away, so we can only see stellar parallax with modern technology.

Johannes KeplerJohannes Kepler

Kepler, an Austrian by Kepler, an Austrian by birth, became Brahe’s birth, became Brahe’s assistant about a year assistant about a year before Brahe’s deathbefore Brahe’s death

Kepler, a brilliant Kepler, a brilliant mathematician, analyzed mathematician, analyzed Brahe’s 20 years of Brahe’s 20 years of astronomy data and astronomy data and discovered some discovered some interesting relationships in interesting relationships in the datathe data

Kepler’s laws of Planetary MotionKepler’s laws of Planetary Motion

The 1The 1stst law states that law states that planets travel in elliptical planets travel in elliptical orbits, rather than circular orbits, rather than circular orbits. This eliminates the orbits. This eliminates the need for those pesky need for those pesky epicyclesepicyclesThe 2The 2ndnd law states that a law states that a planet travels faster planet travels faster closer to the sun, so that closer to the sun, so that it covers equal triangle it covers equal triangle shaped orbit sections shaped orbit sections during the same amounts during the same amounts of time. of time.

So, Kepler’s 2nd law shows that the triangles above are all equal in area. Closer to the sun the planet goes faster, covering a larger section of its orbit, but the sun-planet distance is shorter.

The 3The 3rdrd law of planetary motion law of planetary motion

Let’s look for a minute at a solar system model running – Let’s look for a minute at a solar system model running – hyperlinkhyperlinkYou probably noticed that the planets travel at different You probably noticed that the planets travel at different speeds.speeds.The inner planets travel more quickly than the outer The inner planets travel more quickly than the outer planetsplanetsKepler’s 3Kepler’s 3rdrd law give a mathematical relationship for this. law give a mathematical relationship for this.This relationship is: period(of revolution)^2 = distance This relationship is: period(of revolution)^2 = distance (from the sun to the planet)^3(from the sun to the planet)^3Simply put: pSimply put: p22=r=r3 3 (p is in years and r is in AU’s – what’s (p is in years and r is in AU’s – what’s an AU? Watch the an AU? Watch the LinkLink.).)

KeplerKepler

Interestingly enough, Interestingly enough, Kepler figured out Kepler figured out howhow this all worked this all workedHowever, he had no However, he had no idea idea whywhy it was so. it was so.That explanation That explanation would have to wait a would have to wait a little while for the little while for the concept of Gravity as concept of Gravity as proposed by Isaac proposed by Isaac NewtonNewton

Galileo GalileiGalileo Galilei

Galileo was one of the Galileo was one of the first scientists to use first scientists to use experimentation to figure experimentation to figure things out in science.things out in science.He came up with the He came up with the concepts behind inertia, concepts behind inertia, among other thingsamong other thingsKepler and Galileo were Kepler and Galileo were contemporaries and contemporaries and actually corresponded.actually corresponded.

Galileo’s astronomical observationsGalileo’s astronomical observations

Galileo may have been the 1Galileo may have been the 1stst scientist to use the scientist to use the newly developed telescope to make astronomical newly developed telescope to make astronomical observations. These included:observations. These included:– The discovery of the four largest moons of JupiterThe discovery of the four largest moons of Jupiter– The discovery that planets are not just “points of light.”The discovery that planets are not just “points of light.”– The discovery that the moon is not a smooth ball, as The discovery that the moon is not a smooth ball, as

proposed by others.proposed by others.– The discovery of sunspots (which probably caused him The discovery of sunspots (which probably caused him

blindness later) and observations of them to estimate blindness later) and observations of them to estimate the rotational period of the sunthe rotational period of the sun

Galileo’s observationsGalileo’s observations

And the discovery of And the discovery of the phases of Venus, the phases of Venus, which showed that which showed that Venus must orbit the Venus must orbit the sun, not the earthsun, not the earth

This was evidence This was evidence supporting the supporting the heliocentric model of heliocentric model of the solar system (study the solar system (study the sketch here.)the sketch here.)

Galileo vs the ChurchGalileo vs the ChurchMany of his observations went against the Many of his observations went against the accepted geocentric model.accepted geocentric model.The heliocentric model had been banned by the The heliocentric model had been banned by the churchchurchGalileo was told to drop the heliocentric theory Galileo was told to drop the heliocentric theory from his writingfrom his writingWhen he didn’t, he was tried by the Inquisition When he didn’t, he was tried by the Inquisition and was sentenced to house arrest until he died.and was sentenced to house arrest until he died.He continued to work until his deathHe continued to work until his deathThe church exonerated Galileo in 1992, more The church exonerated Galileo in 1992, more than 350 years after his death. than 350 years after his death. LinkLink

Sir Isaac NewtonSir Isaac Newton

One of the greatest One of the greatest scientists who ever scientists who ever lived, although not a lived, although not a “nice man.”“nice man.”Newton’s 3 laws of Newton’s 3 laws of motion motion (CP Newton’s Laws vi(CP Newton’s Laws video), deo), (Other 3(Other 3rdrd law demo law demo video)video)Orbits controlled by Orbits controlled by gravity and inertia gravity and inertia (sketch here)(sketch here)Law of GravitationLaw of GravitationGravity activityGravity activity

His law of Universal Gravitation explained how the force decreases with distance. This is similar to how light spreads out from a source. This is somewhat complicated. See if you can understand the link here. (It’s hard.)

Newton continuedNewton continued

Newton also revised Kepler’s 3Newton also revised Kepler’s 3rdrd law to include the force law to include the force of gravity. This allows us to find the mass of an object of gravity. This allows us to find the mass of an object from the orbits of its satellites. For example, we can find from the orbits of its satellites. For example, we can find the mass of Jupiter by calculating how long it takes one the mass of Jupiter by calculating how long it takes one of its moons to go around it. (possible lab) We can even of its moons to go around it. (possible lab) We can even use a more complicated version of this law to find the use a more complicated version of this law to find the masses of galaxiesmasses of galaxiesNewton also made other great contributions to science in Newton also made other great contributions to science in the areas of optics. He also invented Calculus, and the the areas of optics. He also invented Calculus, and the reflecting telescope. Linkreflecting telescope. LinkHe would not have been surprised by space travel. He He would not have been surprised by space travel. He understood how we could actually leave the earth with understood how we could actually leave the earth with his “cannon model”. his “cannon model”. LinkLink

The EndThe End