Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11
Chaisson & McMillan, Astronomy Chapter 1 History of Modern Astronomy History And Planetary Motion And The Motion of the Planets The
The planets wander on the sky along the ecliptic ecliptic Mostly, West to East Mostly, Occasionally they loop back – retrograde motion Explaining Planetary Motion Explaining
What theory might work? What Can everything go around the Earth? Can Geocentric System Geocentric
Aristotle advocated a Geocentric system (everything goes around earth) (everything He insisted that all orbits must be circles based on the innate perfection of spheres – a heavens of nested crystalline spheres heavens Can simple circular orbits around the earth explain everything? explain Retrograde Motion and Epicycles and
To get the planets to appear to go backwards, epicycles (or more smaller circles were needed) circles Recall that the data was poor – iinferred by looking nferred at the sky with the naked eye The pinnacle of Epicycles: The Ptolemy Ptolemy
Ptolemy (~140AD) attempted to fit all observations to date observations This required 80 circles and the centres of the centres of large circles were offset from Earth slightly from Not a simple theory Not Failure to meet future tests tests Heliocentric System Heliocentric
Heliocentric systems put the Sun at the centre centre Aristarchus of Samos (310230 B.C) Aristarchus 230 proposed this idea – based on estimates based of the Earth, Sun and Moon separations and deducing the Sun to be very large he decided the Earth goes around the Sun decided Heliocentric System: Slow arrival Slow
Why did Aristarchus’ ideas fade? Why Why did Ptolemy’s iideas last for 13 deas Why centuries? centuries? Aristotlean Ideas Aristotlean
Aristotle was very influential – he was long considered the he preeminent philosopher of the pre eminent ancient world ancient Many of his influential ideas don’t Many stand up to scrutiny (e.g. Four Elements, Geocentric Universe) but were appealing were Stars, statesmen and scientists today are also often given too much credit outside the areas they actually know outside Aristotle, the Church and Geocentrism Geocentrism
Aristotle’s assertion of a perfect Aristotle assertion unchanging universe was very attractive to the Church to JudeoChristian scripture also has Judeo Christian passages indicating the Sun goes around the Earth (Joshua, Psalms, …) The Church became extremely influential at the time of Emperor Constantine (306 AD onwards) (306 The Renaissance in Astronomy: Copernicus Astronomy:
Nicolas Copernicus (14731543) Nicolas 1543) was a Polish Cleric and Astronomer was He took observations of planetary motions He He felt a Suncentred system could explain He system the motion of the planets more simply and believed it to be reality believed He retained the idea of perfect circles and put the Sun at the centre of the universe the Page from “de de Revolutionibus” Revolutionibus by Nicolas by Copernicus Heliocentrism and Heliocentrism and Retrograde motion Retrograde Copernican Revolution? Copernican
Copernicus’ books on his ideas were not Copernicus books published until he was near death published The books were printed in Protestant Nuremberg to get past Catholic censorship censorship The publisher’s preface (Osiander) The suggested the idea was a “device” for for calculating planet positions, not reality calculating The Revolution in earnest… The
Copernicus’ iideas were somewhat Copernicus deas tolerated by the Church, particularly when described as a “device” for for calculations calculations They were not widely known They It took others, who read Copernicus to push the case that the Sun really was at the centre: particularly Galileo the Galileo Galilei, Galilei Italian (15641642) Italian
Galileo was something of a sensationalist of He is credited as the father of modern scientific inquiry and modern astronomy
e.g. He theorized the equal action of gravity on all falling things on He did not invent the telescope but immediately built his own and was first to pursue astronomy with it (1609) to Galileo’s telescope: Galileo The imperfect Heavens
The Church embraced Aristotle’s perfect and perfect unchanging heavenly spheres, but Galileo saw: unchanging Craters and valleys on the Moon Craters Blemishes on sun (sunspots) Blemishes 4 Moons around Jupiter Moons Venus had phases like the moon Venus Stuff Stuff
Observing next week: Sept 20+, 7:30 pm Sept Planetarium next week ~ 5:30 pm Planetarium See Avenue for details – outdoor observing may See outdoor be cancelled due to weather be Lab Friday (Sept 17) in class, due Tuesday Lab Galileo the Heretic Galileo
In 1610 Galileo published his observations: “The Starry Messenger” In 1616 his works and Copernicus’ were In were banned by the Church banned In 1632 New book: “Dialogue between Dialogue two Chief World Systems” iin accessible n two Italian (not Latin) Italian 1633: Inquisition and house arrest 1633: 1992: Catholic Church forgives his crimes 1992: Galileo’s Observations Galileo
Craters and valleys on the Moon Craters The Moon is not a perfect sphere but similar to Earth – another world Galileo’s Galileo Observations
The Sun is not a “perfect sphere”, iit has t small dark regions called sunspots small Sunspots rotate as the sun spins Sunspots Galileo inferred (correctly) that the Sun rotates once every month or so rotates Galileo’s Observations Galileo
Moons orbiting Jupiter – small spheres with small shadows passing across the face of Jupiter the These are the four largest, “Galilean Moons” of Jupiter These moons are orbiting something that is not Earth something Galileo’s Observations Galileo
Galileo saw phases of Venus Galileo All planets are illuminated by the Sun, just like the moon and thus have phases thus Only something orbiting the Sun on an orbit that passes between the Earth and Sun can have the full set of phases
(e.g. new, first quarter, full, etc…) (e.g. Phases of Venus: Heliocentric Phases Venus Phases of Venus: Geocentric Phases
A “Full Venus” never occurs: Wrong! Modern Astronomy Modern
Through looking at the sky with telescopes, Galileo effectively invented modern observational astronomy observational In constantly seeking to test his ideas with observations, Galileo estabilished modern estabilished modern experimental science experimental Through people like Galileo the Renaissance of science was firmly established of The Scientific Revolution The
Galileo’s “new science” emerged at a time of Galileo emerged wealth, trade, education and exploration wealth, There were similar leaps forward in scientific experimentation in other areas: e.g. physiology, architecture, engineering architecture, During the Renaissance mathematics flourished and ancient knowledge was “rediscovered” The nobility actively supported science The Tycho Brahe (15461601) Tycho
Tycho Brahe was an observer, Tycho Brahe possibly the greatest ever Danish nobleman, attended university (1559), studied and travelled extensively extensively Saw eclipse 1560 – fascinated that it fascinated was predicted, started astronomical observations, also worked on alchemy observations, Saw “new star” (supernova) in 1572 Tycho Brahe  Quirks Tycho Brahe Lost his nose in a duel with a fellow student – made a gold/silver replacement Had a pet moose that was allowed to drink in local pubs local When the King of Denmark was his patron (15751597) he was given 1% of Denmark’s (1575 entire income entire Built a castle/observatory: Uraniborg Uraniborg Tycho Brahe’s Tycho Observations
No telescopes No Precise and expensive instruments for measuring positions in the sky: positions
(1/120th of a degree) Took careful observations over long periods Took The observational data was owned by his heirs and was very valuable heirs Brahe and Kepler Kepler
Brahe moved to Prague (1599), to serve the Holy Roman Emperor, Rudolph II as Imperial Mathematician to create astronomical tables Mathematician Brahe hired Kepler (1600), a mathematician, Kepler (1600), to use his 38 years of data to prove the Sun went around the Earth went Brahe died in 1601, Kepler took over as Kepler took Imperial Mathematician and continued to work on the data work Johannes Kepler Kepler (15711630), German
Kepler was poor but bright – he went Kepler was he to university on a scholarship to He embraced Copernican ideas early and published a defence while a math defence while teacher teacher He was an accomplished geometer and mathematician mathematician Key contributions to Optics – explained explained how, lenses for images: how telescopes work work Kepler’s Ideas Kepler on Planetary Motion on
Started with Copernicus ideas Started Closely studied the orbit of Mars Closely Realised that Ellipses could fit the Realised that observations better than circles observations Published “Astronomia Nova”, 1609, 1609, These ideas later known as Kepler’s Kepler First and Second Laws relating to First individual orbits individual Kepler’s Ideas Kepler on Planetary Motion
Published:“Harmonices Mundi “, 1619 Published: 1619 which included the motions of several planets planets This established his Third Law, which relates the properties of orbits of different planets different Made highly accurate predictions, including transits by Mercury and Venus including Kepler’s First Law: Kepler Planets orbit in ellipses with ellipses with the Sun at one focus the Kepler’s First Law: Ellipses Kepler
An ellipse can be defined as a curve from which the sum of the distances to two fixed points (call foci, plural of focus) stays constant focus) Kepler’s First Law: Ellipses Kepler
A circle is an ellipse with both foci at the same point point The deviation from a circular orbit is called eccentricity eccentricity Mostly the planet’s Mostly eccentricities are quite low: The orbits are quite close to circles close Kepler’s First Law: Ellipses Kepler Kepler’s Second Law Kepler An imaginary Line connecting the planet to the Sun sweeps out equal areas in equal times times Kepler’s Second Law: Speed Kepler
Kepler’s Second Law Kepler Second relates to the speed of a planet at different times during its orbit times Planets move faster when closer to the Sun Sun Kepler’s Third Law Kepler
The period of a planet’s orbit period orbit squared iis equal to the semisquared s semi major axis of the orbit cubed of cubed Kepler’s Third Law Kepler
P2 (years) = a3 (astronomical units) An astronomical unit iis the distance s An astronomical from the Earth to the sun from e.g. Something orbiting at 4 times Earth’s orbit would take 8 years to complete each orbit each Kepler’s Third Law: Kepler Third Orbital Periods Orbital Distances in the Solar System Distances
Kepler’s Laws don’t tell us Kepler tell how far an Astronomical Unit is Unit We can use radar to measure the distance to other planets and infer the intrinsic scale of the Solar System Solar Distances in the Solar System Distances
An Astronomical Unit is around 150 Million km 150 Also written as: Also 1.5 x 108 km In Scientific In Notation Notation Kepler’s Laws: Empirical Kepler
Kepler’s Laws fit the observed planetary Kepler Laws motions in an empirical manner motions There is no builtiin theory of why they n There are correct are Ideas of Gravity Ideas
Early ideas (e.g. Aristotle) proposed things fell down because “Earth” at the at centre was the natural state of things centre Kepler’s ideas didn’t relate gravity as Kepler relate felt on Earth to the motions of planets Kepler’s Laws need to be empirically Kepler Laws adjusted for each system adjusted Isaac Newton Isaac (16421727)
Newton was an English mathematian, one of the mathematian one greatest of all time greatest Was one of the inventors of calculus calculus Master of the Royal Mint Master Keen theological scholar with a tendency toward literal interpretation of the bible bible Newton’s Laws of Motion Newton
Newton understood that momentum is conserved: moving objects tend to continue in a straight line unless acted upon upon Laws of Laws Motion
Aristotle proposed that the natural state of things was at rest of Newton’s Laws say they keep going Newton Objects come to rest due to friction: Newton’s Laws describe the ideal case Newton Laws with no friction applied with No friction is a good assumption in space No Newton’s Law of Gravity Newton
Newton hypothesized that every object exerts a gravitational pull on every other object other The more massive an object the stronger the pull stronger
e.g. Gravity is 1/6th what it is on Earth on the what surface of the Moon surface Newton’s Law of Gravity Newton
Newton hypothesized that the pull of gravity falls off with distance gravity The gravitational pull is inversely proportional to the square of the distance: distance:
twice as far, one quarter the pull three time as far, one ninth the pull Newtonian Gravity: Orbits Newtonian “Falling with style”
Gravity doesn’t hold things in orbit Gravity They are always falling They This is why astronauts feel weightless just the same as someone in a plunging elevator elevator Newtonian Gravity: Orbits Newtonian “Falling with style”
The Earth attempts to fall toward the Sun but it “misses” due to its due forward motion and instead follows a nearly circular path circular Circular Orbits Circular
Centrifugal force is the force required to Centrifugal keep an object moving in a circle at a specific separation, r, from the centre specific When gravitational force equals the When centrifugal force, the orbit is circular (r=constant) (r=constant) Elliptical Orbits Elliptical
Centrifugal force stronger stronger than than gravitational force force Centrifugal Centrifugal force weaker than gravitational force force Newtonian Gravity Newtonian
Mathematically: Mathematically: G × Mass Acceleration = 2 distance
G is universal, a constant number – you you can use it for any gravitational system can G = 6.67 × 1011 m3 kg1 s2 (S.I.) (S.I.) Newtonian Gravity Example Newtonian
The Moon has 0.25 the radius of Earth The The Moon has 0.01 the Mass of the Earth Earth G × Mass Acceleration = 2 distance 0.01 / (0.25x0.25) = 1/6 the gravity at the surface the Newtonian Gravity Newtonian
Newton’s Law of Gravity is Newton Law universal – iit predicts how t gravity will act in every system from the Solar system, to Jupiter’s Moons and for objects Jupiter Moons falling on Earth falling Kepler’s Laws can be derived Kepler Laws from Newton’s Law of Gravity from ...
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 Spring '11
 wadsley
 Astronomy, Galileo Galilei, Isaac Newton, Kepler's laws of planetary motion, Heliocentrism

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