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The Copernican Revolution
The Birth of Modern Science




                              1B
                              1B
What do we see in the sky?
       The stars move in the sky but
        not with respect to each other
       The planets (or “wanderers”)
        move differently from stars
         ◦ They move with respect to the stars
         ◦ They exhibit strange retrograde
           motion
       What   does all this mean?
       How can we explain these
        movements?
       What does the universe          1B
        look like?
Timeline        Galileo
                1564-1642               Newton
                                        1642-1727

              Tycho
Copernicus   1546-1601
                             Kepler
1473-1543
                            1571-1630




                                            1B
Geocentric
(Ptolemaic) System
  The  accepted model for
   1400 years
  The earth is at the center
  The Sun, stars, and
   planets on their spheres
   revolve around the earth: explains daily movement
  To account for unusual planetary motion epicycles were
   introduced
  Fit the Greek model of heavenly perfection – spheres are the
   perfect shape, circular the perfect motion

                                                          1B
Heliocentric (Copernican) System
              Sun at center (heliocentric)
              Uniform, circular motion
                  ◦ No epicycles (almost)
               Moon orbited the earth, the
               earth orbited the sun as another
               planet
              Planets and stars still on fixed
               spheres, stars don’t move
              The daily motion of the stars
               results from the Earth’s spin
              The annual motion of the stars
               results from the Earth’s orbit 1B
In the heliocentric model, apparent
 retrograde motion of the planets is a direct
 consequence of the Earth’s motion




                                           1B
Geocentric vs. Heliocentric
How  do we decide between
 two theories?
Use the Scientific method:
 ◦ These are both explanations
   based on the observation of
   retrograde motion
 ◦ What predictions do the models
   make?
 ◦ How can these predictions be
   tested?

                                    1B
Phases of Venus

Heliocentric
 predicts that Venus
 should show a full
 phase, geocentric
 does not
Unfortunately, the
 phases of Venus
 cannot be
 observed
 with the
 naked eye      1B
Geocentric vs. Heliocentric
Against   heliocentric
 ◦ It predicted planetary motions and events no better
   than the Geocentric system
 ◦ The earth does not move (things do not fly off)
 ◦ The earth is different from the heavens (from
   Aristotle – the heavens are perfect and unchanging)
   and cannot be part of the heavens
For   heliocentric
 ◦ Simplified retrograde motion, but epicycles were
   necessary to account for the planets’ changing speed
 ◦ The distances to the planets could be measured.
   These distances were ordered, and therefore
                                                  1B
   aesthetically pleasing to the philosophy of the day
Stellar Parallax
Parallax  caused by the motion
 of the earth orbiting the Sun
Not observed with the naked
 eye
The heliocentric model predicts
 stellar parallax, but Copernicus
 hypothesizes that the stars are
 too far away (much farther than
 the earth from the Sun) for the
 parallax to be measurable 1B
 with the naked eye
Misconceptions
1.   The Copernican model has a force between the sun and
     the planets. Actually, the natural motion of the celestial
     spheres drove the planetary motions.
2.   The Copernican model was simpler than the Ptolemaic
     one. In fact, though Copernicus eliminated circles to
     explain retrograde motion, he added more smaller ones
     to account for nonuniformities of planetary motions.
3.   The Copernican model predicted the planetary motions
     better. Because both models demanded uniform motion
     around the centers of circles, both worked just about as
     well – with errors as large
     as a few degrees at times.

                                                            1B
Galileo Galilei
Turneda telescope toward the heavens
Made observations that:
 ◦ contradicted the perfection of the heavens
    Mountains, valleys, and craters on the Moon
    Imperfections on the Sun (sunspots)
 ◦ Supported the heliocentric universe
    Moons of Jupiter
    Phases of Venus – shows a full phase



                                                   1B
Tycho Brahe
 Had  two sets of astronomical
  tables: one based on Ptolemy’s
  theory and one based on
  Copernicus’.
 He found that both tables’
  predictions were off by days
  to a month.
 He believed that much better
  tables could be constructed
  just by more accurate observations.
 Tycho’s homemade instruments improved
  measurement precision from ten minutes of arc (which
  had held since Ptolemy) to less than one               1B
The skies change
Tycho observed 2 phenomena that
 showed the heavens DO change:
 ◦ In November 1572, Tycho noticed
   a new star in the constellation
   Cassiopeia
 ◦ Comet of 1577
   Prior to this sighting,
    comets were thought to be atmospheric
    phenomena because of the immutability
    of the heavens
   But neither the star nor the comet changed position
    as the observer moved, as expected for atmospheric
    phenomena                                             1B
Johannes Kepler
 Kepler  succeeded Tycho as the Imperial mathematician (but at
  only 1/3 the salary of the nobleman)
 Kepler worked for four years trying to derive the motions of
  Mars from Brahe’s observations
 In the process, he discovered that the plane of the earth’s orbit
  and the plane of Mars’ (and eventually the other planets) passed
  through the sun
 Suspecting the sun had a force over the planets, he investigated
  magnetism
 While this is not true, it did lead him to the idea of elliptical
  orbits
 “With reasoning derived from physical principles agreeing with
  experience, there is no figure left for
  the orbit of the planet except a perfect ellipse.”
                                                             1B
Astronomia nova
Published    in 1609, The New Astronomy was
 just that, it revolutionized the field
It predicted planetary positions as much as ten
 times better than previous models
It included physical causes for the movement of
 the planets
The ideas of the Greeks were gone – the
 heavens no longer were perfect, immutable, or
 different from the earth

                                           1B
Kepler’s first Law
                     The   orbital
                      paths of the
                      planets are
                      elliptical (not
                      circular), with
                      the Sun at one
                      focus.


                                    1B
Kepler’s second law
                An   imaginary
                 line connecting
                 the Sun to any
                 planet sweeps
                 out equal areas
                 of the ellipse in
                 equal intervals
                 of time.

                                1B
Kepler’s Third Law

The  square of a
 planet’s orbital
 period is
 proportional to
 the cube of its
 semi-major axis.
Kepler orbit demonstration:
 http://csep10.phys.utk.edu/guidry/java/kepler/kep

                                           1B
Planetary Properties
Planet    Orbital       Orbital semi-major     Orbital
          eccentricity, axis, a                period,P
          e             (Astronomical units)   (Earth years)
Mercury   0.206         0.387                  0.241
Venus     0.007        0.723                   0.615
Earth     0.017        1.000                   1.000
Mars      0.093        1.524                   1.881
Jupiter   0.048        5.203                   11.86
Saturn    0.054        9.537                   29.42
Uranus    0.047        19.19                   83.75
Neptune   0.009        30.07                   163.7
Pluto     0.249        39.48                   248.0   1B
Other Solar System Bodies
Kepler  derived
 his laws for the 6
 planets known to
 him. The laws
 also apply to the
 3 discovered
 planets and any
 other body
 orbiting the Sun
 (asteroids,
 comets, etc.)
                                 1B
A force for planetary motion
Newton    proposes a force which controls
 the motion of the planets – GRAVITY
The larger the mass, the larger the force
 of gravity
The further the distance, the smaller the
 force of gravity
Kepler’s third law can be derived from
 Newton’s law of gravity
F = GMm/r2 = mg
                                     1B
Gravity




          1B

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Astro101 1b

  • 1. The Copernican Revolution The Birth of Modern Science 1B 1B
  • 2. What do we see in the sky? The stars move in the sky but not with respect to each other The planets (or “wanderers”) move differently from stars ◦ They move with respect to the stars ◦ They exhibit strange retrograde motion What does all this mean? How can we explain these movements? What does the universe 1B look like?
  • 3. Timeline Galileo 1564-1642 Newton 1642-1727 Tycho Copernicus 1546-1601 Kepler 1473-1543 1571-1630 1B
  • 4. Geocentric (Ptolemaic) System  The accepted model for 1400 years  The earth is at the center  The Sun, stars, and planets on their spheres revolve around the earth: explains daily movement  To account for unusual planetary motion epicycles were introduced  Fit the Greek model of heavenly perfection – spheres are the perfect shape, circular the perfect motion 1B
  • 5. Heliocentric (Copernican) System Sun at center (heliocentric) Uniform, circular motion ◦ No epicycles (almost)  Moon orbited the earth, the earth orbited the sun as another planet Planets and stars still on fixed spheres, stars don’t move The daily motion of the stars results from the Earth’s spin The annual motion of the stars results from the Earth’s orbit 1B
  • 6. In the heliocentric model, apparent retrograde motion of the planets is a direct consequence of the Earth’s motion 1B
  • 7. Geocentric vs. Heliocentric How do we decide between two theories? Use the Scientific method: ◦ These are both explanations based on the observation of retrograde motion ◦ What predictions do the models make? ◦ How can these predictions be tested? 1B
  • 8. Phases of Venus Heliocentric predicts that Venus should show a full phase, geocentric does not Unfortunately, the phases of Venus cannot be observed with the naked eye 1B
  • 9. Geocentric vs. Heliocentric Against heliocentric ◦ It predicted planetary motions and events no better than the Geocentric system ◦ The earth does not move (things do not fly off) ◦ The earth is different from the heavens (from Aristotle – the heavens are perfect and unchanging) and cannot be part of the heavens For heliocentric ◦ Simplified retrograde motion, but epicycles were necessary to account for the planets’ changing speed ◦ The distances to the planets could be measured. These distances were ordered, and therefore 1B aesthetically pleasing to the philosophy of the day
  • 10. Stellar Parallax Parallax caused by the motion of the earth orbiting the Sun Not observed with the naked eye The heliocentric model predicts stellar parallax, but Copernicus hypothesizes that the stars are too far away (much farther than the earth from the Sun) for the parallax to be measurable 1B with the naked eye
  • 11. Misconceptions 1. The Copernican model has a force between the sun and the planets. Actually, the natural motion of the celestial spheres drove the planetary motions. 2. The Copernican model was simpler than the Ptolemaic one. In fact, though Copernicus eliminated circles to explain retrograde motion, he added more smaller ones to account for nonuniformities of planetary motions. 3. The Copernican model predicted the planetary motions better. Because both models demanded uniform motion around the centers of circles, both worked just about as well – with errors as large as a few degrees at times. 1B
  • 12. Galileo Galilei Turneda telescope toward the heavens Made observations that: ◦ contradicted the perfection of the heavens  Mountains, valleys, and craters on the Moon  Imperfections on the Sun (sunspots) ◦ Supported the heliocentric universe  Moons of Jupiter  Phases of Venus – shows a full phase 1B
  • 13. Tycho Brahe  Had two sets of astronomical tables: one based on Ptolemy’s theory and one based on Copernicus’.  He found that both tables’ predictions were off by days to a month.  He believed that much better tables could be constructed just by more accurate observations.  Tycho’s homemade instruments improved measurement precision from ten minutes of arc (which had held since Ptolemy) to less than one 1B
  • 14. The skies change Tycho observed 2 phenomena that showed the heavens DO change: ◦ In November 1572, Tycho noticed a new star in the constellation Cassiopeia ◦ Comet of 1577  Prior to this sighting, comets were thought to be atmospheric phenomena because of the immutability of the heavens  But neither the star nor the comet changed position as the observer moved, as expected for atmospheric phenomena 1B
  • 15. Johannes Kepler  Kepler succeeded Tycho as the Imperial mathematician (but at only 1/3 the salary of the nobleman)  Kepler worked for four years trying to derive the motions of Mars from Brahe’s observations  In the process, he discovered that the plane of the earth’s orbit and the plane of Mars’ (and eventually the other planets) passed through the sun  Suspecting the sun had a force over the planets, he investigated magnetism  While this is not true, it did lead him to the idea of elliptical orbits  “With reasoning derived from physical principles agreeing with experience, there is no figure left for the orbit of the planet except a perfect ellipse.” 1B
  • 16. Astronomia nova Published in 1609, The New Astronomy was just that, it revolutionized the field It predicted planetary positions as much as ten times better than previous models It included physical causes for the movement of the planets The ideas of the Greeks were gone – the heavens no longer were perfect, immutable, or different from the earth 1B
  • 17. Kepler’s first Law The orbital paths of the planets are elliptical (not circular), with the Sun at one focus. 1B
  • 18. Kepler’s second law An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time. 1B
  • 19. Kepler’s Third Law The square of a planet’s orbital period is proportional to the cube of its semi-major axis. Kepler orbit demonstration: http://csep10.phys.utk.edu/guidry/java/kepler/kep 1B
  • 20. Planetary Properties Planet Orbital Orbital semi-major Orbital eccentricity, axis, a period,P e (Astronomical units) (Earth years) Mercury 0.206 0.387 0.241 Venus 0.007 0.723 0.615 Earth 0.017 1.000 1.000 Mars 0.093 1.524 1.881 Jupiter 0.048 5.203 11.86 Saturn 0.054 9.537 29.42 Uranus 0.047 19.19 83.75 Neptune 0.009 30.07 163.7 Pluto 0.249 39.48 248.0 1B
  • 21. Other Solar System Bodies Kepler derived his laws for the 6 planets known to him. The laws also apply to the 3 discovered planets and any other body orbiting the Sun (asteroids, comets, etc.) 1B
  • 22. A force for planetary motion Newton proposes a force which controls the motion of the planets – GRAVITY The larger the mass, the larger the force of gravity The further the distance, the smaller the force of gravity Kepler’s third law can be derived from Newton’s law of gravity F = GMm/r2 = mg 1B
  • 23. Gravity 1B