ISP205 Section 2
Lecture#3: Tuesday, January 11

Handouts:

Topics:

1. Review

   1. Ecliptic: path of the sun along the celestial shpere (west to east)

   2. Planets and Moon move on or close to the ecliptic. Planets show retrograde motion.

   3. Milky way (galactic equator) almost vertical

   4. Precession: North celestial pole moves around every 26000 years. 
      

2. History of Astronomy - Development of heliocentric solar system model
   (From Aristotele to Keplers Laws)

   1. Aristotele (384 BC - 322 BC)

      • Description of nature from a priori philosophical speculation, not observation

      • Found earth is spherical, but thought it is in the center of the universe
        

   2. Aristarchos of Samos 310 BC – 230 BC

      • Determined distance from earth to sun from angle between earth-moon and moon-sun
        (Redshift Presentation: The Greeks, Pg5,6)

      • First heliocentric model (earth moves around the sun)
        

   3. Eratosthenes 276 BC - 196 BC

      • First determination of the size of earth from angle of sunlight at different positions on earth
        (Redshift Presentation: The Greeks, Pg 14,15,16,20)
        

   4. Claudius Ptolemy (100AD – 160AD) (Redshift Presentation: The Greeks, Pg 23)

      • Wrote Almagest: Description of sun and planetary motions with epicycles around the earth
        (Redshift Presentation: Pg 24)

      • Assumed earth at rest because he could not measure parallax. This is because the stars
        are much further away than he could imagine and parallax is therefore small and can 
        only be measured with large telescopes.
        Parallax: shift of an objects position due to the movement of the observer
        (Demo: Parallax)(Redshift Presentation: The Greeks Pg 11)

      • Wrote Tetrabiblios: Basis of natal astrology 
        

   5. Nicolas Copernicus (1473 – 1543) picture

      • Rediscovered Aristarchos idea of heliocentric solar system (probably quote in Archimedes writing)

      • Realized that it describes planetary motions in a much simpler way

      • Still used a few epicycles as he still assumed circular orbits
        

   6. Galileo Galilei (1564-1642) picture

      • Founded modern scientific method: A hypothesis can only be tested by careful observations

      • First man to direct a telescope (developed 1604) to the sky

      • Astronomical discoveries:

        • Milky Way consists of individual stars

        • Moon has a landscape (mountains etc.) like earth (Redshift Presentation: C&G Pg 9)

        • The sun has dark spots (Redshift Presentation: C&G Pg 10)

        • Moons orbiting Jupiter (Redshift Presentation: C&G Pg 11)

        • Venus goes through a full set of phases 
          (Redshift Presentation: C&G Pg 14) and picture

      • Founder of modern theory of mechanics (discussed later when we talk about Newton)

      • Tireless promoter of heliocentric solar system model, but could not explain absence of
        parallax, and his main argument was based on a wrong theory of the tides.
        

   7. Tycho Brahe (1546-1601) picture

      • Most accurate and complete naked eye observer of the sky in history
        Catalog of 1004 stars

      • Had his own observatory on danish island Hveen (picture)

      • Compromise model of solar system to explain absence of parallax: sun orbits around the 
        earth, but all other planets orbit around the sun. Accepted by catholic church.
        

   8.   Johannes Kepler (1571-1630)

      • Mathematician (and Astrologer) and Assistant of Brahe

      • Used Brahes Data to find the first accurate (!) description of planetary motion:
        Keplers three laws:

        1. The orbit of the planets around the sun is an ellipse, with the
           sun being at one focus. (see picture)
           

        2. The straight line joining a planet and the Sun sweeps out equal
           areas in space in equal amounts of time 
           (Redshift Presentation: Keplers laws Pg 8)(see picture)
           

        3. The orbital period of a planet is related to the semi major axis of its elliptical orbit. 
           If we measure period P in years and the semi major axis A in AU (average distance earth-sun) then simple (assuming semimajor axis of earth is 1 AU which is a good approximation):
            (semi major axis)³ = (period)²  or A³=P²
           

   9. Summary Evidence of heliocentric solar system

      1. So far only hints that make this theory plausible:

         • Simpler, and with Kepler only accurate description of planet motions

         • Landscapes on Moon: Earth could be a celestial object as well

         • Moons around Jupiter: something that moves can still have other objects
           orbiting around it

         • Phases of Venus: Other planets orbit the sun
           

      2. Direct evidence only found later:

         • James Bradley 1728:
           found Abberation of starlight (proof of moving earth)
           stars describe small ellipses on sky over the course of a year
           (Demo: Abberration)
           

         • F.W. Bessel 1838
           first detection of stellar parallax (from 61 Cygni)