AT imeline of the Early Astronomers

							A Timeline of the Early
     Astronomers
Aristotle (384 -322 B.C.E)
             • Born in Greece.
             • Studied with Plato.
             • Studied the natural world
               including objects, matter,
               living organisms,
               medicine, weather,
               climate.
             • Believed the Universe
               had no beginning and
               would never end –
               eternal.
Aristotle’s Universe
           • 55 crystalline spheres
             in which celestial
             objects were
             attached.
           • Each moved at
             different velocities.
           • Earth was at the
             center of these
             spheres.
           • No forces in the
             cosmos.
           • Geocentric model.
Hipparchus of Rhodes (190-120 B.C.E.)
                   • Lived and worked in Rhodes,
                     Greece.
                   • Worked with geocentric model.
                   • Measured precession –
                     Thuban was his North Star.
                   • Measured distance to Moon
                     during a solar eclipse.
                   • Measured location and
                     catalogued over 850 stars
                     which became the basis for all
                     astronomers after him.
                   • Used eccentrics, epicycles and
                     deferents to explain the
                     variable motion of the sun and
                     planets through the zodiac.
Ptolemy (Circa 90 C.E. – 168 C.E.)
               • Studied in the Great Library
                 of Alexandria.
               • Wrote the Almagest, the first
                 professional astronomy
                 textbook.
               • Accurately predicted
                 planetary motion.
               • Refined the Geocentric
                 model.
               • Remained as accepted
                 wisdom until Copernicus’
                 heliocentric model in 1543.
         Geocentric Model
• Any model of the universe must explain
  motions of celestial objects.
• Daily motion.
• Annual motion of Sun, planets, stars.
• Comets.
 Some Problems With Geocentric
            Model
• Sun speed not constant throughout a year.
• Planets speed not constant.
• Mercury and Venus always close to the
  Sun.
• Planets brightness not constant. Brightest
  when opposite the Sun.
• Retrograde.
  Retrograde Motion of Mars
over the course of a few months.
  Eccentric, Epicycle and Deferent
• Eccentric –                      • Epicycle and deferent –
   – Places the Earth not at the     Used in geocentric
     exact center of planet’s        model.
     circular orbit.                  – Explains retrograde motion
   – Explains why planets               of the planets.
     appear to move faster in         – The deferent was a large
     part of their orbit.               circle made by an orbiting
   – Used in the geocentric             planet, sometimes
     model.                             centered on Earth,
                                        sometimes not.
                                      – The epicycle was a small
                                        circle made by the planet
                                        along the circumference of
                                        the deferent
Copernicus (1473 – 1543)
            • Polish
            • Earth rotates on its axis
            • Revolves around a stationary
              sun.
            • Developed Heliocentric
              model.
            • Sun at the center of the
              cosmos.
            • Initially neither accepted nor
              rejected by fellow
              astronomers.
            • Problem, no observable
              parallax.
            • Eventually sparked a
              firestorm during the
              Renaissance years.
The Heliocentric Model
  Copernicus’ Heliocentric Model
• Sun is at the center.   • Circular orbits.
• Stars are much          • Uniform speeds.
  farther than the Sun.   • Planets closer to Sun
• Earth spins on axis.      move faster.
                          • Example: Mercury
                            moves faster Mars.
   Copernicus Model Solves (some)
   Problems with Geocentric Model
• Apparent motion of Sun
  is caused by Earth’s
  revolution
• Retrograde is an optical
  illusion.
• Eliminated epicycles
  and equants.
• More complicated.
• Violated Aristotelian
  physics.
• Still retained concept of
  spheres and no forces.
• Did not explain varying
  speed of planets in
  orbits.
Tycho Brahe (1546 – 1601)
             • Danish.
             • November 1572
               “stella nova”, a “new
               star”.
             • In Cassiopeia, bright
               enough to be seen
               during the day!
             • No parallax.
             • “Don’t hold it in!”
             • Silver nose.
• Used parallax to measure
  distance to a comet.
   – Farther than the Moon.
   – Aristotle – comets were an
     atmospheric phenomena.
• Found no parallax for the
  stars.
   – Either Earth is stationary or
     stars are too far away.
   – Incorrectly chooses that
     Earth is stationary.
• His model is a
  combination of
  Geocentric and
  Heliocentric
   Tycho Brahe’s Hybrid Model
• Could not accept that “the
  earth, that hulking, lazy
  body” could be moving
  around the sun. “unfit for
  motion”.
• Moon and sun revolve
  around the earth, all other
  planets revolve around
  the sun.
• Although not always
  correct, his observations
  led to development of
  modern Astronomy.