Renaissance Astronomy

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RenaissanceAstronomy
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The end of an era

• The decline of Greek civilization marked the end
  of scientific inquiry in the western world
• No new theories were proposed as alternatives to
  a geocentric universe
• Observations, if made at all, were to refine the
  accepted model of the solar system
• No observations were accurate enough to
  contradict Ptolemys model

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Islamic Astronomy

• Translation the ancient works (including Ptolemy)
  into Arabic
• Built observatories
• Named stars (ie. Betelgeuse, Altair, Aldeberan)
• Attempted to verify the existing theories

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Rebirth of Western Astronomy

• Rediscovery of the ancient texts (now in Arabic)
• Growth of Universities in the 13th century
• Began as places to gather and hear scholars
  lecture on the wisdom of the ancients
• Students recited and commented on previous works
• Eventually evolved to criticism and analysis

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The Copernican Revolution

• Nicholas Copernicus (c1500 A.D.)
• Had a rich uncle, didnt need to work
• Devoted his life to academics (medicine,
  astronomy, Greek, philosophy, mathematics)
• Published works on economics and trigonometry
• Astronomy was his main interest
• De Revolutionibus Orbium Caelestium
• A heliocentric model

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The Heliocentric Model

• New model motivated by the fact that the
  Ptolemaic model did not quite agree with the best
  available observations
• Driven by a desire to create a simpler model
  without so many circles
• Still needed to account for the daily motion of
  the sky, the yearly movement of the sun through
  the stars, and the motion of planets

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The Heliocentric Model

• The sun exists at the center of the universe
• Planets orbit the sun in perfect circles
• Retrograde motion due to overtaking slower
  planets
• The Moon orbits the Earth
• The Earth rotates on its axis once per day
• The stars are very far away (at least 1000 AU)

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The Immediate Impact of Copernicus

• Copernicus was recognized as a great astronomer
  because of his mathematical results about the
  positions of the Sun, Moon, and Planets
• The heliocentric model was either ignored or
  disregarded
• The heliocentric model couldnt predict anything
  Ptolemys geocentric model wouldnt
• It was just prettier

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The Great Observer

• Tycho Brahe (c1575)
• Danish nobleman
• At 26 he wrote a book describing his observations
  of a new star (nova)
• Established as a quality astronomer
• To keep him in Denmark, the king gave him an
  island and all its residents, and paid for the
  construction of an observatory

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Tychos Observations

• Whenever possible, his instruments were made of
  metal instead of wood, made very large to mark
  off small fractions of degrees, and were placed
  in permanent mountings
• Observations of positions were twice as accurate
  as any before him (to within 1)
• Carried out regular observations over a period of
  many years and replaced the error-ridden body of
  ancient observations on which theories had to rely

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Tychos Theories

• Concluded the Earth was stationary
• Reasoned hed be able to observe stellar parallax
• Would be able to if stars were no further away
  than 7000 AU
• Never observed parallax and could not accept
  larger dimensions of the universe
• Eventually arrived at a model where the sun and
  moon orbit earth, but all other planets orbit the
  sun
• Copernicus mathematical models on planetary
  motion were too good to ignore

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Kepler and Planetary Orbits

• Tycho had a falling-out with the new king and
  moved to Prague
• Johannes Kepler joined his staff and took over
  for him after he died
• Kepler was a brilliant mathematician
• Tried to account for Tychos observations of the
  planet Mars

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Fitting Observations to Theories

• First approach was strictly Copernican (perfect
  circular orbits
• Didnt quite fit
• Neither did epicycles
• Neither did a lot of other geometric orbits
• Finally abandoned the 2000 year-old requirement
  of perfect circular orbits
• Settled on ELLIPTICAL ORBITS

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Keplers Laws

• First Law
• A planet moves on an elliptical orbit with the
  Sun at once focus

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Major axis
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Semi-major axis
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Perihelion
Aphelion
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Keplers Laws

• Second Law
• An imaginary line from the Sun to the planet
  sweeps out equal areas in equal times
• A planet travels faster at perihelion and slower
  at aphelion

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Keplers Laws

• Third Law
• The square of the sidereal period of a planet is
  proportional to the cube of the semi-major axis
  of its orbit
• P2 a3

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Examples

• Saturn has a semi-major axis of 9.5 A.U.
• What is its period?
• It takes Neptune 164 years to complete one orbit.
• What is the size of its semi-major axis?

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The Telescope

• Invented in the early 1600s by an unknown Dutch
  lens maker
• Two lenses combine to magnify distant objects
• Early telescopes were poor quality and do not
  compare favorably to a good pair of modern
  binoculars

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Galileo Galilei (1564-1642)

• In the summer of 1609, Galileo heard about the
  telescope
• Built one himself
• Made important observations of the stars, Moon,
  Sun, and planets

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The Stars

• Found previously unknown stars
• The Milky Ways glow was due to many stars too
  faint and too close together to be distinguished
  by the unaided eye
• The stars remained the same size when viewed
  through the telescope
• Must be very far away
• One of Tychos main arguments against the
  heliocentric model was dismantled

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The Moon

• Celestial objects were generally assumed to be
  perfect, smooth spheres
• Galileo found the Moon has mountains, valleys,
  and plains
• This suggested the Earth was not as unique as was
  once thought

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The Sun

• Galileo found the Suns surface was not perfect
• Dotted with dark spots (sunspots)
• Sunspots move across the disk and repeat about
  every four weeks
• Reasoned that the Sun rotates once every month or
  so
• If the Sun rotates, why not Earth?
• Side note never look at the Sun through a
  telescope.

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Planets

• Jupiter
• Jupiter has moons! Galileo found four moons
  orbiting Jupiter, naming them (in order of
  distance from Jupiter)
• Io
• Europa
• Ganymede (largest, about 10 bigger than Mercury)
• Callisto
• Earth is not the center of ALL motion

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Planets contd

• Venus
• Venus exhibits phases like the Moon
• Has a larger angular size at new phase than full
  phase
• Clearly, Venus orbits the Sun!
• Goodbye, Ptolemaic Model!
• Tychos Model could still account for this, but
  it soon also fell out of favor because Gravity
  could account for the motions in a heliocentric
  model

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Reaction of the Church

• The Church had largely ignored the heliocentric
  model until Astronomers began to think of it as a
  realistic picture rather than a mathematical
  model
• The Church believed this contradicted biblical
  teaching declared it heretical
• Galileo was told he could no longer hold, teach,
  or defend the Copernican system either orally or
  in writing

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What did Galileo do?

• For many years he worked on other scientific
  problems
• Pope Paul V died, replaced by friendlier Urban
  VIII who told Galileo he could write about it so
  long as it was treated only as a hypothesis
• Galileo wrote a book
• Dialogue Concerning the Two Chief World Systems
• Takes the form of a conversation between
  Simplicio (Ptolemaic) and Salviati (Copernican)
  and Sagredo (neutral)
• Very demeaning to the old model (and the Church)

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The Church gets Pissed

• Galileo was brought before the Inquisistion
• Found guilty of disobeying the order of 1616 that
  he not advocate the Copernican theory in writing
• Not convicted of heresy because he denied the
  truth of the new theory
• Book was banned (as was De Revolutionibus)
• Galileo was imprisoned for the remainder of his
  life
• Ban was lifted in 1822 and Galileo was finally
  cleared by the Church in 1993