September 13
The Heliocentric Alternative

"..a sure bond of harmony for the movement and magnitude of the orbital circles such as cannot be found any other way."

Nicolaus Copernicus, De Revolutionibus Orbium Coelestium


Reading: Check of the website on Foucault's pendulum and how it shows that the Earth must be rotating.
Also, please read the web pages on Tycho Brahe and Johannes Kepler

Problem Set #3 is due Thursday, 15 September at 1:00 pm

Sign up for a time slot for Observing Lab #2.

In Class:

Question to Ponder

The Sun and the Moon have very nearly the same angular size when viewed from the Earth. (This is in part why solar eclipses are so short in duration and so spectacular.) From this information, we can conclude that
  • a) the Sun and Moon have the same physical size.
  • b) the Sun and the Moon are the same distance away.
  • c) the ratio of each object's size to its distance is the same.
  • d) none of the above.

Reasons Geocentrism Lasted so Long

  • It made a good story; easy to understand
  • Didn't contradict everyday experience (THE EARTH DOESN'T MOVE!)
  • More accuracy wasn't really needed

The Sociopolitical Environment Changes During the Renaissance

  • Rise of affluent European nation-states
  • Longer distance trade; navigation issues
  • Exploration of New World, sea routes to Asia
  • Weakening of Church control over academia (especially in northern Europe)

Nicolaus Copernicus (1474-1543)

  • Father of modern heliocentric theory
  • Religious administrator by trade, but deeply interested in mathematics and astronomy
  • Tried to reconcile his own observations with Ptolemaic system, but without luck
  • Considered a heliocentric system as an alternative
  • Published De Revolutionibus orbium coelestium, On the Revolutions of the Celestial Orbs
  • His heliocentric theory was no less complex and convoluted than Ptolemy's geocentric model:

  • Moreover, it didn't even predict planetary positions any better than Ptolemy's!

Pros and Cons of the Copernican Heliocentric Model

  • Cons:
  • Didn't predict planetary positions any better than geocentric model
  • Parallax caused by the Earth's revolution around the Sun should be observed
  • Aristotelean physics says that the Earth cannot be moving
  • Pros:
  • The Sun is biggger than the Earth (Aristarchus)
  • The Earth is just like the other planets (Galileo)
  • Not everything orbits the Earth (Galileo)

The Parallax Problem

  • If the Earth moves around the Sun, we should see "turnings and passings of the fixed stars," e.g., parallax shifts over the course of the year.
  • Lookit the special parallax web page to see how one mght observe parallax for nearby stars.
  • In fact we do observe parallax shifts for nearby stars, but the "nearby" stars are so far away that the shift was completely imperceptible for pre-telescope (as well as 200 years of post-telescope) astronomers.
  • Parallax was not conclusively measured until 1838, and only then because telescope technology had advanced enough to detect the tiny shifts.

Galileo's Astronomical Reasons to Believe in Heliocentrism

  • Galileo was the first to use a telescope to systematically study the sky.
  • Detected sunspots -- the Sun is not pure and pristine.
  • Discovered mountains and craters on the Moon -- again not a pristine or perfect environment.

  • Detected Jupiter's Moons -- not everything orbits the Earth.

    These are Galileo's own notes of the movement of the "Medicean Stars" (a.k.a. the moons of Jupiter) relative to Jupiter from night to night.

  • Detected phases of Venus -- concludes from patterns of phases that Venus must orbit the Sun, not the Earth. The most important observations here is that Venus appears in both crescent and gibbous phases when close (in angle) to the Sun. This can only happen if Venus orbits the Sun.

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