Heliocentrism vs Earthcentrism

Helios - Sun

A large number of you are ElEd and English majors

Tutoring - come to the ARWC in Good Library to sign up for peer tutoring or tutor tutoring.

On writing assignment, some of you said for 4.) stars move *eastward*, that's also correct.

9 did not hand in a writing assignment--some of you I know were here??

Greek model - Earth centered

Retrograde motion:
retrograde motion

By the 1500s, The Greek model with Earth at the center, as capped off by Ptolemy (2nd century AD), had been around for nearly 1800 years. It did a good job of explaining the observations of...

  • motion of constellations across the sky
  • motion of sun and moon around Earth
  • retrograde motion of the planets against the background of stars: planets orbit in epicycles around their basically circular orbits (around Earth).

Metaphysical content of the Greek model: Imperfect earth below with irregular mountains/valleys/oceans   VS   Perfect realm above with perfectly round moon, $amp; planets orbiting in precise combinations of circles.

Ptolemy's innovation was the equant (the $\cdot$ in the picture): From this point, the planetary epicycles appear to move at a constant angular speed.

Copernicus proposes heliocentrism (1543)

...in De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres).

Copernicus' talk with God

What instrument is Copernicus shown using to make observations with?

Copernicus made the rather modest claim that... it was easier to do the calculations of planetary positions if you pretended that the planets were all orbiting the sun in circles.

Can heliocentrism explain retrograde motion?

See this animation of how Copernicus' model deals with retrograde motion.

According to Owen Gingerich (Copernicus scholar *and* GC graduate) Copernicus' heliocentric model of the "universe" tended to be passed over by the scholars of the time, who were more interested in his new, equant-free, simpler methods for calculating planetary position.

Copernicus vs. Ptolemy

Brahe's Wall QuadrantTycho Brahe designed a new instrument (still not a telescope) which allowed him to make extraordinarily precise measurement of the planets' and stars' positions.

Whose model did his observations agree with better?

Brahe's measurements disagreed with both Copernicus and Ptolemy

Copernicus - Brahe ~ 8 arc min = 8/60 of one degree

Johannes Kepler

A student of Brahe's, Johannes Kepler, took this 8 arc min discrepancy seriously.

After 16 years of effort, he found that Brahe's measurements agreed more exactly with planetary orbits that were ellipses, not circles around the sun.


Astronomia Nova, 1609.

All this time, Kepler, Brahe and other astronomers earned money partially through their astrological publications.

Galileo's "Starry Messenger" 1610

Sidereus nuncius- Orion

Opticians in the Netherlands hit on a way of arranging lenses to make a 'spyglass' of obvious use for sailors. Galileo Galilei heard about this, and set about building one in order to look up at the night sky and started making observations in 1608. His first telescope had 3x magnification, and he eventually achieved 30x telescopes. The observations published in Sidereus Nuncius in 1610 were made with a ~20x device.

One of his first observations was the multiplicity of stars not seen by the naked eye. (Above, his sketch of Orion). Milky areas of the sky turned out to be thick with dim stars which could not be seen with the naked eye.

The face of the moon

Moon - Sidereus Nuncius Galileo sketched the Moon as seen through his telescope. It appeared to have mountains and craters--very much like the earth itself.

He looked closely at the moon and found irregularities at the edge of the terminator. These changed in time in a way that could be interpreted as light illuminating higher elevations before lower elevations were illuminated. The moon appeared to have mountains and craters, and was indeed just as 'imperfect' as the earth. Plato and other Greeks had argued that the heavenly spheres were perfect orbs, devoid of the kind of imperfections of earth.

Detail from a painting by Ludovico...

The changing picture of the universe was rapidly picked up by some artists, if not by church authorities.

The moons of Jupiter

Jupiter - Sidereus NunciusGalileo observed 3 or 4 'little stars' that moved fast, always close to Jupiter, and always in the same plane. Could these be moons circling Jupiter? Just like the moon circles Earth?

He received permission from papal authorities to publish Sidereus Nuncius ("Starry Messenger"). But between the time of receiving permission, and publication in March of 2010 he decided to insert (sneak) his latest observations of Jupiter into the manuscript.

The Greeks viewed the Earth as a special place, quite different from the far-away perfection glimpsed in the heavens. This picture was breaking down.

Phases of Venus

Venus - UV image from HubbleWith the naked eye, Venus appears as a 'dot' of light. It's not possible to make out the shape of that dot.

Shine an overhead projector lamp on a ball to see "phases" even in a well light room.

Spend more time on phases using Moon as example. Then do writing exercise with Venus. Show picture of earth, moon, with a "V" on it depicting field of view. Why do you *ever* see a full moon? (means orbit of moon is tilted....).

Galileo's telescope revealed that the shape was at times "horned" like the crescent moon. That is, it appears to be reflecting light (from the sun) rather than being a source of light.

[Writing - Phases of Venus]

Galileo was able to observe the phases of Venus with his telescope. His observation of a complete set of phases for Venus convinced many that, at least the inner planets rotated around the sun rather than the moving on epicycles around the earth.

Roots of a scientific worldview

  • Pythagoreans believed in natural harmonies--that the Universe is organized by patterns or principles which can be uncovered by observation.
  • European civilization picked this up, but certain discoveries of the Greeks hardened into a worldview: A belief in a dichotomy between earth and the heavens.
  • Copernicus, Galileo, and other contemporaries showed that the Earth is a planet similar to other planets--that the natural world would appear to be the same everywhere.

The story of Galileo pointedly illustrates how new observations, or higher precision measurements might overturn theories. In what sense might our current theories be said to be true or false?

The nature of science

Science is a human attempt, frequently collective, to organize experience using rational thought. It is a process, not a set of facts.

Direct experience includes observation, measurement, and experimentation.

A theory is a well-confirmed set of ideas that explains what is observed.

A scientific theory must be falsifiable.

Suggested Exercises

Conceptual Exercises in Chapter 1: 7, 10, 11,13, 20, 22

Image Credits

Matthew Spinelli, NASA / JPL,Galileo Galilei, Sidereus Nuncius, Transl Albert Van Helden, Copernicus' Conversation with God painted by Jan Matejko (1872), Tycho Brahe's Wall Quadrant


"Direct experience includes observation, measurement, and experimentation."

Give one example of each of these in the story told about astronomy so far.  Were all involved?


The story of Galileo pointedly illustrates how new observations, or higher precision measurements might overturn theories. In what sense might our current theories be said to be true or false?