Experiment II-6

Earth's Orbit

Galileo's success


Patience required! The photographs needed for this experiment may take a couple minutes to fully download.

Galileo Galilei (1564-1642) used his observations with a telescope he had built to try to convince everyone that we live in a Copernican universe with the Earth moving around the Sun.  While he used the scientific methods and logic developed by the ancient Greeks, he rejected the widely believed notion that the Greeks correctly understood all phenomena.  He had observed numerous heavenly imperfections and several moons orbiting Jupiter, all contrary to the accepted Greek explanations of the heavens.  But unfortunately Galileo lacked any evidence that the Earth actually moves!

Using three party dialogues published in Italian designed to bypass scholars who by tradition wrote and read Latin, Galileo embarked on a propaganda campaign to use the evidence he did have to convince his readers of matters for which he had no evidence.  The character of Salviati speaks for Galileo's views while Simplicio presents the ancient ideas of the scholars.  Then the objective, intelligent, and supposedly impartial Sagredo, having heard both arguments, points out why Galileo's views are superior.  Not only were Galileo's dialogues more interesting to read that the dry textbook style of the scholars, but they were also very persuasive.

Still a close reading reveals an absence of evidence that the Earth actually moved as required by Copernicus' explanation.  As a result of the controversy Galileo created, the dominant Catholic Christian Church headquartered at the Vatican in Rome held a legal trial called an Inquisition to determine which of the explanations of heavenly motion was correct.  After calling for the presentation of all available evidence about any motion of the Earth, the Inquisition found no evidence that the Earth really orbits the Sun.  Because the accepted physics of the time requiring all bodies to fall downward, the Earth was required to be in the center of the Universe.  The Inquisition concluded that both the explanations of Eudoxos and Copernicus are false.

When an acquaintance of Galileo was elected as new Pope to head the Church, Galileo met with him and gained his permission to write an impartial book on the subject.  Galileo called the book The Flux and Reflux of the Seas (meaning the tides) but the Pope suggested a better title would be Dialogue on Two Great World Systems and supported its publication.  Several Censors hesitatingly allowed publication.  But the dialogue's clear intent to promote the Copernican view quickly resulted in the banning of the book and attempts to retrieve all sold copies.  The Inquisition investigated whether Galileo had violated the previous findings about the Copernican system.  Galileo claimed he never really believed that the Earth orbits the Sun.  In the end Galileo was found guilty of perjury of having held and believed a doctrine that is false and placed under house arrest and banned from every discussing the motion of the Earth again.

In the last years of his life, Galileo wrote one final book using his persuasive dialogue style to present Discourses and Mathematical Demonstrations Concerning Two New Sciences.  In the second new science, Galileo proposed a new physics which would not require the Earth to be the center of the Universe.  It was published far from the influence of the Catholic Church in Leyden, Holland.  By removing the last major reason opposing the motion of the Earth, Galileo succeeded in convincing much of the educated world that the Earth does in fact move as Copernicus proposed.  This was all accomplished without any evidence for the Earth actually moving!


sun cameraWith much of the population convinced that the Earth orbits the sun, it becomes appropriate to measure and plot the path of the Earth.  (For those persisting in the belief in the Ptolemeic system, the same path also fits the orbit of the Sun around the Earth.)  The size of a distant object such as the Sun varies inversely with the object's distance:  At smaller distances, an object appears larger.  This property of geometry can be used to determine the relative distance between the Sun and the Earth.  The direction of the Sun compared to the celestial sphere has been accurately recorded over three millenia.  Since the Earth apparently follows the same orbit each year, available information from any year will be adequate for determining the orbit.  Below we have information obtained from 1963!

The monthly photographs below were taken by a fixed camera that uses a mirror to track the Sun.  The angles around the ecliptic are arbitrarily determined by the tradition of assigning 0° to the location of the Sun at the Vernal Equinox on March 21.


  1. Many monitors allow adjustments.  This may accidentally distort measurements such as we propose to do.  Measure the lengths of the horizontal and vertical gray bars on the January photograph below.  Adjust your monitor until these two measurements are identical.  (If the suns in the photographs are too large to allow measurement of their diameters, go to smaller photographs available here.)
  2. Carefully and as precisely as possible, measure the diameter of the Sun in each photograph.  Note the date and location of the Sun in the sky.
  3. On a large piece of paper, locate a dot in the center to represent the Sun.
  4. Arbitrarily draw a ray outward from the sun to represent the direction of the Earth at the Vernal Equinox (0°).
  5. Use a protractor to clockwise determine the direction from the Sun to the Earth on each photograph date.
  6. Choose a convenient constant about ten times one of your diameter measurements. (The larger the constant, the larger the plot you will construct.)
  7. Divide each measured diameter into the constant to determine the distance the Earth is from the Sun.
  8. Carefully place a dot for the Earth out from the Sun's dot the calculated distance for each photograph.
  9. Use an appropriate method to plot a circle that passes through all the Earth points.  Does an off-center circle fit the orbit of the Earth?
  10. Determine the dates of closest approach called perihelion and greatest distance called aphelion.  Does this fit your expectation based on the warmth of the seasons?
  11. What is the ratio of the closest approach to farthest distance?
January photo
February photo
march photo
April photo
May photo
June photo
July photo
August photo
September photo
October photo
November photo
December photo



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created 4/18/2003
revised 5/6/2003
by D Trapp
Mac made