Cosmological Systems, Part I

This OER was designed by the OU Academy of the Lynx (oulynx.org) in conjunction with the "Galileo's World" (galileo.ou.edu) exhibition at the University of Oklahoma.

This activity is designed to be completed in 5 minutes by a typical visitor to the exhibition. For adaptations to other age levels and pedagogical settings, visit the "Cosmological Systems Educational Cluster" below.

Introductory Cosmological Systems Activity

In 1623 Galileo Galilei published his illustrations of the phases of Venus which he had discovered through his telescope. See those here, 

The phases prove that Venus revolves around the Sun, as in the Copernican model. Today we often regard the discovery that Venus shows phases as proof of Copernicanism, but this is mistaken, for other systems in discussion at the time also predicted that Venus would show phases.

In 1660 the Jesuit polymath, Athanasius Kircher, discussed 6 varieties of cosmological systems and provided a pictorial depiction of each of them. See Kircher's image here,

Using Kircher's pictorial representation of the six world systems, answer the following question: Which of Kircher’s six world systems were compatible with Galileo’s discovery of the phases of Venus?

1. Ptolemaic?
2. Platonic?
3. Capellan?
4. Tychonic?
5. Semi-Tychonic?
6. Copernican?

Observations to empirically distinguish between a multiplicity of systems proved elusive. 

• Do Venus and Mercury show phases? How many of these systems predicted the phases of Venus and Mercury? Why was Galileo’s discovery of the phases of Venus not sufficient to prove Copernicanism?
• Do the orbs of Mars and the Sun intersect? If so, the spheres must be fluid instead of solid.  The parallax for Mars and the Sun was very difficult to assess.
• Do comets display retrograde motion at least some of the time, as implied by a moving Earth?
• Do the stars appear to shift in position (stellar parallax), as implied by a moving Earth? If not, then then stars must be so far away that (if Copernicus is right) they would be larger than the orbit of the Earth. Some Copernicans like Lansbergen accepted this inference as a manifestation of divine omnipotence.

Phases of Venus: Answers

Phases of Venus:  Answer

4 of the 6 systems in Kircher’s illustration predicted, or were consistent with, the phases of Venus:

• The first system in Kircher’s diagram is the Ptolemaic. This was disproved by the phases of Venus.
• The second is the Platonic, which switched the positions of Venus and Mercury. It, too, was disproved by the phases of Venus. 
• Third, in the middle row, left, is the Egyptian system proposed in antiquity by Martianus Capella and never forgotten. In it, Venus and Mercury revolve around the Sun, not around the Earth.
• Fourth is the Tychonic system, in which the Sun carries the planets around it while it in turn revolves around the central Earth. The Tychonic system was published in 1588 before the telescope. 
• The 5th system, lower left, is Riccioli’s variant of the Tychonic, where the two outermost planets revolve around the Earth instead of the Sun.
• And the 6th is the Copernican, lower right.

Historical Background for the Cosmological Systems.

The phases of Venus disproved the Ptolemaic system, but they did not prove Copernicanism. Even in antiquity, the Ptolemaic or Earth-centered system was not the only one under discussion.  Heraklides of Pontos argued in the 4^th century B.C.E. that the Earth rotates on its axis.  Plato switched the positions of Venus and Mercury.  Philolaos reported that some of the Pythagoreans held to a Sun-centered system, which was rejected when Aristarchus demonstrated the absence of stellar parallax.  In late antiquity, Martianus Capella argued that Venus and Mercury revolve not around the Earth, but around the Sun.

Regiomontanus, the greatest European astronomer of the 15^th century, proved in the Epitome of Ptolemy’s Almagest that astronomers might transpose the positions of the Earth, Sun and other planets without affecting planetary predictions. The following century witnessed a proliferation of transpositions as various astronomers created many diverse systems of the world. Read more about Regiomontanus here, https://galileo.ou.edu/exhibits/almagest-ed-regiomontanus.

Kircher illustrated 6 chief world systems debated at the time, although it is not exhaustive. For example, his illustration does not include systems with a rotating Earth, nor the interesting case of the Jesuit, reported by Riccioli, who created a system with the Moon in the center as a form of protest.

Further OER's on Cosmological Systems

Use the following OER's to further explore the Galileo's World exhibition.

• Galileo's World iPad Exhibit Guide, itunes.apple.com/us/book/galileos-world-exhibit-guide/id1032005948?mt=1.
• Galileo's World iTunes U Course, itunesu.itunes.apple.com/enroll/FDS-EYK-MRL.
• Galileo's World website, galileo.ou.edu.

Cosmological Systems Educational Cluster

We want to create variations on this activity that connect the Cosmological Systems Activity to a variety of ages. Use the following chart and hyperlinks to find the one to best fit your group.