Astrophysics can be said to have been founded by Johannes Kepler around the year 1600. He based his break-through science on data of the position of Mars in the sky and disproved the ancient ideas about circular orbits and epicycles.

But why wasn't this done far earlier, by using observations of the Moon? Wasn't it pretty obvious to a careful astrologer a thousand years ago, that the Moon does not have a circular orbit and does not describe epi-cycles? It is the easiest celestial object to observe, visible both night and day. Moon calendars may have been designed tens of thousands of years ago, there's no lack of observational data. Kepler instead used a few oppositions of Mars which take place only once every two+ years. Since the Moon is the one object which does orbit Earth, in a geocentric world view it should've been the perfect test of circular and epi-cyclical theories about its orbit. Its nearness causes a daily parallax between moonrise and moonset, but that wouldn't be beyond a genius like Kepler or many mathematical astrologers before him.

What made the orbit of the Moon difficult for the ancients to understand?

  • $\begingroup$ A curious historical quirk is that Kepler liked circular orbits and he tried "egg shaped" orbits reluctantly because he couldn't get circles to work. He also had better data than anyone previously. Brahe took very careful measurements, not with telescope but with one of these: airandspace.si.edu/exhibitions/explore-the-universe/online/etu/…, so the measurements Kepler had to work with were about 10 times more accurate than anyone previously. Galileo liked circular orbits too. $\endgroup$
    – userLTK
    Nov 14, 2015 at 11:20

1 Answer 1


Wasn't it pretty obvious to a careful astrologer a thousand years ago, that the Moon does not have a circular orbit and does not describe epi-cycles?

The ancient Greek model of the motion of planetary bodies remained unchallenged for almost two millennia, so obviously not.

Hipparchus' model did a fairly good job dealing with the elliptical motion of the Moon; it did even better with the planets. The Moon's motion is tough to model because of perturbations by the Sun, Venus, and Jupiter. Ptolemy discovered what would eventually be called evection, the largest of the perturbations caused by the Sun. There was one problem with Ptolemy's model: It had the Moon swinging in and out by a huge amount. If Ptolemy's model was correct, we would see the Moon changing in diameter by a factor of two over the course of a bit over half an orbit. Copernicus much later came up with a scheme that fixed this problem and still relied on those old concepts of deferents, equants, epicycles, etc.

While Newton pointed the way to describing the Moon's orbit, it wouldn't be until 200 years after Newton's death that a decent (one that matches observations) model of the Moon's orbit was developed.

  • $\begingroup$ I'm not convinced that perturbations by Venus and Jupiter is a good explanation to why the orbit of the Moon is hard to understand. Kepler showed that Copernicus' world view was mathematically identical to that of Ptolemy. Deferents and equants, well, we do have center of masses in the middle of empty space today. The "thing" around which everything turns. The ancients actually made up focus points in the circle before they understood that it is an ellipse. And what does a moon if not describing an epicycle? Galileo literally saw epicycles in the sky when he discovered the moons of Jupiter. $\endgroup$
    – LocalFluff
    Mar 22, 2015 at 23:12
  • $\begingroup$ I must add that the Moon is special since it actually orbits Earth. If you're geocentric, then Luna is your friend. But it is an exception, one of a kind. Any conclusions drawn from observing it will likely go bad. Astronomy is hard. $\endgroup$
    – LocalFluff
    Mar 22, 2015 at 23:18
  • $\begingroup$ @LocalFluff - The perturbations from the Sun are the primary reason the orbit of the Moon is hard to understand. Venus and Jupiter just add to that. It's orbit deviates significantly from an ellipse; these deviations were known in Kepler's and Newton's time. With enough epicycles, Ptolemy's construction can create any observed shape. Two sufficed for the observational data available up until Brahe's time. $\endgroup$ Mar 22, 2015 at 23:32

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