What were the challenges for the ancients to observe the orbit of the Moon (instead of Mars)? 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?
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.