these seemed important at 1st reading ..

Someone could get a rich education following your links ljk .. unfortunately I won't always have the time but at the least will bookmark the abstracts I want for later reading ..

we claim that the concept of orbit is revolutionary and its introduction into astronomy dramatically changed the practice of physics as well as that of astronomy. Kepler insists that the path of a planet moving in space is what needs to be explained by physical or causal reasoning. Thus the goal of astronomy is to account for the path of a planet in three dimensions in physical terms. The concept of orbit, that is, the path together with its physical causes—formulated as physical laws— becomes the central issue, which henceforth an astronomer had to address. Previously, astronomers were primarily expected to produce a model and then to apply it to compute the positions of a planet in longitude at any given time, past or future. By and large, in the period before Copernicus, cosmology, that is, the arrangement of the planets in space and the nature of their substance, received separate treatment apart from positional astronomy. However, the two subjects, cosmology and astronomy, overlapped to some extent in discussions of orbs.

The orb of a planet is a spherical shell whose inner radius corresponds to the least distance of the planet from the Earth and whose outer radius corresponds to the greatest distance of the planet from the Earth. The ratios of these distances were assumed to be the same as the ratios of maximum to minimum distances in Ptolemy’s models. Since the shells are all hollow, they can be nested in the manner of Russian dolls (or the layers of an onion, to use a medieval analogy). The nesting principle introduced by Ptolemy is that the minimum distance of a planet is equal to the maximum distance of the planet closer to the Earth in this order from nearest to farthest: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, and the Fixed Stars. Since the relative distances of a given planet can be computed from its model, only one absolute value is needed to ax all the other values.

Kepler was the arst to draw the Ptolemaic and Copernican models to scale and he noticed that for Copernicus there were signiacant gaps between adjacent planetary orbs, that is, the minimum distance of a planet from the Sun was not equal to the maximum distance of the planet closer to the Sun where the order is: Mercury, Venus, Earth, Mars, Jupiter, and Saturn .. Kepler considered these gaps to be a problem. He exclaimed: “What extravagance of Nature [Naturae luxuries] is this, so out of place, so pointless, so little like herself?” .. With this in mind he began his search for a new concept that would allow him to resolve both the geometrical and physical aspects of the problem that he had formulated, and the arst fruits of this search were already reported in his Mysterium cosmographicum.

We have documented that Kepler initially used orbis as a geometric term devoid of material existence. Then he employed a number of terms for the path of a planet in space such as iter and via. The arst signiacant occurrence of orbita is found in the letter to Fabricus in February 1607 and it is contrasted, as the true path, with spira (coil) which refers to the apparent path of a planet as seen by an observer on the Earth (without physical causes). Indeed, the data in the historical record available to us suggest that Kepler began to use the term orbita for the true path of a planet in 1607. This term is then applied in a consistent way in the Astronomia nova, connoting not just the true path but, as Stephenson put it, “a trajectory of a planet moving through space, acted upon by physical forces” which, we may add, is cast in the form of laws expressing physical causes. However, an explicit deanition only appears in the Epitome of Copernican Astronomy.