Retrograde motion of the Earth and planets does not prove Copernicanism.

Usually the 3rd most cited reason to 'prove' Heliocentricity. It is an invalid claim.

Introduction

A long-standing problem, known to ancient astronomers, is that of planetary retrograde motions. Mercury, Venus, Mars, Jupiter, and Saturn, appear as bright stars that move along paths inclined to the ecliptic by only a few degrees. Their observed periods of motion with respect to the Sun range from 116 days for Mercury, 584 for Venus and 780 days for Mars.

These planets move west to east through the stars like the Sun and moon do, but from time to time the planets halt their eastward motion and appear to move backward, east to west, before resuming their normal eastward motion. In principle, each of the planets, as viewed from Earth, will create a retrograde motion, although some, due to their proximity to Earth, will have more pronounced retrogrades. This is true of Venus and Mars, the latter’s path being the most eccentric.

Retrograde motion does not support heliocentricity and raises quite a few questions. After stellar parallax and stellar aberration, it is the most cited ‘proof’. None of them, however, ‘confirm’ heliocentricity.

Complexity

In the Ptolemaic model (or variations of the geo-centric model), complex epicycles were developed to explain this backward motion of planets. This complexity was used as proof against geo-centricity, and it is indeed an issue with Ptolemy’s model. Copernicus, however, uses more epicycles than Ptolemy, a fact dismissed by Copernicans as irrelevant (~90 vs 48), but it is entirely relevant, given the foundation for their theory is its supposed simplicity.

The Copernican-heliocentric model emphasises the speed of planets closer to the Sun, giving the appearance of retrograde motions of those planets precessing at a slower velocity. In the Tychonic model, which is a mix of geo-helio centricity, the retrograde movement can be straightforwardly explained, as outlined below, incorporating Copernican arguments. All of these models contain their own complexity.

Explanation of Retrograde Motion

Various astronomy texts and other science publications have consistently appealed to this phenomenon as a proof for heliocentrism! Science textbooks illustrate the occurrence with elaborate diagrams, websites use sophisticated java script animations, both stating that only the heliocentric model has an explanation for retrograde motion (source; Physics in my view).

[Heliocentric explanation of Retrograde motion - Video]

Model’s and views

Sunworshippers explain retrograde motion in a simple and straightforward way. The planets closer to the Sun move more quickly than planets farther away from the sun. Hence, the Earth travels more quickly than the ‘superior planets’, or planets that are further from the Sun than the Earth.

Moving more quickly and traveling in a smaller orbit, the Earth overtakes the ‘superior planets’ each synodic period (or the time between 2 successive appearances in the sky, of a planet). From the perspective of the Earth, as this occurs, the superior planets appear to move backward. In the same way, as one car passes other cars, the passed cars appear to move backward.

In a similar manner ‘inferior planets’, or planets orbiting closer to the Sun than the Earth does, go through a retrograde motion as they overtake the Earth each synodic period. This is of course relative, not absolute motion but on the Earth, we cannot discern this (source: Britannica).

Heliocentric Retrograde Motion

Figure 1: The Earth and Mars are revolving counterclockwise around the sun. The red line represents the appearance of Mars’ motion against the fixed stars, as viewed from Earth.

Figure 2: As Earth overtakes Mars in their respective orbits around the sun, Mars appears to move backward against the fixed stars.

Figure 3: As Earth begins to revolve downward, Mars is moving more laterally, giving the appearance that Mars is resuming its forward course against the fixed stars.

There is nothing inherently wrong in the heliocentric model to explain retrograde motions. It is however just an explanatory model, not a proven phenomenon.

Other models explain the phenomena

Unfortunately for the Copernican triumphalists, both the geocentric model, or the geo-helio-centric model (the Tychonic) explain the phenomenon of retrograde motion just as well as the Copernican model. The video below is a good animation clip of how geocentrists explain the retrograde motions of the planets. Since the Copernican, Ptolemaic and the Tychonic models can incorporate the same geometrical distances between the planets and the Sun, all models in principle, can account for retrograde motion, and they will do so in identical geometrical proportions.

Geocentric Retrograde Motion

In the Tychonic system, all the planets orbit the Sun, and this ‘constellation’ orbits the Earth. In this model the inner planets appear retrograde for an observer on Earth, when they are in front of the Sun and going in the opposite direction, because their apparent radial motion is greater.

The outer planets appear retrograde when they are opposite the Sun, because in effect they are being pulled along while in their slow orbit of the Sun. Since the motion that dominates is that of the Sun, these planets are on the opposite side of the Earth from the Sun, and they appear to be in retrograde motion for an observer on Earth as given in the images below (Britannica).

Figure 4: The Sun is revolving counterclockwise around the Earth as Mars is revolving around the sun. The red line represents Mars’ motion against the fixed stars.

Figure 5: As the Sun begins to move further in its orbit and carry Mars with it, Mars will appear to slow its speed and reverse its course against the fixed stars.

Figure 6: As the Sun moves even further in its orbit, Mars moves to the left, thereby causing it to appear to resume its forward course against the fixed stars.

As the above diagrams illustrate there is no difference to the observed phenomena between the Heliocentric or Tychonic models. The geometric maths are also equivalent in both cases.

Bottom Line

Earth and planetary retrograde motions can be explained by helio, geo, or geo-helio-centricity. The Copernican system incorporates more epicycles than the Tychonic or Ptolemaic, so simplicity cannot be used in its defense. It is mechanically very difficult to prove that any of the 3 models are valid or incorrect given the geometrical mathematics which can be used to support all 3 interpretations of the quite discernible retrograde motions of the planets.

Given this model equivalency, heliocentrists cannot honestly claim planetary retrograde motion as confirmation of Copernicanism. Retrograde motion remains an unsolved astronomical truth, one known to the ancients who never believed that planets stopped and regressed, but who used ingenious maths and astronomical logic to explain what they viewed in the cosmos.

All hail.

Sources

Martin, Douglas R. 1984, “Status of the Copernican theory before Kepler, Galileo, and Newton,” American Journal of Physics. 52, 982.

Britannica 1- Britannica's entry on the geocentric model, providing historical background and detailing Ptolemy's contributions

Britannica 2 - Britannica article specifically discussing Ptolemy's use of deferents and epicycles to explain retrograde motion and the adjustments made to account for observational discrepancies

Britannica 3 - Britannica article explaining the Tychonic system

Britannica 4 - Britannica article on the absence of observable parallax as one of the key arguments Tycho used to support his system

Socratic.org - A concise summary of how Ptolemy's model accounted for retrograde motion using epicycles and deferents

Physics in My View article on the Tychonic model