A synodic cycle is simply the period from one conjunction of 2 celestial bodies to its next conjunction.
The Neptune-Pluto synodic cycle has not been studied by astrologers as much as some of the other synodic cycles. For example, the Jupiter-Saturn synodic cycle has long been regarded as important in mundane cycles. Jupiter's sidereal period (amount of time to revolve around the Sun and therefore to return to its previous zodiacal position) is very roughly 12 years and the period sidereal period for Saturn is very roughly 29 years. The duration of time between the return of the two planets to a conjunction (disregarding the multiple conjunctions that may occur from a geocentric perspective due to retrogradation) is roughly 20 years.
To some extent one may regard Neptune as the most outer planet, while Pluto may be regarded as half-planet and half-asteroid, the leader of the Kuiper Belt objects (although its leadership may be regarded as being shared by other large objects like UB313, etc.). My point here is not to tiptoe into the highly debatable topic of what Pluto's astronomical status is, but rather to simply point out that if one places certain restrictions on the definition of a planet, such as the allowable eccentricity and inclination of the orbit, then Neptune falls within this classification while Pluto does not.
NEPTUNE PLUTO SYNODIC CYCLE:
If we regard Neptune as the outermost planet, then we might look to Uranus-Neptune synodic cycle to establish a slow rhythm of hundreds of years within which the rise and fall of civilizations and the ongoing drama of human history unfolds. I do believe that a strong argument can be made for the relevance of the Uranus-Neptune synodic cycle to historical periods, but, very interestingly, the relationship of historical cycles to the Neptune-Pluto synodic cycle also appears to be very powerful.
Oppositions occur at the midpoints of these time periods, approximately these years: 325 BC, 165, 658, 1152, 1645, and 2140.
My hypothesis is that within 50 years of a conjunction there is a paradigm shift, a new way of envisioning how the word works and what humans can achieve. Within about 30 years of the midpoint of the cycle there is a grand achievement and fulfillment of the dream implicit in this paradigm. During the first half of the cycle there is a building of the tools to achieve this achievement and in the last half of the cycle the details of the grand achievement are worked out and applied on a massive scale. Each cycle requires very roughly 495 years.
Transition from the Babylonian to the Greek World:
Around 578 BC the transition from the Babylonian to the Greek world view occurred, along with the beginnings of horoscopic astrology, the Pythagorean vision being articulated, etc. This vision is fufilled around 325 BC with the articulation of astrological methods, Euclidean geometry, etc.
Around 82 BC is a new ethical vision and more simplified and elegant linearity in thinking that is embodied in Christianity, and reaches a fulfillment around 165 AD, which is when Vettius Valens and Ptolemy produced their works, the foundations of western astrology. Around 412 BC we have the beginnings of a new historical period.
I will leave it as an exercise for those with greater expertise in medieval and ancient history to determine how these cycles my correlate with historical cycles towards the end of the first millenium and the beginning of the second millenium.
Beginnings of a new empiricism:
Around the 1399 we have the beginnings of a new empiricism in thinking with Ockham's razor followed by other developments in critical thinking that reaches a culmination within 30 years of 1645 when Kepler places the crowning glory of the Pythagorean vision, Platonic idealism, and critical thinking and analysis in the discovery of the formula for planetary orbits. Galileo uses the telescope and performs an experiment on gravity that Newton explicates in extraordinary detail in his theory of gravity. The achievements of Galileo, Kepler, and others become the framework for Newton et al to apply this new scientific awareness on a scale undreamed of before.
Within about 10 years of 1892 Newton's giant clock is replaced by Einstein's theory of relativity. The enormity of Einstein's discovery requires yers to digest and is augmented by quantum theory and, for example, Bell's Theorem, which articulates a new mystical almost non-rational workings of the world. Also around 1892 are the seeds of a new religious perspective in the development of theosophy and the arrival of Hindu sages on American soil, etc.
The new insights of Einstein and others merge mysticism and religion into a cohesive, comprehensible, but mysterious vision of what the world is.and what humans can achieve. Within around 30 years of 2133, I predict that this vision will reach fuilfillment in the development of formula and mechanisms for the energy needs of humans to operate without fossil fuels, astrology will be validated through empirical studies and demonstrated to be a field that has objective as well as subjective validity.
In the following centuries until 2385 many of our greatest minds while be busily working out the details of a new intellectual domain as fresh, exciting, and extraordinary as Newton's monumental work in the 1700's. These bold forecasts, of course, cannot be validated in our lifetimes, and therefore are useless from the perspective of what we can hope to unravel regarding the relationship of history to astrology at this time, but I provide them as an example of how one might use one's understanding of these larger cycles combined with one's own predelictions (and perhaps biases) to envision how our generations fit within the larger unfolding patterns of history.
One must not, however, regard this one synodic cycle as the only guidepost for understanding historical cycles. There are other synodic cycles, the movement of fixed stars through zodiac signs, and other astrological variables, as well as non-astrological variables that work together to build the cycles and trends of human history.
In the article "Minor Aspects and Modern Trends" also on this website I present correlations of outer planet aspects with events and developments in the 20th century. The information provided in that article show the finer details that are revealed through an analysis of the more intricate relationships between outer planets, while this article shows how the overall general patterns of history are reflected in the large synodic cycle of Neptune and Pluto.