Re: Find Solar Calendar Seasons spreadsheet posted

In any given era the longest cycle mean year having a stable calendar season will match the mean year (in terms of mean solar days) at the ecliptic longitude of the Earth orbital aphelion, and the shortest cycle mean year having a stable calendar season will match the mean year at the ecliptic longitude of the Earth orbital perihelion. The advance of perihelion (and aphelion) together with the tidal slowing of the Earth rotation rate cause calendar seasons to evolve and migrate as the millennia pass.

A leap cycle will not have any calendar seasons in an era in which its mean year is a few seconds shorter than the mean year at the ecliptic longitude of aphelion. With tidal slowing of the Earth rotation rate, however, eventually the mean year at aphelion will equal the cycle mean year, so a calendar season will appear at the ecliptic longitude of aphelion. With progressive tidal rotation slowing that calendar season will split into a more stable season that will migrate ahead of aphelion (to earlier solar longitudes) as well as a less stable season that will migrate after aphelion (to later solar longitudes). As tidal rotation slowing continues, eventually the average length of the solar cycle will approximately equal the cycle mean year, and then both calendar seasons will be optimally stable and perihelion and aphelion will be situated approximately midway between them, with perihelion having the less stable calendar season behind it (prior solar longitude) and the more stable calendar season ahead of it (later solar longitude). Further tidal rotation slowing will cause the calendar seasons to converge towards perihelion, disappearing in later years when the mean year at the ecliptic longitude of perihelion becomes shorter than the cycle mean year.