that it must be 366+ (about which more later)

In the conversation with Petr9 on the Astro-Seek forum, occurred the following exchange:

*Petr9 asked:*
'Btw do you have any idea why there are these 2 slightly different Sidereal days values?:

"366.2421992"

https://www.google.com/search/…;

- used by OPA astrology

"366.2563992"

https://www.google.com/search/…;

- used in Astro Gold and Janus"

*FraterAC replied:*
"366.2421992 is the sidereal days value based on the MC returning to a point in the Zodiac (ecliptic) - so it´s based on a tropical year.

366.2563992 is a sidereal days value based on the MC returning to the location of a fixed star (or adjusted for precession) - so it´s based on a sidereal year.

The difference in time -- 20 min 26.88 seconds -- is the average amount of time it takes for the Earth to rotate 50.3" of arc -- the average amount of annual precession.

Obviously over time this difference is going to add up. Is it going to add up enough to make a difference in the 90 or so days we consider the amount of time needed to compute secondary progressions for an average lifetime? Probably not. Tertiary progressions would cover 3 to 4 years after the birth, making a difference of about 3´ of arc. Is that significant? Maybe.

Minor progressions (lunar revolution = 1 year) do not use a day as a basis so are not affected (by this question, anyway)."

I had been using the first value, 366.2421992. This factor assumes the "year" being considered is the

*tropical* year. Which seemed reasonable on the face of it, BUT

*the tropical year is in motion*. The vernal point is precessing at the rate of approximately 50.3" per year. Therefore, the factor 366.2421992 IS NOT a complete circle, and cannot logically be the equivalent of one full 360 degree revolution of the Earth on its axis.

Therefore, it's recommended, and I will be using in the future, the factor 366.2563992 which represents the sidereal year, which IS one full 360 degree circle.

Admittedly, the difference is very small. In a lifetime of 75 years it will amount to approximately a minute of arc on the Midheaven. But this factor does preserve the fractal integrity of the calculations (and may have other effects).

Note that this does not have anything to do with whether one is of the Siderealist school, or uses the tropical Zodiac. The considerations above are relevant in either case.