Precession Part 3: technical stuff

Precession correction logo.J

Precession diagram-JPrecession Correction Part 3:  technical stuff will go into some detail about just what precession correction is.  First, we will refer to the diagram at the left.  Depicted is the Earth (blue globe) circling the Sun (yellow symbol in the center).  The Earth is at the Aries Point, zero degree of Aries at the Spring Equinox of the Tropical Zodiac.  The Earth circles the Sun in a counter-clockwise direction in this diagram.

If we could look through the exact center of the Sun we would be looking at the start of the sign Libra.  Of course, this is a mathematical point so we might not see it at all.  However if there was a planet there such as Mars positioned on the zodiac plane, we would see Mars at zero Libra.

Were we to really be able to look through the center of the Sun to the starry background itself, we would see the stars associated with five degrees of Pisces.  This is because the tropical zodiac, influenced by the Earth’s polar axis’ slow (25,000 year) wobble, slides backward along the Earth’s orbital plane by 1/72 of a degree from year to year.  This results in a shift backwards along the orbital path of one degree every 72 years.  This slide is depicted in the diagram by the Aries positions in following years — shown slightly out of scale for illustrative purposes.

Precession Factors 001

To the left is a table of precession-correction factors for each year up to 80 years.  This is the table that I used for many years before computer software came along to do the precession correction work for me.

If you were to manually calculate a solar return chart for a given year, year # 40 for example, you would first ad 0d 33′-30.6″ (zero degrees, 33 minutes, 30.6 seconds) to your natal Sun’s position to obtain the precession-corrected position.  Then you would calculate your Solar Return (the directions for doing so are not provided here).  With most purchasable software the ability to create a p.c. Solar Return is provided.  This function is not always available with Internet software.  The free-to-download and free-to-use RIYAL software is an excellent software choice for those who have some technical experience with software and Internet downloads.  Full download, set up and usage examples are provided on this blog site for using RIYAL software.  The Solar Returns and Progressed Daily Angles charts show on the Ninthouse blog site are all done with RIYAL.

Note:  When using most software (Solar Fire, Kepler or Sirius, Halloran, Matrix, etc.) for Solar Returns, the natal Sun will be shown with its uncorrected position while the transiting Sun’s position will be shown with its precession-corrected degree-minute-seconds value.  RIYAL uses the natal chart (tropically expressed) as if it was its own personal Sidereal Zodiac and shows all transiting positions in the same format.  This is the only correct (as I see it) way to do this.  However, if you compare transiting planet positions as shown in a printed ephemeris the positions will differ slightly as they are calculated in the uncorrected tropical zodiac.  


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