The Equation of Time

The “Equation of Time” makes it possible to pass from a System based on the MEAN Solar Time, to a System based on TRUE Solar Time.  The Equation’s formal definition is the difference between clock time (Greenwich Mean Time) and sun time (apparent solar time).   It was created as a result of the true annual time discrepancy caused by combination of the Earth’s tilt (angle of obliquity) and the fact that the Earth’s orbit around the Sun is an ellipse and not a circle.  These two effects are illustrated in the below graph from the National Maritime Museum and the Royal Observatory Greenwich.  The purple line reflects the obliquity and the dark blue line the unequal motion:

Somewhat of a misnomer today, the term “equation” meant “correction” back in the Eighteenth Century when the Equation of Time was formulated

Clock time hinges on the belief that there are exactly 24 hours in each day while sun time is based on the idea that when the Sun reaches its zenith each day that is “noon” and the time elapsed between each “noon” varies based on the time of the year and the Sun’s position relative to the Equator.  While the difference between the duration of the solar day is never as great as one minute, it is cumulative, thereby causing as much as a 16 minute discrepancy at certain times of the year.  Astrologically speaking this is a vast variation and one that many of us have been unknowingly arriving at or very close to by the time-consuming task of rectifying charts based on life events.  Imagine having the ability to show a client the scientific reason and formula for having to “correct” their birth time.  Better yet, imagine having the ability to go to a website, put in place of birth, date and clock time and having the Equation completed for you in a matter of seconds!

Such sites are readily available, for example:
http://www.srrb.noaa.gov/highlights/sunrise/azel.html, http://www.spot-on-sundials.co.uk/calculator.html

Although this time difference is of little use in modern navigation, apart from determining the local apparent noon, it is critical in medical astrology, particularly in the cases of Scorpio and Aquarius charts where the extreme 15 to 16 minute discrepancies occur.

The below graph depicting the Equation of Time is provided by Patrick Powers from his sundial page:

For those of you without easy computer access, or just the desire to know how to work the root formula, following is the work of Guy de Penguern, a well-known French medical astrologer.  He states:

The exact relation between GMT (Greenwich Mean Time) and True Local Time is:  Local Time = GMT ± Longitude ± ”E”, where “E” is the Equation of Time.  The Equation of time is tied to the four year cycle of the Gregorian Calendar hence varies slightly from one year to another.  It is because of this leap year differential that the four days out of a year that watches tell the time “exactly” right range a day either way of December 25, April 15, June 14 and September 2.

Table F below gives the Equation of Time for a date at 12noon GMT of a leap year “L”, and Table G gives a slight correction to make to Table F when it is not a leap year, thus “L+1”, “L+2”, “L+3”, representing the 1st, 2nd and 3rd years following a leap year.

For a non leap year, read in January and February, the date + one day.  So, for January 12, 1975, read Table F:  F=8m29.

Please note that “E” can be as much as ± 16 minutes which can put off the M.C. by as much as 04o.

Table F
Table G

References

http://www.sundials.co.uk/equation.htm
http://en.wikipedia.org/wiki/Equation_of_time
http://www.nmm.ac.uk/server/show/conWebDoc.351

Information compiled by Richard Nelson and Ginny Warner, July 2007.

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