Good morning,

Particularly in the last (20th) century CE much ado was made about 'true' arcs of various kinds in astrology, ex. gr. 'true' nodes, 'true' solar arcs. 'True' always sounds good and implies that other methods are 'false'.

I have searched various reference books and the internet until now in vain to find WHEN, HOW and BY WHOM such notions were introduced to astronomy and astrology. Who can help? Perhaps Professor Martin Gansten?

Best regards,

lihin

I don't think astronomers relate "true" to "false". It is "true" vs "mean" or vs "apparent".

Behind the "mean sun" and the "true sun" is the mean anomaly and the true anomaly (kepler's equation for an ellipse). That's a first and it would be interesting to know the terminology used by Kepler. There is an equivalent in the traditional Ptolemaic epicycle theory.

Then there is the true or "geometric" vs "apparent" position of a star or planet, behind which is the discovery of stellar and planetary aberration as mentioned in previous posts. The use of higher precision instrumentation is responsible for this discovery.

Then there is the true aka "osculating" vs mean orbital elements such as the true node and apogee, behind which is a refinement of Newton's principles applied to the calculation of orbits and perturbed orbital motion.

But the osculating orbital elements of the Moon were not used nor were available until advances in computer programming in the 1970's (n-body numerical integrations which included the Moon). It's only then that you begin to see use and talk of the "true" node in astrological circles. The alternative name would have been "osculating", and it's not difficult to understand why astrologers adopted the word "true" instead.

There's also the "true" horizon vs apparent vs visual. I'm not sure if ancient authors already spoke of it, but I would start by looking at the history of the discovery and calculation of parallax.

I think the association of "true" with "false" arises only in the mind of astrologers who have no idea of how the object to which the label "true" is applied is or can be or has been calculated.

J.

Good evening,

Thank you, Se?or Juan, for your pertinent comments. Interesting might be that 'apparent time' in English is called 'heure vraie' (true hour, both meaning 'sundial' time) in French. One can see once again that the separation of astronomy from astrology has perhaps resulted in confusion.

But when did the notion for example of 'true solar arc' get introduced in astronomy and astrology? Renaissance? By whom? Herr Johannes Kepler? Signor Placido di Titi?

The Aristotelian notions of 'essential' and 'accidental' are still used in astrology and are in my humble experience useful. But they are not synonymous with 'true' and 'mean' or 'apparent'.

Best regards,

lihin

lihin wrote:But when did the notion for example of 'true solar arc' get introduced in astronomy and astrology? Renaissance? By whom? Herr Johannes Kepler? Signor Placido di Titi?

Implicit are the notions of "true motion" and "mean motion": one is non-uniform and "eccentric" (Ptolemy's equant, or Kepler's ellipse), the other uniform and circular. You calculate the mean arc and then apply to it the "equation of the center" to convert it to the true arc.

I am not an expert on ancient sources, but from the little I have read these are standard designations when describing the Almagest, the Toledan Tables... all the way up to Kepler. To know if the terms "true" and "mean" were actually used by ancient writers and are not just the standard terminology of modern historians, we need someone who can read in the original language (e.g. the Almagest), and translate the term literally.

J.