Exact Angles Are Measured. Orbs Are Chosen.
An aspect has two halves that behave completely differently: an angular separation you can measure, and an allowance somebody decided on. Telling them apart is the single most useful habit in reading an aspect table.
HellenisticRenaissance & early modernModern WesternAstronomical factTraditional doctrineEditorial synthesis
In this lesson
- Separate the measured and chosen components of an aspect
- Cite where the classical aspect angles are set out
- Explain why aspect lists differ between astrologers and software
- Explain why an aspect list should state the orb allowance it used
Before this lesson: From a Longitude to a Sign and a Degree
Two bodies are 124.17 degrees apart. Is that a trine? The first half of the answer is arithmetic. The second half is a decision somebody made, and it is not written in the sky.
The measured half
The angular separation between two ecliptic longitudes is a subtraction, wrapped onto the circle. Given the same longitudes, every correct implementation returns the same separation; it is as checkable as the longitudes themselves.[1]
The classical angles are equally definite. Ptolemy's Tetrabiblos sets out the configurations between parts of the zodiac as one hundred and eighty degrees, one hundred and twenty, ninety, and sixty: the opposition, trine, quartile and sextile.[2]
The chosen half
Almost no pair is ever separated by exactly one hundred and twenty degrees. So every practical system allows a margin (an orb) within which a pair counts as making the aspect. How wide that margin should be is a convention.
Schools have defined it in incompatible ways: as a property of each planet, as a property of each aspect, wider for the luminaries, narrower for minor aspects. No measurement adjudicates between them, because there is nothing to measure.[3]
What follows for a reader
Two aspect lists for the same chart can differ entirely while both are arithmetically correct. If one lists a square that another omits, the usual explanation is a wider allowance, not a different sky.
Changing the allowance never moves a single separation (those are measurements), but it changes which pairs are listed. That is why an honest aspect list names the allowance it used, and why two lists can disagree without either being wrong.
In brief
An aspect combines a measured angular separation with a chosen allowance. The classical angles (180, 120, 90 and 60 degrees) are set out in Ptolemy's Tetrabiblos, which notably does not list the conjunction among them. The orb has been defined incompatibly by different schools and no measurement settles it, so aspect lists legitimately differ and should name their allowance. This platform publishes no aspect interpretation, and its guest Birth Chart currently shows no aspects.
Check your understanding
Check your understanding:
- Which half of an aspect can be checked against an independent calculation?
- Why is the conjunction absent from the Tetrabiblos chapter on configurations?
- If two aspect lists for one chart differ, what is the most likely reason?
References
Numbered citations in the text link to the records below; each record links to the full source registry.
- Sean E. Urban (editor); P. Kenneth Seidelmann (editor). Explanatory Supplement to the Astronomical Almanac. Sean E. Urban and P. Kenneth Seidelmann. University Science Books. 2013. Astronomical fact View source record
- Claudius Ptolemy. Ptolemy's Tetrabiblos, or Quadripartite: Being Four Books of the Influence of the Stars. J. M. Ashmand (translator). W. Foulsham & Co. Ltd, London. 1900. Historical primary source View source record
- Nicholas Campion. History of Western Astrology Volume II: The Medieval and Modern Worlds. Bloomsbury Publishing Plc. 2009. Historical scholarship View source record