Start with a declared angular measure
An aspect can be tested as a relationship between two stated coordinates. In the common zodiac-longitude calculation, take the absolute difference, reduce it on the 0°–360° circle, then use the smaller of that result and 360° minus it. The result can be compared with a selected exact angle: 0° conjunction, 60° sextile, 90° square, 120° trine or 180° opposition. Swiss Ephemeris distinguishes ecliptic longitude from latitude, distance, right ascension and declination, so a longitude aspect is not a measurement of physical nearness or a complete description of sky position.
Swiss Ephemeris programming interface · coordinate outputs ↗
Exactness and orb are separate decisions
An orb is the tolerance around an exact angle; it determines whether a near-match is reported. For an invented example, positions at 358° and 2° are 4° apart across the 0°/360° boundary, not 356° apart. They count as a conjunction under a declared 5° orb and do not under a declared 3° orb. Deborah Houlding’s annotated edition of William Lilly’s Christian Astrology identifies a moiety technique for calculating orbs in chapter 19. This documents a historical procedure, rather than a universal limit. Record the aspect angle, the coordinate frame and the orb before comparing outputs.
William Lilly, Christian Astrology · aspects and planetary configurations ↗Swiss Ephemeris programming interface · degree normalization ↗
Sign relationships use a different rule
A sign-based method first assigns each position to a 30° sign and then asks how many signs separate the pair. Ptolemy’s Tetrabiblos I.13 describes opposition, trine, quartile and sextile as both sign intervals and 180°, 120°, 90° and 60°; I.16 calls signs one or five signs apart disjunct or alien. Thus the invented 358°/2° pair is within a 5° longitude conjunction, while its Pisces/Aries labels are adjacent signs and fall under Ptolemy’s stated disjunct category. The two results answer different questions; neither rule should silently replace the other.
Ptolemy, Tetrabiblos I.13 and I.16 · aspects of signs and disjunct signs ↗
Read the geometry within its tradition
A calculation establishes only that positions meet the declared rule. Ptolemy calls trines and sextiles harmonious, and quartiles and oppositions disharmonious within his sign-based account. Lilly’s account also considers application and separation. Such meanings are interpretive inheritances, not consequences that follow from the angle alone. Choose the guide for the active method, cite it when making a historical claim, and keep a degree-based output distinct from a sign-based reading.
Ptolemy, Tetrabiblos I.13 · aspect categories and rationale ↗William Lilly, Christian Astrology · aspects and planetary configurations ↗
Word origin · etymology
Latin through Middle English.
Latin aspectus, from aspicere, means a looking at. The name retains the old metaphor of planets seeing one another.
How we know · epistemology
Measured angle plus selected rule
Angular separation is computable, but selecting meaningful angles, tolerances and interpretations is conventional. A detected aspect establishes the stated geometry, not a causal connection between the represented life topics.
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Terms used in this explanation
These links connect subjects and terminology. They do not establish a shared historical origin.
Sources & provenance
Use the citations beside each section to distinguish a calculation convention, a historical teaching and an editorial example.