Semisextile

Definition

A minor aspect of 30 degrees between two planets — equivalent to one zodiacal sign apart and to half of a sextile (60°/2 = 30°). The semisextile belongs to the twelfth-harmonic family generated by dividing the 360° circle by 12. It is one of the four Keplerian minor aspects that, in Holden's framing, have been adopted by most modern astrologers.

In Tradition

Across the modern Western minor-aspect tradition (Holden on Kepler; Marks), the semisextile is read as a Keplerian minor aspect of mild qualitative inflection. Holden documents it as one of the four Keplerian minor aspects (30°, 45°, 135°, 150°) adopted by most modern astrologers, alongside the semisquare, sesquiquadrate, and quincunx.

In Practice

Astrologers note semisextiles between planets one sign apart, applying narrow orbs typical for minor aspects (Marks specifies that minor aspects within 30 minutes should be considered close, and those within 15 minutes treated as at least as important as wider major aspects). Marks reads the semisextile as a potential compensator when a chart has few trines or sextiles: 'If a chart has few trines or sextiles, close semi-sextiles, quintiles, and biquintiles will take on added significance, for they will indicate potential opportunities.' The aspect is not used in horary or traditional electional work, where the Ptolemaic aspects (conjunction, sextile, square, trine, opposition) govern judgment. The semisextile links signs of adjacent elements that are otherwise unconnected by Ptolemaic geometry, contributing to the modern reading of the aspect as a register of small-scale qualitative adjustment.

Historical Origin

The 30° aspect is attributed to Johannes Kepler in the seventeenth century; it is not part of the Ptolemaic five and is not recognized in Ptolemy's Tetrabiblos. Holden's *A History of Horoscopic Astrology* documents Kepler's introduction of the additional minor aspects (18°, 24°, 30°, 36°, 45°, 72°, 108°, 135°, 144°, 150°), of which 30°, 45°, 135°, and 150° entered standard modern practice. Marks's *The Art of Chart Interpretation* preserves the canonical modern-Western minor-aspect classification framework.

Etymology

Origin: Latin. Meaning: Semi-sextile = 'half-sextile,' from Latin semi- ('half') + sextilis ('sixth'). The aspect-name encodes its half-of-sextile (30° = 60°/2) twelfth-harmonic structure..

Further Reading

• Johannes Kepler, Harmonices Mundi
• James H. Holden, A History of Horoscopic Astrology
• Tracy Marks, The Art of Chart Interpretation