Cusp

Definition

A cusp is a boundary line in the chart, and the word is used in two distinct ways. First, it can mean the boundary between two neighbouring zodiac signs, which falls every 30 degrees around the ecliptic. Second, it can mean the line that begins a house in any of the quadrant-based house systems — Placidus, Koch, Regiomontanus, Campanus, Alchabitius, Porphyry. In whole-sign houses there is no separate house cusp; the sign boundary does the job. In quadrant systems the house cusps land at varying degrees inside the signs and depend on the place's latitude and the time.

In Tradition

In Hellenistic and traditional astrology, only the sign a planet is actually in matters for sign-based judgments — there is no "cusp blend" in classical doctrine. House cusps are a different story: their meaning carries weight across every major school. A planet near a house cusp is read as taking part in that house's affairs, though astrologers differ on whether the cusp is a sharp line (whole-sign) or allows a small leading orb (quadrant systems).

In Practice

Astrologers mark the sign cusps at every 30-degree point — 0 Aries, 0 Taurus, and so on. A planet within roughly one degree of a sign boundary is sometimes handled specially: a planet in the late degrees is read as finishing the themes of the sign behind it, one in the early degrees as opening the new sign. House cusps are computed from whichever house system the chart uses. Holden's history catalogs the main quadrant systems — Alchabitius, Regiomontanus, Campanus, Placidus, Koch. A planet within a small orb of a quadrant house cusp is commonly read as already participating in the house that cusp begins.

Historical Origin

House-cusp doctrine is surveyed in Holden's history of horoscopic astrology, which traces the sequence of quadrant house systems from medieval Alchabitius (10th c.) through Regiomontanus (1467) to Placidus (17th c.) and modern Koch. The whole-sign system, attested in the Hellenistic primary sources Valens, Paulus, and Hephaestio, treats sign boundaries as house boundaries, so the distinction never arises. The popular "born on the cusp" idea in modern Western astrology is a by-product of Sun-sign popularization, with no classical warrant.

Further Reading

• James H. Holden, A History of Horoscopic Astrology
• Robert Hand, Whole Sign Houses: The Oldest House System