Dichotome, or Dicotome

Definition

Dichotome, also spelled dicotome, is an older astronomical and astrological term for a celestial body appearing half illuminated. It is most commonly applied to the Moon at its first- or last-quarter phase, when the Moon is approximately 90 degrees from the Sun as viewed from Earth. The term comes from a root meaning “cut in two.”

Astrological Meaning

In traditional astrology, a dichotome Moon is interpreted through both its square relationship to the Sun and its place in the lunation cycle. At the first quarter, the Moon is waxing and light is increasing, suggesting development, outward effort, or a point requiring decisive action. At the last quarter, the Moon is waning, which may emphasize reassessment, adjustment, release, or completion. These meanings are not identical to those of every Sun–Moon square; phase, motion, sign, house, dignity, and other aspects also shape the interpretation.

Why It Matters

Recognizing this term helps readers understand historical astrological texts and distinguish lunar phase terminology from general aspect language. In chart interpretation, it adds context to the relationship between the Sun and Moon without determining an outcome by itself. The waxing or waning condition can clarify whether a cycle is building toward fullness or moving toward renewal.

Related Encyclopedia Entries

• Traditional Astrology
• Astrological Terminology
• Lunar Phases
• First Quarter Moon
• Last Quarter Moon
• Sun–Moon Aspects