Inferior Planet

Definition

An inferior planet is one whose orbit around the Sun lies entirely inside Earth's. In practice that means Mercury and Venus. Because they orbit closer to the Sun than we do, they never stray far from it in the sky — Mercury stays within about 28 degrees of it, Venus within about 48 — they show phases through a telescope, and their retrograde cycle is built around the inferior conjunction, when the planet passes between Earth and the Sun, rather than around opposition.

In Tradition

In both traditional and modern Western astrology, "inferior planet" is a basic class defined by orbit, and it shapes how a planet phases and when it is visible. Astrologers agree that an inferior planet appears to retrograde because Earth passes it on the outside — a different geometry from the superior-planet cycle. Traditional Western practice further holds that inferior planets prefer the occidental phase, setting after the Sun, where they are read as stronger — the mirror of the superior planets' liking for the oriental phase.

In Practice

An astrologer recognizes an inferior planet by its orbit: Mercury and Venus, whose average orbital radii both lie inside Earth's 1 AU. Their cycle relative to the Sun runs from superior conjunction (the planet behind the Sun, moving direct), through greatest eastern elongation, which is the evening-star phase, the retrograde station, inferior conjunction (the planet between Earth and the Sun, retrograde), the direct station, greatest western elongation, the morning-star phase, and back to superior conjunction. Each phase carries its own interpretive signature, and the morning-star and evening-star phases of Mercury and Venus are tracked separately when the planet stands in for a person or matter as a significator.

Historical Origin

The inferior-planet classification goes back to Ptolemaic astronomy and the geometric model of the Almagest, came down through the Arabic-Latin transmission, and remains the standard orbital classification in modern Sun-centered astronomy. Ptolemy discusses oriental and occidental phasing for the inferior planets in the Tetrabiblos, and Bonatti keeps the doctrine in Liber Astronomiae. The modern Western synthesis appears in Sullivan's Retrograde Planets and Hand's Planets in Transit.

Etymology

Origin: Latin. Meaning: From inferior (lower, beneath) — planets with orbits below (inside) Earth's.

Further Reading

• Claudius Ptolemy, Tetrabiblos
• Erin Sullivan, Retrograde Planets