Mercury (astrology)

Definition

Mercury in astrology is the planet of speech, reason and exchange, and the one that keeps closest to the Sun. Ptolemy made it the changeable planet: drying at some times and moistening at others, common to both genders, and joined in character to whatever planet it meets. Modern astrologers read its sign and house for how a person thinks and communicates.

Primary source

Mercury in general is found at certain times alike to be drying and absorptive of moisture … and again humidifying, because he is next above the sphere of the moon … and to change quickly from one to the other…

— Ptolemy, Tetrabiblos I.4, Of the Power of the Planets, trans. F. E. Robbins (Loeb Classical Library 435, Harvard University Press, 1940). Open-access copy

Deep Understanding

The planet carries the name of Hermes, the messenger god, whom the Romans called Mercury. The Greeks also called it Stilbon, "the gleaming". It never appears far from the Sun, so it is seen only low in the twilight. Ptolemy builds its character on that closeness and on its speed. It dries because it stays near the Sun's heat, it moistens because its sphere lies just above the Moon's, and it swings quickly from one to the other because of how fast it moves near the Sun.

The same doubleness runs through the rest of his scheme. He reports that the ancients gave Mercury and the Sun a common nature that takes on the character of whichever planets they join. He calls Mercury common to both genders, diurnal when it rises before the Sun as a morning star and nocturnal when it follows as an evening star. He gives it Gemini and Virgo as its houses, the signs next to the houses of the two luminaries, and the dry sign Virgo as its exaltation. In Book III he reads the qualities of the soul that concern reason and the mind from the condition of Mercury, and the sensory, irrational part from the Moon.

The modern tradition reads the natal Mercury for thinking, speech, learning and trade. Popular astrology also watches its retrograde periods, which come three or four times a year for about three weeks, when the planet seems to move backward against the stars. Gnostic readers counted its sphere among the seven of the hebdomad. The god behind the planet is the same Hermes from whom Hermes Trismegistus, the legendary author of the Hermetic writings, takes his name, and the alchemists named their philosophical mercury after him too.

Often confused with

Mercurial Water: the alchemists' philosophical mercury, the solvent of the Great Work, where Mercury here is the planet and its place in a chart. The metal quicksilver, which the alchemists paired with this planet, is a third thing again.

In Practice

Mercury is hard to see. It shows only low in the west after sunset or low in the east before sunrise, and only for a few weeks at a time. Look up the date of its next greatest elongation and search for it then. Seeing how tightly it keeps to the Sun makes Ptolemy's reasoning about its drying heat easy to follow.

Stefan's Alkimia. (2026, September 30). Mercury (astrology). Pleroma Guide. https://pleroma.guide/en/lexicon/mercury

Stefan's Alkimia. “Mercury (astrology).” Pleroma Guide, 30 Sept. 2026, pleroma.guide/en/lexicon/mercury.

Stefan's Alkimia. “Mercury (astrology).” Pleroma Guide. September 30, 2026. https://pleroma.guide/en/lexicon/mercury.

@online{alkimia2026mercury,
  author = {{Stefan's Alkimia}},
  title = {{Mercury (astrology)}},
  organization = {{Pleroma Guide}},
  date = {2026-09-30},
  year = {2026},
  url = {https://pleroma.guide/en/lexicon/mercury},
  langid = {english}
}