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The Babylonian picture was not two fish. This stretch of sky held MUL.KUN.MEŠ, "the Tails", along with MUL.SIM.MAḪ, "the Great Swallow" — a bird figure covering the western fish and part of what is now Pegasus — and Anunitum, a name of Ishtar, covering the northern one. A swallow and a goddess were reorganized into a matched pair of fish somewhere in the later transmission, which is a useful reminder that these figures were edited, not received.

The Greek story explains the pair and, unusually, points straight back at real religion. Aphrodite and Eros, fleeing the monster Typhon, leapt into the Euphrates and became fish — or were carried to safety by two fish — tying their tails together so they would not be separated. Hyginus tells it in De Astronomia. The setting is not Greek: this is the territory of Derceto / Atargatis, the Syrian goddess whose sacred fish at Hierapolis and Ascalon were kept in temple pools and not eaten, a food taboo Lucian describes in De Dea Syria. So the catasterism is Hellenistic literary tidying, but the fish taboo underneath it was live cult practice.

Christian readers later found the fish congenial — the ichthys was an early Christian sign, and the equinox moved into Pisces around the beginning of the Christian era. The tidy conclusion drawn from that, an "Age of Pisces" corresponding to Christianity, is modern esotericism. No ancient or medieval Christian writer made the argument; it belongs to the same nineteenth- and twentieth-century astrological-ages scheme that produced the Age of Aquarius, and it depends on constellation boundaries not drawn until 1930.

Date	Event
c. 1000 BCE (contested)	MUL.APIN records the Tails, the Great Swallow and Anunitum in this region of the Moon's path.
5th century BCE (contested)	Pisces becomes the twelfth of twelve equal 30° signs, closing the circle at the equinox.
c. 2nd century CE	Lucian's De Dea Syria describes the sacred, uneaten fish of Atargatis — the cult practice standing behind the Greek myth.
c. 150 CE	Ptolemy catalogues Pisces in the Almagest and assigns Jupiter the rulership in the Tetrabiblos.
1st century BCE (boundary-dependent; often given as c. 68 BCE, but the figure depends entirely on the 1930 IAU boundaries)	Precession carries the March equinox point out of the stars of Aries and into Pisces, where it still is.
present	The Sun spends most of tropical Pisces against the stars of Aquarius, entering the constellation Pisces only around 11 March. The equinox will eventually cross into Aquarius — estimates cluster around the 26th century CE, but the date is an artefact of where the boundary was drawn.

Pisces is the end of winter and the last mutable sign, dissolving the year into the equinox that starts it again. In Rome, February was the final month of the old calendar and the month of februa, the purification rites that cleared the year before it turned; the Christian season of Lent occupies much the same weeks with much the same intention. In the Mediterranean this is when the rains taper, the almond flowers and the sea becomes navigable again. A sign of thaw, blur and boundlessness is a reasonable name for the part of the year when nothing is definite yet.

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