The Roman astrologers poet, Marcus Manilius wrote a poem in five books called the Astronomica, in Latin in the 1st century CE. It is one of the oldest major astrological texts that has survived. It contains many techniques that are not found in other authors.
In the work, the signs of the zodiac are ruled by the classical planets in the typical fashion, but there is also an aside in which Manilius assigns additional Roman gods to signs, possibly for the purpose of religious practice or to teach them by analogy. Interestingly, in the passage we find what is likely to be the first association of Neptune to the sign Pisces. Modern astrologers assert that Neptune is a ruler of Pisces, in addition to, or instead of, its traditional ruler, Jupiter.
Neptunian rulership of Pisces would break with the established scheme and rationale of planetary sign rulership, so I don’t advocate it in this sense. However, many modern astrologers struggle with understanding the relationship between signs and newly discovered modern planets (of which there are plenty these days if we include the “planets” of the Kuiper Belt recently downgraded to dwarf planets by some astronomers) and asteroids. I remember passages in a work by the 17th astrologer Morinus, in which he discussed another type of planetary strength called “analogical strength” which involves a planet being in a sign with similar significations. For instance, Saturn which signifies earthly resources being strong for such in the 4th house as it also signifies earthly resources.
The Neptune-Pisces association, and some of the other god-sign associations named by Manilius have a similar “analogical strength” to them, with Pallas associated with Aries, and Ceres associated with Virgo. It would seem natural for asteroid Pallas, named for the goddess known for her military strategy (Pallas Athena), to be analogically strengthened in the sign Aries, a fire sign of Mars. Similarly, for the planet Ceres, named for the goddess of agriculture, to be analogically strengthened in the sign Virgo, an earth sign whose constellation is a virgin holding a plant (palm front or sheaves of wheat). Associations of Vesta to Capricorn, Diana to Sagittarius, Apollo (Phoebus) to Gemini, Vulcan to Libra (as Vulcan made the scales), and Juno to Aquarius, may also have some use. Less interesting are associations of Venus (Cytherean) to Taurus and Mars to Scorpio, which are the same as the natural rulerships. He also associates the god Mercury with Cancer and Jupiter with Leo, which are more puzzling associations, especially in the Mercury-Cancer instance.
The complete passage is recounted below (Manilius, Astronomica, 2.433-452, Goold trans., 1977, p. 117-119):
What step must one take next, when so much has been learnt? It is to mark well the tutelary deities appointed to the signs and the signs which Nature assigned to each god, when she gave to the great virtues the persons of the gods and under sacred names established various powers, in order that a living presence might lend majesty to abstract qualities. Pallas is protectress of the Ram, the Cytherean of the Bull, and Phoebus of the comely Twins; you, Mercury, rule the Crab and you, Jupiter, as well as the Mother of the Gods, the Lion; the Virgin with her sheaf belongs to Ceres, and the Balance to Vulcan who wrought it; bellicose Scorpion clings to mars; Diana cherishes the hunter, a man to be sure, but a horse in his other half, and Vesta the cramped stars of Capricorn; opposite Jupiter Juno has the sign of Aquarius, and Neptune acknowledges the Fishes as his own for all that they are in heaven. This scheme too will provide you with important means of determining the future when, seeking from every quarter proofs and methods of our art, your mind speeds among the planets and stars to that a divine power may arise in your spirit and mortal hearts no less than heaven may win belief.
Manilius, M. (1977). Astronomica. (G. P. Goold, Trans.). Cambridge, MA: Loeb Classical Library.
Featured image attributed to 3268zauber, titled “Neptunbrunnen (1885) in Baden-Baden”