The difficulty is, that in the system that we know from Ptolemy, Firmicus Maternus, Hephaestion of Thebes, and others, the Lion is the house, not of Jupiter, but of the Sun;55
and we can scarcely venture to date the gem earlier than the astrological writings that conform τo that system. The statement that Leo is the house of Jupiter cannot be reconciled with the usual system; the most that can be suggested is that the planet has a special interest in the sign of the Lion. Thus in a fragment of Hephaestion of Thebes, Jupiter is joint lord of the house (συνοικοδεσπότης) of Leo κατὰ τὸ τρίγωνον, that is, because when in his own house, Sagittarius, Jupiter is in trine aspect to Leo.56
Again in the Egyptian and Chaldean systems of terms (ὅρια) as described by Ptolemy,57
the first terms of Leo are assigned to Jupiter. Further, Firmicus Maternus (2, 4, 1–2), explaining the division of the twelve zodiacal signs into thirty-six parts (ten degrees each), over each of which a decan presides, adds that the decans themselves are allotted to individual stars; further, “Si in ipso decano stella fuerit, licet sit in alieno domicilio, sic est habenda, quasi in suo sit domicilio constituta; ‹in› suo enim decano constituta haec eadem perficit, quae in signo suo constituta decernit.” The second decan of Leo is allotted to Jupiter (2, 4, 3); therefore, if Jupiter is in the second decan of Leo, his powers are the same as if he were in his own house, Sagittarius and Pisces. Yet all this is not the same as saying that Leo is the house of Jupiter, and attempts to explain the inscription in these ways would not be entirely convincing.
It is more likely that we have to do with a mixture of the classical doctrine of the planetary houses with an earlier system to which that doctrine was unknown, but which nevertheless assigned to each of the twelve great gods the guardianship of a zodiacal sign.58
That earlier system finds expression in a passage of Manilius (2, 433–452), from which it suffices to cite a few verses (433–435, 439–441):
His animadversis rebus quae proxima cura?
noscere tutelas adiectaque numina signis
et quae cuique deo rerum natura dicavit
Lanigerum Pallas, Taurum Cytherea tuetur,
formosos Phoebus Geminos; Cyllenie, Cancrum,
Iuppiter, et cum matre deum regis ipse Leonem.
Here the signs are spoken of as “protected” or “ruled” by their guardian deities, not as being their “houses”; but the transition from one system to another might naturally entail some confusion in terminology.
Another possible explanation depends upon an identification of Helios and Zeus, which is well known to students of Greek religion in the Hellenistic and Roman periods.59
If Zeus is the Sun, the Lion may be called the house
55 Ptol. Tetrab. 1, 17; Heph. Theb., p. 53, 30 (ed. Engelbrecht); Firm. Mat. 2, 2, 3.
56 Cat. Cod. Astrol. Graec., I, 91, 11–12.
58 See Housman's account of this system in the preface (p. xvi) to his edition of Manilius, Book II.
59 See Cook, Zeus, I, 186 ff.