How can the use of "-aeus" as an adjective suffix in "Herculaeus" be explained?

Question

Apparently, the English word "Herculean" has an old spelling variant "Herculæan". This seems to correspond to a Latin variant of the adjective "herculeus/Hercŭlĕus" spelled "Herculæus" (example: "Magnetem qui Herculæus vocatur ubique dispersit, quo nullus est admirabilior, aut utilior lapis in rerum natura." –Universæ naturæ theatrum, Jean Bodin, 1596).

I am a little confused by this; I think I understand the formation of the more commonly found "Hercŭlĕus" (correct me if I'm wrong, it seems to be "Hercŭl-" as in "Hercul-is" or "Hercul-em" + the adjectival suffix -ĕus), but I'm not sure if "Herculæus" should just be considered an error, or has some basis to it that I can't currently see.

I know that in medieval Latin, there was a lot of interchange between spellings with "e", "æ" and "œ", so a word might be spelled with any of these with no regard to its etymology. However, I am not entirely sure if that's the whole explanation for the use of "Herculæus" as an adjective.

I know that there are a number of Latin adjectives that did end in etymological "aeus", ultimately coming from Greek adjectives in -αιος (such as trochaeus from τροχαῖος). As far as I can tell, the "a"/alpha in -aeus adjectives originates from the thematic vowel of the first declension, but what I don't know is to what extent this suffix may have been able to be applied analogically to nouns of other declensions. (Also, I know there were similar adjectives in Greek terminating in -ειος, Latin -ēus~ῑus,, which seem to be formed when an "e"-like vowel came before the adjective suffix. So the Greek name of Hercules, Ἡρακλῆς, was the source of the word ἡράκλειος that became Latin Heraclius.)

In spoken Latin, at least, I would expect there to be a difference in stress between "Herculĕus", stressed on the antepenult, and "Herculaeus", stressed on the penult. But maybe these rules aren't relevant when dealing with medieval Latin. I was wondering if if the "ae" spelling is used in any texts from a period when Latin was still a spoken language, and if so, if that should be taken as evidence for analogical extension of -aeus as an adjective suffix, rather than just variation in the letters used to spell the -ĕus adjective suffix. Also, if there are other words where "-aeus" is suffixed to a non-first-declension noun like this, I would be interested in knowing about them.

Answer 1

The adjective Herculeus is well attested in Classical Latin. I think “Herculaeus” is merely a bad spelling in Mediaeval or Humanistic Latin.

Answer 2

As far as I can tell, the answer that fdb provided to my question is completely correct; I just wanted to add some relevant information that I discovered recently when I was studying the inflection of nouns (particularly names) ending in -ēs.

Nouns/names ending in -es

Many names ending in -es come from Greek nouns of the first declension (which means that from an etymological perspective, they are built on a stem ending in "a"). Greek masculine nouns of the first declension end in -ης or -ας. Not all Greek masculine nouns ending in -ης belonged to the first declension; some (including Ἡρακλῆς, in fact) belonged to the third declension.

The declension of a name in Latin does not necessarily correspond to the declension of the corresponding name in Greek. In Latin, names ending in -es were usually declined as third-declension nouns (with genitive singular forms in -is), regardless of whether they came from Greek names of the first or third declension.

However, masculine names ending in -es did sometimes take some non-third-declension forms:

• masculine names ending in -es sometimes had second-declension genitive singular forms in -i. It seems like this might be related in some way to the use of -ου in Greek as a termination for the genitive singular of masculine first-declension nouns. That said, there isn't a perfect correspondence between the existence of a Greek genitive singular in -ου and a Latin genitive singular in -i: Diomedes, from the Greek third-declension noun Διομήδης, Διομήδους, had a genitive singular form in -i alongside a regular third-declension genitive singular form in -is.

• masculine names ending in -es sometimes had Greek-style accusative singular forms in -en (-ēn).

The name Hercules

Currently, it seems plausible to me that the spelling "Herculæus" is related to the use of -ης as a termination for Greek first-declension masculine nouns, even though (as mentioned above) Greek Ἡρακλῆς is not a first-declension noun.

The Latin name Hercules is ultimately from Greek, but apparently not a direct borrowing. Wiktionary says that the Latin form Hercules comes from an Etruscan form "Hercle". Although Hercules seems to have a somewhat complicated and variable declension, it is not classified as a first-declension noun in any of its forms. But in the nominative singular, it does end the same way as a first-declension noun like Hermes.

Greek first-declension nouns sometimes had derivatives spelled with ae in Latin: the Lewis and Short entry on Hermes mentions the related word Hermaeum, and from the Greek name Δίρκη, there was the Latin adjective Dircaeus. I think that the spelling Herculæus might have been used partly because of analogy with forms like those.