Constantis vs. constantes et similia

Question

_{Following up on @brianpck's suggestion in this question:}

In this passage:

Maxume vero sunt admirabiles motus earum quinque stellarum quae falso vocantur errantes; nihil enim errat quod in omni aeternitate conservat progressus et regressus reliquosque motus constantis et ratos.
_{Source: M. Tullius Cicero, de Natura Deorum 2.51. O. Plasberg, Ed.}

why does Cicero write constantis rather than constantes? I expected constantes since it modifies the accusative plural reliquos motus.

This Wiktionary table suggests that many third-declension adjectives allow both -ēs or -īs for their accusative plural.

Why and when is this legitimate? And considering that the genitive is constantĭs, didn't writing constantis confuse Roman readers, especially if they didn't mark the vowel length in -īs?

Answer 1

I don't know enough to give a detailed answer, so this is just some basic information to start with.

Speaking generally, -īs is an "i-stem" accusative plural ending, so it's expected to go along with -ium in the genitive plural, -ī in the ablative singular, and -ia in the neuter nominative/accusative plural.

But the actual use of "i-stem" forms wasn't consistent in Latin: some case/number combinations were more likely than others to use an i-form. For example, the i-stem accusative singular ending -im was very rare; on the other hand, the i-stem neuter nominative/accusative plural ending -ia was used for nearly all adjectives (provided that they had a neuter nominative/accusative plural form at all).

The masculine/feminine accusative plural seems to be one of the cases where the i-form was not too uncommon, but also not mandatory for any word. Unfortunately, I don't have a good sense of how frequent accusative plural -īs was: I've found a few web pages that call it an "older" or "poetic" variant, but I can't tell you how accurate that characterization is. (Specifically, this page and this Reddit discussion.) Allen and Greenough's Latin Grammar calls -īs the "regular accusative plural" for i-stem nouns and says it is "common, but not exclusively used in any word" (§77); Section 117b and Section 118 specifically mention -ns present participles as words that can take -īs in the masculine/feminine accusative plural.