Paion quell'acque liquidi zaffiri,
Non turbate da nembi o da procelle;
E luminosi raggi in loro rimiri
Percossi lampeggiar de l'auree stelle;
E i lor torti viaggi e i torti giri
Da quelle a queste, o pur da queste a quelle;
E con ogni altra più serena imago
L'errante luna e 'l sole errante e vago.
Those waters look like liquid sapphires,
Undisturbed by clouds or by storms;
In them you can see luminous rays
That flash by reflecting the golden stars,
And their crooked paths and turns from
One another, and the other way round;
And together will all serene images,
The wandering moon and wandering sun.
These further 'explanations' seem to suggest that the first river symbolizes the starry universe with its divine perfection and its complex motions; a subject that is developed in the whole Day/canto 4 in Tasso's long poem Il Mondo Creato.
In line 4, percossi is the past participle of percuotere, "to hit," that in this case -- see Dante -- is probably referred to the action of light when it reaches a body and is reflected by it. Lines 1 and 2 also echo Dantean phrasing.
In line 8, the untranslatable adjective vago entails a pun since it means at the same time "wandering" and "beautiful." This double meaning will be fully developed by Giacomo Leopardi, e.g. in his poem Vaghe stelle dell'Orsa.
Anyway, it must be frankly admitted that, in spite of the poetical beauty of this octave, the whole symbolism (of which we have only read a part so far) is a bit heavy to read, even if philosophically interesting.