Al bel de tuoi capelli
L'oro ogni pregio tiene
Ch'al paragon piu impallidir si vede.

Son cosė forse i velli
De l'Auriga celeste
Quando di raggi il mondo infiamma e veste?

Febo vinto si rende
Mentre piu chiaro luce.

Dunque di che risplende?
D'un vivo raggio de l'eterna luce!

In English the lyrics of the madrigal mean something like:

It is gold that gives
light to your hair,
even if gold looks less bright.

Is the hair of the celestial driver
as beautiful as yours,
when he dresses the world in his flames?

No, Phoebus declares himself defeated
even when he is at his best.

So, where does the light of your hair come from?
From a ray of the eternal light!

Thanks to Andrea Friggi for the English translation.