In Borges’ “La cifra” and the obverse of his “Poema“, both translated here earlier, the poet refers to Virgil. In the “Poem”, among the things brought with the hexameters of Virgil’s Æneid is “[l]a amistad de la luna“, “the friendship of the moon”, and “La cifra” opens with “[l]a amistad silenciosa de la luna / (cito mal a Virgilio)“, “the silent friendship of the moon / (I misquote Virgil)”.
He is making reference to line 255 in the second book of the Æneid, from the scene where the Greeks slip out from the Trojan horse to sack the city of Troy, making their way “tacitæ per amica silentia lunæ“, “unnoticed through the friendly silence of the moon”.
Borges’ interest seems only in the expression quite abstracted from the story’s context through his own fondness for the moon as evidenced in many of his other works. He returns to the expression many times. In his brief essay on Dante’s Purgatorio, I.13 (from Nueve ensayos dantescos ), he refers again to “el famoso hexámetro de La Eneida: «a Tenedo, tacitæ per amica silentia lunæ»” (“the famous hexameter from The Æneid: «from Tenedos, unnoticed through the friendly silence of the moon»”), comparing to it verse 60 of canto I of Dante’s Infierno, “mi ripigneva là dove ‘l sol tace“—”pushed me there where the sun is silent”.
In Borges’ defence, though he needs none, his misquotation isn’t as severe as that perpetrated by José Lezama Lima in Paradiso: “Pero con esperada frecuencia volvíamos al ternario, a unir sol, tierra y luna, aunque yo casi siempre me inclinaba a la luna silentiæ amicæ.” (In Gregory Rabassa’s translation, this reads “But with expected frequency we returned to the ternary, to unite sun, earth and moon, although I almost always leaned toward the silent friendship of the moon”, though his translation is incorrect for the Latin.)