So far as I know, the manuscripts' fraternis in Prop. 2. 34. 52 ‘aut cur fraternis Luna laboret equis’ has never been doubted. I offer an emendation of it in this note.
Luna laboret ought to allude to lunar eclipse, but you cannot see it through the fog of fraternis equis. In C.Q.xliii (1949), 26–7, Shackleton Bailey dealt with the traditional claim for it, that the moon is eclipsed, not by the sun, by the presence of her brother's horses, but by their absence, just as in Virgil the sea and Ixion's wheel stand still when the winds' presence is no longer felt: ‘cum placidum ventis staret mare’ (Ecl. 2. 26), ‘Ixioni vento rota constitit orbis’ (G. 4. 484.). Simply, he saw no parallel between the unambiguous absence of those winds’ blasts and the alleged absence of the sun's horses here.