I went back in memory over the happenings of the night before.

At least ten minutes had intervened between the first heralding sigh and the intensification of the moonlight in the courtyard.

And this glow grew for at least ten minutes more before the first burst of the crystal notes.

Indeed, more than half an hour must have elapsed, I calculated, between the moment the moon showed above the horizon and the first delicate onslaught of the tinklings.
"'Edith!' I cried.

'I think I have it! The grey rock opens five minutes after upon the moonrise.