§ Of the Sun and Moon and the Hiding of Valinor The Silmarillion

Now Varda purposed that the two vessels should journey in Ilmen and ever be aloft, but not together; each should pass from Valinor into the east and return, the one issuing from the west as the other turned from the east. Thus the first of the new days were reckoned after the manner of the Trees, from the mingling of the lights when Arien and Tilion passed in then- courses, above the middle of the Earth. But Tilion was wayward and uncertain in speed, and held not to his appointed path; and he sought to come near to Arien, being drawn by her splendour, though the flame of Anar scorched him, and the island of the Moon was darkened.