Tilion was the youth whom the Valar chose from among the Maiar to steer the island of the Moon. In the days of the trees in Valinor, Tilion had been a hunter from the company of Oromë. He was said to be less mighty than Arien, the Maia chosen to guide the Sun.
Tilion was reckless at times, and did not always follow the correct path, causing the Moon to be at times in the sky at the same time as the Sun, or not appear at all.
In writings by Tolkien which did not form a part of the published Silmarillion, Tilion is said to have been in love with Arien, and because he sought her out he came too close to the immense heat of the Sun, and the Moon was scarred.