The Valar chose Tillion from among the Maiar to steer the island of the Moon.
In the days of the trees in Valinor, the young Tilion had been a hunter from the company of Oromë. In terms of power, he is said to be less mighty than Arien. Arien was chosen to guide the Sun, while Tilion was chosen to guide the Moon.
Tolkien portrays Tilion as reckless at times, and as one who does not always follow the correct path. This causes the Moon to not always be in the sky at the same time as the Sun or to not appear at all.
In writings by Tolkien alluded to in 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.