The Song of the Sun and Moon.

Following the destruction of the Two Trees of Valinor, Yavanna and Nienna, at the bidding of Manwë put forth all their power to heal the Trees. However they could not avail to heal their mortal wounds.

But before the end, Telperion bore one great flower of silver, and Laurelin a single fruit of gold.

The silver flower and the gold fruit was given to Aulë, and Manwë hallowed them. By the craft of Aulë and his people great vessels were wrought to hold and preserve them. Then Varda set them to voyage upon appointed courses above the Earth, and they became the Moon and the Sun respectively.

Arien the Maia was chosen to guide the vessel of the Sun, and Tilion was chosen to guide the Moon.
Encyclopedia entry originally written by Earendil_the_Mariner