[davem]

Just adding to Raynor's answer, Tolkien's source is clearly Norse myth:

From Snorri's Prose Edda:

XI. Then said Gangleri: "How does he govern the course of the sun or of the moon?" Hárr answered: "A certain man was named Mundilfari, who had two children; they were so fair and comely that he called his son Moon, and his daughter Sun, and wedded her to the man called Glenr. But the gods were incensed at that insolence, and took the brother and sister, and set them up in the heavens; they caused Sun to drive those horses that drew the chariot, of the sun, which the gods had fashioned, for the world's illumination, from that glowing stuff which flew out of Múspellheim. Those horses are called thus: Early-Wake and All-Strong; and under the shoulders of the horses the gods set two wind-bags to cool them, but in some records that is called 'iron-coolness.' Moon steers the course of the moon, and determines its waxing and waning. He took from the earth two children, called Bil and Hjúki, they that went from the well called Byrgir, bearing on their shoulders the cask called Sægr, and the pole Simul. Their father is named Vidfinnr. These children follow Moon, as may be seen from the earth."

XII. Then said Gangleri: "The sun fares swiftly, and almost as if she were afraid: she could not hasten her course any the more if she feared her destruction." Then Hárr made answer: "It is no marvel that she hastens furiously: close cometh he that seeks her, and she has no escape save to run away." Then said Gangleri: "Who is he that causes her this disquiet?" Hárr replied: "It is two wolves; and he that runs after her is called Skoll; she fears him, and he shall take her. But he that leaps before her is called Hati Hródvitnisson. He is eager to seize the moon; and so it must be." Then said Gangleri: "What is the race of the wolves? Hárr answered: "A witch dwells to the east of Midgard, in the forest called Ironwood: in that wood dwell the troll-women, who are known as Ironwood-Women. The old witch bears many giants for sons, and all in the shape of wolves; and from this source are these wolves sprung. The saying runs thus: from this race shall come one that shall be mightiest of all, he that is named Moon-Hound; he shall be filled with the flesh of all those men that die, and he shall swallow the moon, and sprinkle with blood the heavens and all the air; thereof shall the sun lose her shining, and the winds in that day shall be unquiet and roar on every side. So it says in Völuspá:

Eastward dwells the Old One in Ironwood,
And there gives birth to Fenrir's brethren;
There shall spring of them all a certain one,
The moon's taker in troll's likeness.

He is filled with flesh of fey men.
Reddens the gods' seats with ruddy blood-gouts;
Swart becomes sunshine in summers after,
The weather all shifty. Wit ye yet, or what?"

Compare The Sil account:

The maiden whom the Valar chose from among the Maiar to guide the vessel of the Sun was named Arien, and he that steered the island of the Moon was Tilion. In the days of the Trees Arien had tended the golden flowers in the gardens of Vana, and watered them with the bright dews of Laurelin; but Tilion was a hunter of the company of Orome, and he had a silver bow. He was a lover of silver, and when he would rest he forsook the woods of Orome, and going into L6rien he lay in dream by the pools of Este, in Telperion's flickering beams; and he begged to be given the task of tending for ever the last Flower of Silver. Arien the maiden was mightier than he, and she was chosen because she had not feared the heats of Laurelin, and was unhurt by them, being from the beginning a spirit of fire, whom Melkor had not deceived nor drawn to his service. Too bright were the eyes of Arien for even the Eldar to look on, and leaving Valinor she forsook the form and raiment which like the Valar she had worn there, and she was as a naked flame, terrible in the fullness of her splendour......

But Morgoth hated the new lights, and was for a while confounded by this unlooked-for stroke of the Valar. Then he assailed Tilion, sending spirits of shadow against him, and there was strife in Ilmen beneath the paths of the stars; but Tilion was victorious. And Arien Morgoth feared with a great fear, but dared not come nigh her, having indeed no longer the power; for as he grew in malice, and sent forth from himself the evil that he conceived in lies and creatures of wickedness, his might passed into them and was dispersed, and he himself became ever more bound to the earth, unwilling to issue from his dark strongholds. With shadows he hid himself and his servants from Arien, the glance of whose eyes they could not long endure; and the lands near his dwelling were shrouded in fumes and great clouds.