The Prose Edda 1.11

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."