The Prose Edda 1.10

"Nörfi or Narfi is the name of a giant that dwelt in Jötunheim: he had a daughter called Night; she was swarthy and dark, as befitted her race. She was given to the man named Naglfari; their son was Audr. Afterward she was wedded to him that was called Annarr; Jörd was their daughter. Last of all Dayspring had her, and he was of the race of the Æsir; their son was Day: he was radiant and fair after his father. Then Allfather took Night, and Day her son, and gave to them two horses and two chariots, and sent them up into the heavens, to ride round about the earth every two half-days. Night rides before with the horse named Frosty-Mane, and on each morning he bedews the earth with the foam from his bit. The horse that Day has is called Sheen-Mane, and he illumines all the air and the earth from his mane."