The Silmarillion - J.R.R. Tolkien, Ted Nasmith,  Christopher Tolkien

The trees have fallen and the Silmarils lost.  Teleperion bears one last flower of silver, and Laurelin a single fruit of gold, which were taken and put into vessels to hang in the sky as great lamps.  With these lamps they resolve to illuminate Middle Earth, bringing light to the people's there and hindering Melkor's (literally) dark deeds.

Good news: the Valar have a solid idea that they need to actually pay attention to Melkor and the danger he poses.

Bad news: with the arrival of humans imminent (plus the waking of the dwarves), waging war on Melkor might take out the life they're charged with preparing the world for.

Isil the Sheen the Vanyar of old named the Moon, flower of Telperion in Valinor; and Anar the Fire-golden, fruit of Laurelin, they named the Sun.  But the Noldor named them also Rana, the Wayward, and Vasa, the Heart of Fire, that awakens and consumes; for the Sun was set as a sign for the awakening of Men and the waning of the Elves, but the Moon cherishes their memory.
cliffs and clouds rising above a sea at dawn

It's almost shocking to me to see that the waning of the Elves starts so early.  But the Sun and the Moon are hung in the sky, each steered by a Maiar.  Arien for the Sun and Tilion for the moon.  Arien... is kind of amazing, mightier than her hunter counterpart, a spirit of fire, undecieved by Melkor, and with eyes too bright for even the Eldar to look upon.  As the Sun Melkor dares not come near her and her might.  Like I said, kind of amazing.

The original plan was to have the Moon and the Sun in the sky at the same time, crossing opposed with their lights mingling.  Like many myths of the Sun and the Moon, the path the follow now is the result of an attraction between them.  In this case Tilion, wayard and uncertain in speed, seeks to come near Arien, drawn in by her splendor while yet the flame of Anar scorches him and the Moon itself.  Even when Varda decrees a change in path, a course across and under the world, Tilion's pace remains unsteady.

The withered husks of the trees still stand in Valinor, and while Melkor will not come near the Sun, his failed attacks against Tilion unsettle the Valar.  And so they fortify their home and mount continuous guards, closing all egresses save one, for the Eldar must at times need to breath the air from the land of their birth as carried by the breeze, and for their kin they refuse to sunder entirely from.