In Middle-earth, there is a solar system with Arda being the main setting of Tolkien’s Legendarium. This Arda would include Middle-earth and the Undying Lands. It was introduced in 1957 when Tolkien decided that instead of the Sun and the Moon being made with the Two Trees, they must be coeval with Arda and known by the Elves.
Even before this, Tolkien had ideas of a solar system in Middle-earth. and brought this same concern of verisimilitude from time and time again. This means that he wants to being natural realism to Middle-earth, particularly the matters of Elvish population and migration, as well as some of the broader aspects of his created world such as the solar system and the concept of day and night in Middle-earth.
Solar System
They would also neither be considered a flat or round earth, but rather the Solar System would represent the Sun, Moon, and Wayward Stars and called Arda – though Arda would be used for the habitation of Middle-earth. Tolkien describes a cosmogonic mythology representing Arda.
“Since the Eldar are supposed to be wiser and have true knowledge of the history and nature of the Earth than Men (or then Wild Elves) their legends should have a closer relation to the knowledge now possessed of at least the form of the Solar System (= Kingdom of Arda); though it need not, of course, follow any ‘scientific’ theory of its making or development…the cosmogonic mythology should represent Arda as it is, more or less: an island in the void ‘amidst the innumerable stars.’” (Morgoth’s Ring, “Myths Transformed”)
Thus, he wanted a solar system that was for Middle-earth, and describes how this is made. We can infer that Arda is representative of Earth, a fact of life for beings.
“It was partly astronomical and “scientific,” but closed with a mythological or poetic talent. Even before their first acquaintance with the Valar they had evidently constructed a picture mytho-astronomical of the world, which was in some respects far nearer to our recent knowledge and theory than might be expected. This “picture” endured in their minds and colored their myths even after the learned and most scientific among the High-elves who dwelt with the Valar had, or so it may perhaps be presumed, learnt far more the scientific truth …their “imagination was thus not properly a flat-earth cosmology; and it was geocentric only as regards the Sun, Moon, and certain stars.” (NoME, “Dark and Light”)
Thus, the inhabitants of Middle-earth would have a picture in their minds of the Solar System, and there would indeed be places where they would know are outside of Arda, the Earth.
Tolkien introduces the Earth, en(en)dor (Q.), as a ‘Middle-land’, conceived as a spheroid – set by the Maker without a pictorial myth of the supports. However, it is possible that part of outer space is alluded to by the Poles or the extreme ends of his solar system, as “it wasn’t possible for terrestrial animals, nor Elves and Men, without wings, to reach the West and East Poles or the uttermost North or South, because it was cut by a deep circular chan[nel]” (NoME, “Dark and Light”)
He would also show an elliptical Earth with a major axis for the extreme ends, the East-West axis, and indicate a central high point. However, k, Kuiviénen, the ‘Water of Awakening,’ would lie far off in Middle-earth, east of Endon. Despite seeming to not show the other planets, Elmo, or Eärendil, for Venus, was wayward and altered with regard to other fixed stars, or “far-stars.” These Stars would be representative of the remote parts of the Great Tale of Eä which do not concern even the Valar.
Day and Night
The words for night, twilight, and day require an understanding of a primitive imagination of the shape of the Earth, Arda, and the Sun, Anar, as well as the changes in Eldarin tongue.
“[They would be] originally governed by the primitive Quendian imagination of the passage of the Sun; and also by their imagination of light.” (NoME, “Dark and Light”)
Additionally, fuin would be the word for light in Sindarin, while phuinē would be the vapor like darkness – though fuin also became night. While dark was an ethereal substance, it was more abundant and prevalent than light. Tindomë followed darkness and began starlit still, while undomë would follow light and become dark.
Additionally, the Sun and the Moon came from the Two Trees of Valinor, Telperion and Laurelin. Thus, the making of the Sun and Moon would occur long before the coming of the Elves and before the death of the Two Trees.
After the First Battle, Varda would set certain stars for the dwellers in Arda. Then, the Valar would counteract Melkor’s darkness with the Moon, as a subsidiary light to mitigate night, which Melkor made. However, it was not to drive night away, but rather to have an alternation between day and night, because “in Eä according to the Tale nothing can endure endlessly without weariness and corruption.” (Morgoth’s Ring, “Myths Transformed”)
Varda would preserve some of the Primeval Light which was her original chief concern, and the Two Trees were made. However, the Valar were driven out of Middle-earth by Melkor and his evil spirit, as Melkor decided to dominate the Children of Iluvatar (God). He was also jealous of the light that the Children of Ilúvatar had, and wanted it all to himself. Varda would arise with Manwë to strive with the Cloud of Unseeing, which led to the Great Wind of Manwë. There,…
“The stars shine out clear even in the North (Valakirka) and after the long dark seem terribly bright…it is in the dark just before that the Elves awake. The first thing they see in the dark is the stars…Men awake in an Isle amid the floods and therefore welcome the Sun which seems to come out of the East.” (Morgoth’s Ring, “Myths Transformed”)
Thus, upon the first awakening, the Elves would first see the dark and the stars and the Men would first see light.
And lastly, after the end of the Two Trees, Varda would desire the light to be mingled. However, they would end up creating twilight instead as Irmo and Estë asked her to reconsider as “sleep and rest had been banished from the Earth,” thus inferring that there is day, twilight and dark in Arda and Middle-earth. (Silmarillion, “Of the Sun and Moon and Hiding in Valinor).
