Then Ilúvatar spoke, and he said: 'Mighty are the Ainur, and mightiest among them is Melkor; but that he may know, and all the Ainur, that I am Ilúvatar, those things that ye have sung, I will show them forth, that ye may see what ye have done. And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'
Then the Ainur were afraid, and they did not yet comprehend the words that were said to them; and Melkor was filled with shame, of which came secret anger. But Ilúvatar arose in splendour, and he went forth from the fair regions that he had made for the Ainur; and the Ainur followed him.
But when they were come into the Void, Ilúvatar said to them: 'Behold your Music!' And he showed to them a vision, giving to them sight where before was only hearing; arid they saw a new World made visible before them, and it was globed amid the Void, and it was sustained therein, but was not of it. And as they looked and wondered this World began to unfold its history, and it seemed to them that it lived and grew. And when the Ainur had gazed for a while and were silent, Ilúvatar said again: 'Behold your Music! This is your minstrelsy; and each of you shall find contained herein, amid the design that I set before you, all those things which it may seem that he himself devised or added. And thou, Melkor, wilt discover all the secret thoughts of thy mind, and wilt perceive that they are but a part of the whole and tributary to its glory.'
And many other things Ilúvatar spoke to the Ainur at that time, and because of their memory of his words, and the knowledge that each has of the music that he himself made, the Ainur know much of what was, and is, and is to come, and few things are unseen by them. Yet some things there are that they cannot see, neither alone nor taking counsel together; for to none but himself has Ilúvatar revealed all that he has in store, and in every age there come forth things that are new and have no foretelling, for they do not proceed from the past. And so it was that as this vision of the World was played before them, the Ainur saw that it contained things which they had not thought. And they saw with amazement the coming of the Children of Ilúvatar, and the habitation that was prepared for them; and they perceived that they themselves in the labour of their music had been busy with the preparation of this dwelling, and yet knew not that it had any purpose beyond its own beauty. For the Children of Ilúvatar were conceived by him alone; and they came with the third theme, and were not in the theme which Ilúvatar propounded at the beginning, and none of the Ainur had part in their making. Therefore when they beheld them, the more did they love them, being things other than themselves, strange and free, wherein they saw the mind of Ilúvatar reflected anew, and learned yet a little more of his wisdom, which otherwise had been hidden even from the Ainur.
Because the Sun, Moon, stars, and other objects orbited the planet in the cosmology of Middle-earth they were considered part of Arda, leading to Arda sometimes being called the 'Solar System' of Middle-earth. The term for the Earth itself, excluding these celestial bodies, was Ambar or Imbar.
Originally the Earth (Arda) was flat, the continents were surrounded by a mighty ocean (or perhaps by space), Ekkaia or Vaiya, the Encircling Sea, and separated by Belegaer, called the Great Sea and the Sundering Seas. In the First Age, the area in the northwest of Middle-earth was occupied by the country of Beleriand, but this was destroyed during the War of Wrath.
There was also a separate continent south and east of Middle-earth called the Dark Land. Another separate continent to the east of Middle-earth was the relatively small Land of the Sun, so called because when the world was flat the Gates of the Sun were near there, and it would be scorched.
Information regarding both was vague. To the south were the Hither Lands including Harad, though "Harad" means "South" and while properly used for the region immediately south of Gondor and Mordor was often loosely used to refer to every land to the south. There were many stretches of sun-scorched desert in Near Harad, but there was supposedly a jungle beyond it which few had ever ventured to, from which the great Oliphaunts were found. Men of Far Harad are also described as being black skinned. The vast lands of the East were collectively referred to as Rhûn, though this may have only referred to those immediately east of the Sea of Rhûn and River Running. The Easterlings, like the Haradrim, were a diverse collection of many races. There was even less knowledge of the extreme eastern end of Middle-earth, where the land was supposed to stop at another sea. It is known that in ancient times the great chain of the Red Mountains ran north to south near here, and was roughly analogous to the Blue Mountains of the west (the world was once symmetrical at the dawn of time). The race of Elves first originated near here, though their original home of Cuiviénen no longer existed by the Third Age. It is not known if the Red Mountains survived into the Third Age, either intact or in some reduced form (like the Blue Mountains). It is said that the Blue Wizards went far into the east of Rhûn and beyond to stir up rebellions against Sauron in the lands he held sway over, though the tales of these struggles never reached the west in detail.
In the Second Age, Númenor was raised in the Great Sea for the Edain. This island existed through most of the Second Age, but was destroyed as a result of the pride of the Númenórean people in defying the Ban of the Valar and sailing to Aman in the west.
After the destruction of Númenor, Arda was made round. Aman was taken out of the world, and could only be reached by the Elves, following the straight road that was granted to them. As Aman was taken away from Arda, new lands and continents were created east and west of Middle-earth.