Arda (Q: Realm) was the Quenya name for the world as a whole, containing the continental masses of Middle-earth and Aman in particular. It lay within Eä as a creation of Eru Ilúvatar. Arda is linguistically a derivation from the Valarin word for the same subject, Aþâraphelûn. Though not found in Tolkien's writings, the Sindarin name would likely have been Ardhon.
(Main article: Years of the Lamps)Music of the Ainur, for the purpose of creating a home for the Children of Ilúvatar. Ilúvatar appointed the Valar as its masters, and they invented its geography, for at the time it was largely without feature. They gave it symmetry and form.
As for light source, the Valar decided upon two lamps. One, a blue light, was placed at the north, and it was called Illuin. The other, a golden light, was placed at the very southernmost point, and was called Ormal. Aulë, craftsman of the Valar, forged two towers: Helcar and Ringil, placed at the north and south respectively. The Valar chose as their own home the green isle Almaren.
It was at that time that Melkor made his first attempt, spreading chaos across Arda. He destroyed the Lamps and the Towers, ruining the symmetry of the world. Continents formed: especially Aman, Endor, the Dark Land, the Hither Lands, and the Land of the Sun. So Arda was created.
Before the Shifting
Arda was originally flat, like a plate. Encircling the continents was Ekkaia, the Encircling Sea. Farthest to the West was the great continent of Aman, composed primarily of two geographical features: the Plain of Valinor, and the Pelóri Mountains. Taniquetil was the highest mountain in this range, and in the world. To the east was the great sea of Belegaer.
Across Belegaer, to the north, was Endor, or Middle-earth. The west side of this land mass was called Beleriand, the north Dor Daidelos, and the east Palisor. According to Tolkien’s earlier maps, it was in Palisor that there was the great inland Sea of Helcar, and the shores of Cuiviénen. Middle-earth was dominated by many mountain ranges and forests.
To the farthest east possible was the Land of the Sun, with a great curved mountain range called the Wall of the Sun. In the Ambarkanta it is called the “Dark Land of the Sun”. Corresponding to Taniquetil was the mountain Kalórmë.
2nd Age and Beyond
During the tumultuous War of Wrath, the Valar broke and reshaped much of the world. The most prominent change was the drowning of Beleriand. Also Ossë raised Andor – a large, star-shaped isle – out of Belegaer. The world was stretched and made into the shape of a sphere. The continents shifted.Drowning of Númenor. The most significant changes were that Andor sank and Aman was removed from the Circles of the World – Arda, inaccessible save by the Straight Way.
The Númenóreans were great sailors and explorers. They may have been the ones to discover the Dark Land and the Land of the Sun. Otherwise, Tolkien tells us little about the lands beyond Aman and Middle-earth.
Arda Marred and Arda Unmarred
The Elves held to the concept of Arda Marred (Quenya: Arda Hastaina). In the beginning, at the time of the Music of the Ainur, was Arda Unmarred. According to the idea of Arda Unmarred, the world was originally “unstained” by evil. It became Arda Marred after Melkor entered and introduced chaos. They also clung to the idea that one day the world would become Arda Healed (Quenya: Arda Envinyanta). According to this belief, Arda Healed will not come about until the Dagor Dagorath, the Last Battle.