Middle-earth is the name used for J. R. R. Tolkien's fictional ancient Earth where the stories in his legendarium take place. "Middle-earth" is a literal translation of the Old English term Middangeard, referring to this world, the habitable lands of men. Mythologically, the Endor continent became the Eurasian land-mass after the primitive Earth was transformed into the round world of today. Although Middle-earth's setting is often thought to be another world, Tolkien actually conceived it as a fictional period in our Earth's own past 6,000 to 7,000 years ago.
The Two Trees of Valinor in the fictional universe of J.R.R. Tolkien's Middle-earth are Telperion and Laurelin, the Silver Tree and the Gold Tree that brought light to the Land of the Valar in ancient times. They were destroyed by Melkor and Ungoliant the great spider. Morgoth/Melkor stabbed each of the trees with his spear, and Ungoliant drank them dry. But the last flower of Telperion and the last fruit of Laurelin were made by the Valar into the Moon and the Sun.
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.
See Middle-earth for the main article about Tolkien's fictional universe
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 (The Undying Lands) was taken out of the world into an alien dimension, 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.