Ferdinand Magellan

Among the ancient Greeks, Homer thought the world to be flat. In contrast, Pythagoras postulated that the earth was round. A century later, Aristotle sided with the theory of the roundness of the earth.

Leave it to the clever Greek investigator Eratosthenes to bring it all together with a calculation (and a remarkably accurate one!) about the diameter of the earth, and also the inclination of its axis of rotation to its orbital plane about the sun.

Centuries later, independent work was done by Muslims as Europe descended into the Dark Ages. By the 9th century, many Muslim scholars took it for granted that the Earth was a sphere. The proof, said astronomer Ibn Hazm, "is that the Sun is always vertical to a particular spot on Earth". It was 500 years before that realisation dawned on Galileo. The calculations of Muslim astronomers were so accurate that in the 9th century they reckoned the Earth's circumference to be 40,253.4km --less than 200 km out. Later Europeans mistakenly attribute the work for Nicolaus Copernicus.

Edited just correction : with all respect to the one who answered the question, the Europeans did not thought that Copernicus discovred the Earth's Circumference mistakenly , in fact it he stole it and add it to his books is if it was his own work.

Magellan's expedition was the first to successfully sail around the whole world (although only 18 out of 257 managed to return, and Magellan himself got killed on the way), and quite a few places and animals they found were previously unknown to Europeans.

Among other things, Magellan discovered the Strait of Magellan, which runs between the Atlantic and the Pacific near the southern tip of South America, and a species of penguin called the Magellanic Penguin.

His expedition also established the need for the international date line, you could say he discovered that.

Among things he did NOT discover - he found them early but was not the first European to do so - are:

• The Magellanic Clouds, a pair of galaxies: Described by Magellan's crew, but observed by a couple of Italians about 30 years earlier, and described by a Persian astronomer 500 years prior to that.
• The Pacific: Magellan's expedition were the first Europeans to enter the Pacific through the Strait of Magellan, but Spanish explorer Vasco Nuñez de Balboa had seen the Pacific a few years earlier, by land across Panama.
  
• The Philippines: Parts of the Philippines were already known to the Portuguese, and Arab traders had established commerce there centuries earlier.

The first one we have a record of was a Spanish expedition of five ships and a crew of about 270 led by Portuguese navigator Ferdinand Magellan, who was silly enough to meddle in local politics in the Philippines and was killed.

Four of the ships were lost, and one the Vittoriamade it back to Spain. Its master Juan El Cano and 17 crewmen of the original 43-man crew of the ship survived to return.