You have to remember how little was known about our big old world, way back when, more than 500 years ago. People couldn't have imagined how big and especially how old the world was...and they didn't know how many continents and large islands there might be, nor how large the seas in between the lands were, nor what the best routes from Point A to Point B might be.
When we say “voyage of discovery”—we mean a voyage into the complete unknown, a brave attempt to discover useful knowledge (and riches).
Portugal's King John II knew a lot more than most, because Portuguese sailors had been exploring Africa bit by bit, sailing ships farther and farther south along the continent. Finally, in 1488, a captain named Bartolomeu Dias had rounded the bottom of the continent and started traveling north-east instead of south, as far as Fish River in what is now South Africa. He reported seeing coastline stretching on and on to the north-east...
The next of Portugal's kings, Manuel I, sent out another expedition. He chose Vasco da Gama as the captain—and he told him to travel all the way to India, a land previously explored by overland travelers, and to obtain trading rights for spices.
Vasco da Gama led a fleet of four ships with combined crews of 170 men. They sailed away from Lisbon on this day in 1497. They had no doubt heard about Christopher Columbus's journeys to the “Indies” (although Columbus hadn't actually reached the East Indies, as he had hoped), but these ships were sailing south and east rather than west, and they were determined to return successful.
In actual fact, only 55 of the men returned at all, on just two of the ships. Although the expedition met many dangers and difficulties, including hostile Muslim traders and an unimpressed King of Calicut who demanded more payment than the “trivial” gifts Gama gave him on behalf of King Manuel I, the main reason so few men lived to return was that Gama ignored the locals' knowledge of the monsoons and tried to make an Indian Ocean crossing at the wrong time. Earlier that year, the crossing eastward took just 23 days, but fighting the monsoon winds, the westward crossing took 132 days. Many of Gama's men died, and many of those who survived the crossing were suffering from scurvy. After some recovery time, Gama led the two surviving ships back around the Cape of Good Hope and eventually northward to Portugal.
Even though Gama's relations with the King of Calicut were strained and trading rights hadn't been established, Gama received a hero's welcome back in Portugal. He had of course mapped out more unknown lands and seas, he'd succeeded in reaching India from Europe by sea (which had never been done before), and he had brought back spices and other goods worth sixty times what the expedition cost!
Also on this date: