We usually think of the world as having seven continents, and two of them were named for a man whose birthday is this Saturday.

Amerigo Vespucci, who was born on March 9, 1451, was not the first explorer to reach these continents, nor did he ever suggest that they be named for him. In fact, he died without ever knowing that they were. But the name "America" became applied to the lands of the New World anyway, in part because a young German mapmaker had simply not heard about the voyages of Christopher Columbus.Amerigo Vespucci - a merchant and adventurer from Florence - explored the coast of what is now South America on four voyages. His letters describing his exploits (and some forgeries of those letters that contained even more sensational descriptions) came to the attention of Martin Waldseemuller, who was preparing a book of maps showing all the lands that were known to exist at the beginning of the 16th century.

Waldseemuller drew in the land masses described in Vespucci's letters, and in the introduction to his book, he suggested that these new lands "be called Amerigo . . . or America" in honor of their discoverer. Later, when he learned of Columbus's voyages, he removed the designation "America" from his maps (calling the area "the Land Unknown"), but by then his earlier work had gone through several editions, and people had become accustomed to calling the New World "America."

Unlike Columbus, Vespucci recognized that these new lands were not part of Asia, but were an entirely separate continent. What is a continent? Take a look at the globe. (Here I'll issue another appeal to make an inexpensive, basketball-size globe of the Earth a part of your family's learning resources.)

A continent is usually defined as "a large continuous land mass located on the surface of the Earth." When we help our children locate and identify the seven continents on the globe, some questions are sure to arise.

Listed in order of their size, the continents are Asia, Africa, North America, South America, Antarctica, Europe and Australia. But your children will probably notice that only two of these land masses are "superislands," that is, unconnected to any other continents: Australia and Antarctica. North America and South America are connected at the Isthmus of Panama ("isthmus" comes from an ancient Greek word that meant "narrow passage"). Knowing this, can they tell which continent contains Mexico and the countries of Central America? (Answer: North America)

Africa is connected to Asia at the Isthmus of Suez, and Europe is really just a large peninsula on the giant landmass called Asia. So, you could say that there are just four "large continuous landmasses located on the surface of the Earth," and, indeed, the boundary between Europe and Asia is so artificial that many people today refer to the entire landmass as "Eurasia." But in studying the globe with your children, what you want to concentrate on is not the definition, but the recognition of the positions and the shapes of the seven continents.

If they ask why Antarctica is a continent but the northern polar cap is not, make sure they know that beneath all the ice of Antarctica is actual land, which, like all land, is part of the Earth's crust, but the ice at the North Pole floats in the Arctic Ocean.