(From - Age of Exploration by J.R. Hale p. 46)
At the end of the 15th century, the Spanish and Portuguese began conducting marine voyages beyond the areas known to Europeans and peoples of the Mediterranean. The principal aim of these voyages was to find a sea route to India. The discovery of America by Columbus (who was sent by the Spanish) in 1492, was made during his attempt to circle the globe on his way to India. This attempt failed because America lay in the way, between Spain and India. Columbus was also mistaken in his estimation of the size of the earth. It turned out that the earth was much bigger than he thought, so that his ships would have been unable to reach India, even if he had not happened on another continent by chance. The Portuguese were more successful in finding a sea route to India. In 1498 the ships of Vasco da Gama reached India by sailing around Africa, past the Cape of Good Hope. Magellan was the first who succeeded in circling the earth between the years 1519-1521.
Precise and detailed globes were prepared in wake of geographical discoveries. These globes became a fashionable item in the homes of the rich and the nobility, as well as in libraries.
During this period, more European countries joined the marine journeys in the race for commerce, gold, slaves and the conquest of new territories. This race was also accompanied by a desire to discover the world, and to spread the Christian faith. Numerous European ships sailed across the world at the beginning of the 17th century. A good many of the explorers and sailors were Italian and many ships arrived in Italian harbors known to Galileo.
Effects of the globe on physicsGalileo used the globe as a conceptual tool. The globe demonstrates visually that any movement over the surface of the earth is a circular one. Galileo thought about the characteristics of this movement. His first formulation of the law of inertia was based on a circular movement around the earth. In his book, first published in 1612, which includes a collection of letters on the subject of sun spots, he wrote:
"To some movements they are indifferent, as are these same heavy bodies to horizontal motion, to which they have neither inclination (since it is not toward the center of the earth) nor repugnance (since it does not carry them away from that center). And therefore, all external impediments removed, a heavy body on a spherical surface concentric with the earth will be indifferent to rest and to movements toward any part of the horizon. And it will maintain itself in that state in which it has once been placed; that is, if placed in a state of rest, it will conserve that; and if placed in movement toward the west (for example), it will maintain itself in that movement. Thus a ship , for instance, having once received some impetus through the tranquil sea, would move continually around our globe without ever stopping and placed at rest it would perpetually remain at rest, if in the first case all extrinsic impediments could be removed, and in the second case no external cause of motion were added."
From : Discoveries and Opinions of Galileo - Letters on Sunspots (1613)
Doubleday Anchor Books, Doubleday & Company, INC., Garden City, New York.
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