Living in Community

Boundaries
The Portuguese used lunar eclipses to determine longitude, which was important for making maps and establishing boundaries.

In 1494, Portugal and Spain made dramatic use of longitude to determine boundaries with the Treaty of Tordesillas. Christopher Columbus had explored the western hemipshere in 1492 with Spanish support (though with Portuguese techniques of navigation). The king and queen of Spain wanted to assert Spanish rights to those lands. However, the king of Portugal wanted to protect Portuguese rights to regions of Africa that the Portuguese had explored, and to the routes at sea that led there.

The compromise of the Treaty of Tordesillas drew a line in the Atlantic 370 leagues west of the Cape Verde Islands (a Portuguese settlement off the coast of Africa). The treaty gave Spain rights to all lands west of that line, and gave Portugal rights to all lands east of that line, except that neither country could have rights to any land already under Christian control. The treaty also specified a line in the Pacific exactly halfway around the world from the line in the Atlantic, which gave the other boundary to the regions of Portuguese and Spanish control.

This treaty became more important because when the Portuguese explorer Pedro Alvares Cabral found Brazil in 1500, it was in a region east of the line in the Atlantic. This treaty became the basis for Portuguese rule in Brazil, which lasted for three centuries and had many lasting effects. In the middle 1500s the Portuguese also used the treaty to claim the Molucca islands (now part of Indonesia), which were close enough to the line in the Pacific that it was not easy to determine whether they were on the Portuguese side or the Spanish side.

The Portuguese argued that these locations of places should be determined by astronomical techniques. In particular they used lunar eclipses. Eclipses are good for calculations because we can determine their timing precisely, and because they can be seen from large parts of the world at the same time. Someone in Lisbon can determine at what hour of the night the eclipse happens there, and someone in the Moluccas can determine at what hour of the night the eclipse happens there. Using the difference between these two times, you can determine the difference in longitude between the two places.

Often the Portuguese explorers marked their discoveries by leaving stone markers. In Jakarta, another Indonesian site, they symbolized the importance of astronomy to their explorations by leaving a stone marker with a picture of an armillary sphere.

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