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
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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