Page 65 - Encyclopedia Of World History Vol IV

1366 berkshire encyclopedia of world history

this purpose. Seamen found that a group of clouds over Columbus tried to find his longitude in the New World
the horizon indicated still-unseen land. When visible, from observing a lunar eclipse, which he knew would
Norsemen used the polestar for orientation at night, and also occur in Spain. He compared the local time of his
the sun during daytime. They probably also used these observation with the predicted time it would occur in
celestial bodies for setting out their courses. The mag- Spain.The effort failed because of the inaccurate predic-
netic compass came into use in the north of Europe after tion through imperfect knowledge of the moon’s motion.
it was introduced in the Mediterranean. The Nuremberg astronomer Johann Werner, in 1514,
Navigation with specially developed scientific instru- was probably the first to suggest that longitude could be
ments, and tables with astronomical ephemera, dates determined by measuring lunar distances, the angle
from the time of the maritime expansion of Portuguese between the moon and a fixed star. The moon moves
and Spanish navigators, in the fifteenth century. When across the sky relatively rapidly. At a given time the
the Portuguese embarked on their voyages along the west angle between the moon and a particular star is the same
coast of Africa their courses were generally north-south. for every observer who can see them both. The angle
They found their latitude by observing the polestar at between the moon and a number of stars (called the dis-
night, and during daytime from the sun’s altitude at tance) can be calculated ahead for certain dates and
noon. For their observations they used the mariner’s astro- times, for a particular (prime) meridian. An observer
labe, a navigating instrument that was developed out of measuring the angle would compare the result and his
the astronomical astrolabe, probably around 1445.The local time with the predicted result and time, and the dif-
cross-staff, inspired on the kamal, and used for the same ference between the two times would provide the dif-
purpose, was introduced at sea about 1515. Beyond the ference in longitude.Werner’s method is feasible, but at
equator, which was reached by 1474, the polestar is no the time it was technically impossible to develop suffi-
longer visible. Although the Southern Cross had been ciently accurate instruments and lunar tables. Fernando
seen by Portuguese seamen already in 1454, it was de Magallanes’s astrologer in 1519 unsuccessfully tried
found that there was no southern polestar. It was then to find the longitude in South America by conjunction of
decided to try to improve the method of finding latitude the moon and Jupiter. Failure was blamed on errors in
from the sun’s altitude at noon. As a result the first day- the predicted astronomical ephemera.
to-day declination tables of the sun were compiled. Navigators sometimes used the variation of the mag-
When Columbus sailed towards America in 1492, he netic compass for estimating their longitude. Because of
was provided with such tables in manuscript. Columbus the apparent relation between longitude and variation,
was also the first to observe westerly variation.Variation sixteenth-century scholars designed methods of convert-
is the angle between the directions of the magnetic and ing variation into longitude. For this to be successful in
the geographic North Poles, and depending on the place practice, however, the method requires a vast amount of
on the earth, it can be easterly, westerly, or zero. As the data on the magnetic variation, which was not available
position of the magnetic pole changes slightly over a until the eighteenth century, when Dutch navigators esti-
year, the variation increases or decreases. It is important mated their longitude with variation during crossings of
that, when setting a course, the value of variation is taken the Atlantic and Indian Oceans.
into account. As the attempts to solve the question of longitude at
After the Atlantic Ocean was crossed, and the Indian sea failed, the king of Spain, in 1598, offered a reward
Ocean in 1497, east-west courses were sailed more fre- for anyone who came up with a practical solution. This
quently, and the necessity to measure longitude was felt. example was soon followed by the Dutch States-General,
The difference in longitude between two places can be the governments of France and Venice, and eventually in
expressed in the difference in time between the two. 1714 by the British Parliament, which offered £20,000.