Ultima Thule

In ancient times the northernmost region of the habitable world - hence, any distant, unknown or mysterious land.

Tuesday, October 17, 2006

The Eclipse That Saved Columbus

By Aussiegirl

How frightening to behold a lunar eclipse and not know the physical reason behind it! Here is a beautiful passage from Paradise Lost, in which Milton is describing Satan:
He above the rest
In shape and gesture proudly eminent
Stood like a tower. His form had yet not lost
All her original brightness, nor appeared
Less than archangel ruined, and th' excess
Of glory obscured: as when the sun new-risen
Looks through the horizontal misty air
Shorn of his beams, or from behind the moon
In dim eclipse disastrous twilight sheds
On half the nations, and with fear of change
Perplexes monarchs.

Math Trek: The Eclipse That Saved Columbus, Science News Online, Oct. 7, 2006

The Eclipse That Saved Columbus
Ivars Peterson

By the time Christopher Columbus sailed westward in 1492, navigators were already using hefty volumes containing astronomical tables to guide them across unknown seas. These books often included detailed instructions for manipulating navigational instruments and for computing geographical positions from celestial observations.

Columbus himself probably carried copies of two invaluable books. The "perpetual almanac" prepared by Abraham Zacuto contained more than 300 pages of astronomical tables that had already contributed to such navigational feats as Vasco da Gama's famous expedition from Portugal around the tip of Africa to India. The second volume, called the Ephemerides, had been produced by the prominent German astronomer and mathematician Johannes Müller, who went by the Latin name Regiomontanus.

The astronomical tables that Columbus consulted during his voyage proved useful for determining latitude and, to some degree, longitude. A prediction contained in the tables probably saved his life at a crucial moment during his fourth voyage to the lands he had discovered. [....]

Despite these uncertainties, Columbus was desperate enough to take a chance. On the day before the predicted eclipse, he summoned the leaders of the native inhabitants and warned them through an interpreter that if they did not cooperate with him, the moon would disappear from the sky on the following night.

The natives for the most part were unimpressed; some even laughed. Columbus nervously awaited the outcome of his gamble. Could he rely on tables that had been compiled several decades earlier and that predicted the positions of celestial bodies only for the years between 1475 and 1506? How large were the errors?

Amazingly, the prediction proved correct. As the full moon rose in the east on the appointed night, Earth's shadow was already biting into its face. As the moon rose higher, the shadow became larger and more distinct until it completely obscured the moon, leaving nothing but a faint red disk in the sky.

The natives were sufficiently frightened by this unexpected occurrence and by Columbus's uncanny prediction to beg forgiveness and appeal to him to restore their moon to the sky. Columbus responded that he wished to consult with his deity. He retired to his quarters, using a half-hour sandglass to time how long the eclipse would last. Some time later, when the eclipse had reached totality, he emerged to announce that the moon, in answer to his prayers, would gradually return to its normal brightness.

The next day, the natives brought food and did all they could to please Columbus and his crew. Columbus himself used the timing of the eclipse to calculate his ship's longitude, but his answer proved wildly erroneous. [....]

The success of Columbus's strategem was a tribute to the accuracy of the calculations and predictions made by Regiomontanus, based on Ptolemy's Earth-centered model of the solar system. [....]


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