What Specific Navigational Techniques And Charts Did The Portuguese Explorers, Such As Diogo De Teive, Use To Accurately Determine Their Longitude While Sailing Westward Along The Coast Of West Africa During The Early 16th Century, Given The Limitations Of Astronomical Observations And The Lack Of Reliable Piloting Charts For That Region At The Time?

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During the early 16th century, Portuguese explorers like Diogo de Teive employed a combination of traditional navigational techniques and available tools to navigate the West African coast, despite the challenges of determining longitude. Here's a detailed overview of their methods:

  1. Dead Reckoning: This was a primary technique where they estimated their position by tracking speed and direction. They used a compass for direction and a chip log to measure speed, though this method was prone to inaccuracies over long distances.

  2. Pilotage: Experienced pilots relied on recognizing landmarks, currents, and natural features. They used portolan charts, which, while detailed for the Mediterranean, were less accurate for West Africa but improved with each voyage.

  3. Astronomical Observations: Latitude was determined using the astrolabe and celestial bodies like the sun and stars. Longitude estimation was challenging without precise timekeeping, so they often relied on dead reckoning and travel time estimates.

  4. Magnetic Compass and Natural Phenomena: Observations of the compass needle's variations and natural signs like ocean currents and wave patterns provided additional clues about their position.

  5. Voyage Logs and Charts: Detailed logs were kept, noting celestial observations, landmarks, and other data. These logs helped create more accurate charts for future voyages, gradually improving their navigational aids.

  6. Timekeeping and Celestial Events: Use of hourglasses for timekeeping and observations of the moon and planets relative to the sun and stars were part of their navigational efforts, though these were complex without precise tables.

In summary, Portuguese explorers combined dead reckoning, pilotage, astronomical observations, and evolving charts to navigate. While they lacked precise methods for longitude, their cumulative experience and observations allowed them to sail effectively along the West African coast.