Amazingly, all crewmembers survived except for one, who was murdered by native islanders early in the journey. He accomplished this impressive feat using only a compass and simple sextant. After a brief stop in Tonga, Captain Bligh was able to use the method of dead reckoning to navigate across more than 6500 km of ocean, to the island of Timor in Southeast Asia in just 44 days (Fig. Captain William Bligh and 18 crewmembers loyal to him were set adrift in a large rowboat. In 1789, the crew of the British Royal Navy ship Bounty mutinied against the captain near Tahiti in the South Pacific. Image by Narrissa Spies, courtesy of Google Maps This method of determining location does not take into account the effects of currents, wind, or inaccuracies in the measurement of time. Together with a compass direction heading, the ship’s navigator would estimate the ship’s current position using the estimated speed and time traveled from a previously determined position. The name of the unit “knot,” which stands for nautical miles per hour, arose from this method of measurement. Sailors measured the ship’s speed by counting the number of knots passed during the prescribed time span. When thrown overboard, the chip log stayed in roughly the same place while the knotted line unspooled from the boat for a set period of time (such as the time needed for sand in an hourglass to move from one side to the other). A chip log was a weighted board attached to a uniformly knotted line (Fig. But, to be more precise, ships often used a chip log. Estimating speed can be as simple as noticing the time it takes bubbles to pass along the length of the boat. Image courtesy of Rémi Kaupp, Wikimedia Commonsĭead reckoning is used to calculate distance traveled by estimating a ship’s speed over time. Compasses do not work well close to permanent magnets or in areas with a lot of iron. A magnet has its’ own magnetic field, which can interfere with a compass reading. An example of a permanent magnet is a refrigerator magnet. To magnetize an iron or nickel pointer, it must be repeatedly rubbed on a lodestone, a naturally magnetized rock called magnetite, to impart a permanent magnetic field.Īn object with a permanent magnetic field is called a permanent magnet. The point that pivots to the north is often painted red. They have a magnetized pointer that is held in the middle of the compass with a pin that allows the pointer to spin freely and align itself with the earth’s magnetic field. Simple compasses are sealed round containers. SCUBA divers, for example, use compasses because GPS units do not work underwater. Although Global Positioning System (GPS) units have mostly replaced compasses, compasses are still used by many professionals. Image courtesy of Shyamal, Wikimedia CommonsĪ magnetic compass is an indispensable navigational tool for determining direction of travel (Fig. A magnetic compass is a navigational tool that points towards the earth’s magnetic poles. Perhaps the best-known navigational tool is the compass. Voyagers have used a variety of instruments and techniques to safely navigate the oceans. With this navigational ability, Vikings could venture farther from land, exploring the open ocean, confident in their ability to return home. This information would allow Vikings to gauge latitude. The angle of the shadow cast by the sun would help the navigator establish a heading depending upon the time of day. There is archeological evidence that Vikings used sun compasses (similar to sun dials, see Fig. The high latitude also meant that the nights were very short in the summer, making it difficult to rely on the stars for navigation. Because they were so far north, Vikings tended to do most of their exploring and voyaging during summer months when the weather was good and the sun was out for longer periods of time. Before the Vikings, Europe was navigated mostly along, and within sight of, the coast. Another group of wayfinders noted for their early exploration and seafaring fortitude were the Vikings.
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