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May 12

Anniversary of the Lewis and Clark Expedition: Exploring without a GPS

208 years ago, on May 18th, Lewis and Clark began their expedition to explore the Louisiana Purchase. There were commercial applications for the trek, with the hope of finding a practical route to the Pacific, but the expedition was meant to be a primarily scientific exploration. It was a chance to map the Western United States. It was an opportunity to discover new river junctions, mountain passes, and other wonders. The charting the west was no easy order. After all, Lewis and Clark didn’t have a GPS device. They used an assortment of now dated navigational equipment.


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This nifty device was chiefly used to determine the latitude of a location. By measuring the angle between the horizon and the sun, an approximation of latitude could be determined. The octant got it’s name because of its 45 degree angle, one-eighth of a circle. While useful, it did have its downfalls. It could only be used between April and August, when the sun was greater than 60 degrees above the horizon.

Circumferentor (Surveyor’s Compass)

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The circumferentor was essentially a more precise, larger magnetic compass. It was so precise it could measure to one half of a degree. This was aided by a front and back sighting that extended up to six inches away, ensuring accuracy in sighting. The device could be attached to top of a pole or tripod to help level it. Lewis and Clark tended to use it at night, determining bearings off of the stars.

Two-Pole Chain

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Surveying chains of the 19th century were comprised of iron links approximately eight inches long. There were typically a hundred links in the chain, measuring out to 66 feet. Often, they were used to measure miles and acres, 80 for a mile and ten for an acre. Lewis and Clark used these chains to measure the width of rivers.


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As the locations of the sun and astral bodies are dependent upon the time, 19th century explorers often used a chronometer. These especially precise watches could be used to determine the exact time of the observation. They were typically set to Greenwich Time, the time at zero degrees longitude. Lewis and Clark utilized the timekeeper, but it tended to run a bit haphazardly – often speeding up, slowing down, or stopping altogether. So, they frequently had to use precise measurements of the sun to determine the exact time.

Nautical Almanacs

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These were commonly used by captains at sea. It was the compilation of relevant the data for the location of the sun, moon, stars, and planets at various times of the year. Lewis and Clark kept copies of the almanac close at hand. This way, they could account for the necessary adjustments made to the coordination of the map.

Back in 1804, knowing where you were was no simple feat. Not only did it take a plethora of gadgets, some small and some large, it also demanded the skill to yield them. Fortunately, it’s 2012 and not 1804. All we need to figure out our exact coordinates is a compact GPS enabled device.

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