The year 1803 provided the conservative American president Thomas Jefferson with an opportunity that even he could not pass up. Realizing the critical importance of the port city of New Orleans to the young country, he offered to buy it from France for $9.375 million. When emperor Napoleon Bonaparte counter-offered the entire Louisiana territory for $15 million, he quickly set aside his campaign promise to eliminate the Revolutionary War debt and agreed. England promptly asserted its claim to the upper reaches of the Missouri River, claiming that France had just conveyed more land than it actually owned.
Following the American Revolution, European trading interests sought to circumvent the fledgling United States. England still claimed title to vast regions of North America, and still believed that a faster trade route to the Pacific Ocean existed by way of crossing the continent. By July of 1793, Scottish fur trader Alexander Mackenzie had finally found such a route after more than a decade of exploration. His 1801 published account Voyages from Montreal...to the Frozen and Pacific Oceans in the Years 1789 and 1793 was read with alarm by the American President.
The mission of the Lewis and Clark expedition was to explore the recent Louisiana Purchase by "ascending the Missouri River, crossing the 'highlands' and following the best water communication to the Pacific Ocean." Meriwether Lewis, the personal secretary to President Jefferson, spent two years planning the expedition. He personally contacted his former army superior, Lieutenant William Clark, an experienced outdoorsman, surveyor and mapmaker, offering him joint command of the expedition.
The "Corps of Discovery" shoved off on May 14, 1804 from Wood River, Illinois, after taking a reading of 38 deg. 55 min. 19.6 sec. North latitude. The party consisting of 45 men traveled primarily by keelboat and canoe and made about 10 to 20 miles per day. On July 31, 1804, they took a meridian altitude and calculated the Latitude of their position to be 41 deg. 18 min. 1.5 sec. North. Latitude was much easier for them to measure than Longitude as they had no accurate timepiece for determining Longitude. However, they were able to estimate the distance traveled each day with considerable accuracy, and, combined with compass and latitude, they were able to record quite good positional information.
On January 5, 1805, from 12:57:54 to 02:39:10 AM (at their winter quarters near present-day Mandan, North Dakota) there was a predicted eclipse of the moon. This allowed Clark to reset his timepiece and subsequently measure precise Longitude. On May 20, 1805, Lewis and Clark measured the Latitude of the mouth of the Muscleshell River to be 47 deg. 00 min. 24.6 sec. North. In reality, it is 47 deg. 30 min. North. Their observations were consistently about 1/2 degree too far south.
"August 28th, 1805: Having now crossed more than once the country which separates the head waters of the Missouri from those of the Columbia, we can designate the easiest and most expeditious route for a portage: it is as follows. From the forks of the Jefferson River N60deg.W, five miles to the point of a hill on the right: then S80deg.W, ten miles to a spot where the creek is ten yards wide, and the highlands approach within 200 yards; Southwest five miles to a narrow part of the bottom; then turning S70deg.W, two miles to a creek on the right: thence S80deg.W, three miles to a rocky point opposite to a thicket of pines on the left: from that place West three miles to the gap where is the fountain of the Missouri; on leaving this fountain S80deg.W, six miles across the dividing ridge, to a run from the right passing several small streams; N80deg.W, four miles over hilly ground to the East fork of Lewis' (Lemhi) River, which is here forty yards wide."
From December 7, 1805 to March 23, 1806 the party resided at a wet Fort Clatsop, Oregon. They had hoped to return by ship, however no ships were sighted, so they began their overland return. They arrived in St. Louis, Missouri, on September 23, 1806, over two years and four months after they were last heard from, to the surprise of everyone believing the party had perished.