Using a Sextant.

• a sextant, which cost $77
• an octant
• an artificial horizon
• a surveying compass
• and a scientific pocket watch called a "Gold Chronometer." The watch itself cost $250, a large sum at the time.

So how do you find the latitude of a given point on the earth's surface? Lines of latitude are imaginary circles that start at the equator which is known as 0-degrees of latitude. Imagine that there's a line from the equator at the surface to the center of the earth. Then imagine a line from the center of the earth to the point on the surface where you are standing, say in present-day Nebraska. The angle from 0-degrees to your position is the latitude. So how would Lewis and Clark measure their latitude? They used a sextant and their clock to measure the position of the sun at noon. At noon, they lined up the horizon by looking through the telescope on their sextant, through a half-silvered mirror and through the horizon glass. Then they moved the index mirror on top by moving the micrometer drum until they could see the Sun superimposed on the horizon. Then they read off the angle of the sun on the scale. Using that angle, they looked up the current date in a special reference book — the Nautical Almanac of 1765 — that that gave them the degrees of latitude north of the equator. An octant is the same sort of instrument as a sextant with a larger scale — one-eighth of a circle instead of one-sixteenth. The octant was used during the summer months when the Sun was much higher in the sky.

There was one complication, of course. Lewis and Clark had to find the true horizon, which is easy to do on the sea but almost impossible on land surrounded by trees and hills and mountains. To solve the problem, they used a simple tool called the "Artificial Horizon" that contained a pool of liquid mercury or water covered by a sheet of glass. Since liquid will lie perfectly flat they could use the surface to find the true horizon. This was simple, but still high tech for the time.

Finding the longitude of a given point was a little more difficult. Imagine that there is a plane that is inserted into the earth that runs from the north pole to the south pole. Arbitrarily, navigators had to choose a starting point for their measurements and they chose a line going through Greenwich, England. As you move west, lines of longitude are measured in degrees off of the prime meridian. The 100-degree meridian is located near present-day Cozad.

When Lewis and Clark set off, none of the map coordinates in the Louisiana Purchase had been charted. They measured their longitude positions using a variety of techniques. One involved using the sextant again to find the exact local time and comparing that with Greenwich Mean Time. The second technique involved measuring the angles between the moon and specific stars in the night sky. The stars and moon rotate in the sky and navigators had produced elaborate charts that told the longitude of a given point based on the moon-star angles at specific times on specific dates.

Once they knew their longitude and latitude, Lewis and Clark used their compass and the techniques of dead reckoning to determine the distances between objects on land.

All of these instruments and techniques allowed Lewis and Clark to establish the course of the Missouri River and their route to the Pacific.

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