Mapping Then and Now

Determining Location in 1803–1806

Knowing their coordinates helped the team calculate distances traveled each day and provided a geographic framework for their maps. It allowed them to track their progress and to create a record of what resources (plants, animals, minerals, etc.) existed in the areas they explored.

Determining latitude was far easier than determining longitude, at least in the northern hemisphere. One could measure the angle between the horizon and Polaris (the North Star) to determine one's latitude. For example, if you were at the North Pole, Polaris would be directly overhead, an angle of 90 degrees from the horizon. If you were in Hawaii, Polaris would be fairly low above the horizon, at an angle of about 20 degrees.

Another method for determining latitude involved the use of an astronomical almanac, which contained tables listing the position of the sun, moon, and particular stars on each day. These almanacs included the latitudes of key locations such as the Royal Observatory in Greenwich, England. From the angles and latitude of a known location, one's own latitude could be estimated.

Longitude was more difficult to measure, requiring more observations and calculations (when using astronomical measurements) or an extremely accurate time piece and daily observations of the sun's zenith (its highest point in the sky on a given day). Lewis and Clark had invested in a good chronometer and could use this second, less tedious method to calculate longitude.

1 day’s rotation of earth = 360 degrees = 24 hours 15 degrees = 1 hour 1 degree = 1/15th hour = 4 minutes Measuring the difference in time between local noon and Greenwich noon made it possible to determine longitude. If local noon was 6 hours after Greenwich noon, the team was 90 degrees (6* 15) west of Greenwich.

Because the earth rotates once per day, any given point on the earth's surface travels through a circle of 360 degrees once every 24 hours. Thus, each hour, a given point travels through 15 degrees of longitude (360/24). To determine one's longitude, one compares the time of local noon (when the sun is at its highest point) to the time of noon at a place with a known longitude. The difference in time is converted to the difference in longitude with a fairly simple calculation. For example, if one hour is equal to 15 degrees, then 6 hours would equal 90 degrees, and 6 hours and 30 minutes would be 97.5 degrees.