Variation of the Needle

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Taking observations for the magnetic declination of the compass was one of the less critical tasks that Jefferson ordained for the captains. The purpose of these observations was to allow the captains' river and route survey – made up of the "Courses and distances" recorded daily – to be plotted with respect to True North. There were several other means of fitting the captains' survey to True North besides taking timed observations of Polaris or the sun. If the Expedition's longitudes had been calculated, the most reliable of these other methods would have been to match the river survey between points of known latitude and longitude. This could have been done simply by mathematically enlarging or reducing the plotted survey to fit between the two end points of known latitude and longitude. The amount of rotation needed to make the replotted river survey fit the known end points would have given the average magnetic declination between the two points.

Several methods of determining magnetic declination by direct observation, however, were:

1. In far northern latitudes, especially in winter, take a compass reading on the sun at (or within a few moments of) the time when an observation of the sun's meridian altitude is taken. This gives the magnetic bearing of True South.

2. Take the bearing of Polaris at the time of its upper or lower culmination; at these two times Polaris is Due North. To use this method you need to have a good idea of your latitude and longitude and, from the longitude, the time when these culminations occur in order to make the observation at the correct time.

3. Take the bearing of Polaris and note the time by chronometer. Obtain the correct time of the observation by taking Equal Altitudes observations of the sun. From the true time and latitude, the true bearing of Polaris can be calculated and compared with that observed by magnetic compass.

4. Take the bearing and altitude of the sun simultaneously and note the time by chronometer. Obtain the correct time of the observation by taking Equal Altitudes observations of the sun. From the true time and latitude, the true bearing of the sun can be calculated and compared with that observed by magnetic compass.Taking the sun's altitude provides a means to check the calculations by other mathematics.

Although Clark wrote that the variation of the needle at the mouth of Marias River was 15Ω°E, this was the raw observed bearing, not corrected for Polaris's angular distance from the true pole.

The captains never made any calculations to determine the magnetic declination.1 Nevertheless, their observations, combined with those taken by the Canadian mapper-surveyor-trader David Thompson and magnetic declinations calculated by navigators along the west coast in the late 1700s and early 1800s, provide valuable data to help understand the changes that have occurred to the earth's magnetic field in this region over the past two hundred years.

Date, June 1805Magnetic Declination
9 (Sun, a.m.) 16°55'07" east2
9 (Sun, p.m.) 17°06'12" east
8 (Polaris) 17°11'26" east = 17° east, since their compass could
       be read only to the nearest Ω°.
19053 21° east
19554 18° east
20054 14° east

1. At only two locations during the entire expedition was a "variation of the needle" shown. Both were by Clark – at the mouth of the Marias River, and at Station Camp. Lewis often left a space for that information in the summaries of his observations, but never put any values in those blanks.

2. Average of two observations. The second observation was given as 69°29'45"; it must be 60°29'45".

3. U.S. Coast and Geodetic Survey.

4. National Oceanic and Atmospheric Administration.

Funded in part by a grant from the National Park Service, Challenge-Cost Share Program.