Several methods of determining magnetic declination by direct observation, however, were:
- 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.
- 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.
- 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.
- 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 15½°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 1805||Magnetic Declination|
|9 (sun, am)||16˚55'07" East2|
|9 (sun, pm)||17˚06'12" East|
|9 (Polaris)||17˚11'26" East|
|average||17˚04'15" East (17˚ East as their compass could be read only to the nearest ½˚.)|
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