Magnetic Declination

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Rain showers that fell during the evening of July 28 brought a refreshing coolness to the Three Forks valley and cleared the air of dust and haze. By the morning of July 29 the storm clouds had moved east, revealing a fair sky. Taking advantage of this situation, Lewis, early that morning, took two observations of the sun's altitude with sextant and artificial horizon. Simultaneously, another observer (possibly Clark) took the sun's magnetic bearing with the 6-inch-diameter surveying compass (circumferentor), and another member of the expedition recorded the times of the observations as shown by the chronometer. These observations were followed, less than ten minutes later, by the AM set of Equal Altitudes. At noon Lewis took an observation of the sun's altitude for latitude and three and a half hours later, he took the PM set of Equal Altitudes. Less that fifteen minutes after that set of observations was complete he began the first of two separate observations of the Lunar Distance from the sun. About twenty minutes after completing the second set of observations of the Lunar Distance from the sun, the captains took another set of two observations of the magnetic bearing of the sun, its altitude and the time. Later that night, after measuring the Lunar distance from Antares, the captains also took one observation of the magnetic bearing of Polaris with the circumferentor and recorded the time of the observation.

In the early 1800s, at the latitude of the Three Forks of the Missouri, Polaris circled about 2½° from the earth's rotational pole. Even today, though much closer to the rotational pole, Polaris is due north only twice in about 24 hours. The times when Polaris bears true north are called the upper and lower culmination. A common method of making an observation to determine the local magnetic declination by using Polaris was to find the time of upper or lower culmination of Polaris from special tables (such as Tables Requisite), correct that time for your estimated longitude (estimated Greenwich time) and make the observation at one of the times when Polaris was due north. Lewis and Clark never used this method. Instead, they took the bearing of Polaris with the circumferentor and noted the time of the observation by chronometer, often taking two or three observations several minutes or more apart. The true bearing of Polaris then had to be calculated.

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Table IV, Magnetic Declination at Three Forks Camp
Date With Calculated Azimuth Observed Azimuth Magnetic Declination
(nearest ¼˚)
1805 July 29 Sun Obs 1 100˚58' 085˚ 16˚ East
1805 July 29 Sun Obs 2 102˚02' 086˚ 16˚ East
1805 July 29 Polaris 357˚42' (-2˚18') 013˚ 15½˚ East1
1805 Average       16˚ East
19052       20˚ East
19553       18½˚ East
20054       13¾˚ East

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

  • 1. It would have been much more difficult to align the compass with Polaris at 45° altitude than the sun at 37° in the morning and 27° to 28° in the afternoon.
  • 2. U.S. Coast and Geodetic Survey, 1905, Lines of equal magnetic declination and of equal annual change in the United States for 1905, scale 1:7,000,000
  • 3. National Oceanic and Atmospheric Administration, Declination calculator.
  • 4. ibid.