Magnetic Declination

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Observations for Magnetic Declination or Variation of the Compass

On October 6 the captains made two observations of the sun for magnetic declination (also called variation of the compass). The equipment used were the circumferentor (surveying compass), sextant, artificial horizon, spirit level and chronometer. These observations usually were taken early in the morning or late in the afternoon when the sun was low and could more easily be signted in with the sight vanes of the surveying compass. The sun, however, had to be high enough so that refraction did not produce exaggrated readings for the sextant.

Lewis places the artificial horizon in a convenient position. He takes the sextant from its case and takes some preliminary sightings. Meanwhile, one of the men firmly sets or drives a staff or straight, sturdy branch into the ground in a near-vertical position. Clark takes the surveying compass from its box and places the ball joint on its underside over the stick or staff and secures it with a clamping screw. Next he takes the spirit level and sets it on the upper glass plate of the surveying compass. He loosens the ball-and-socket joint just enough so that he can accurately level the compass. When Clark is sure that the surveying compass is level, he loosens the clamping screw and rotates the compass toward the sun, alighing its sight vanes as near as he can with the sun's center. The chronometer also has been taken out of its case and one of the men stands ready to record the time, sun's bearing, and its altitude. Lewis now is ready and, adjusting the sextant's index arm, matches the sun's reflection from the artificial horizon with the sun's reflection from the sextant's index mirror to the mirror on the horizon glass and into his eye. Clark, all the while, has been "tracking" the sun. When Lewis has an exact match of images, he calls "Now!" The man at the chronometer reads the time at that instant, Clark reads the magnetic bearing shown by the compass needle, and Lewis reads the sun's altitude from the sextant. When this process has been repeated one or more times, the observation is complete.

For the first magnetic observation taken at Clearwater Canoe Camp, Lewis "shot" the sun's upper limb. He did this simply to reduce the effect of refraction. The second observation for Magnetic Declination was part of the Equal Altitude observation that followed the first observation for Magnetic Declination. At this time Lewis "shot" the sun's center.

table of navigational data

The declinations derived should have been closer to each other, but the surveying compass could not be read to closer than ± 1/2°, and an average of 18-1/2° East would be close enough for any hand-held compass.

Funded in part by the Idaho Governor's Lewis and Clark Trail Committee