Magnetic Declination

Page 6 of 6

Variation of the Compass Needle

Beginning about 8:30 a.m. (chronometer time), 1805 November 24, Lewis took three observations of the sun's altitude with his sextant and the artificial horizon. Simultaneous with Lewis's observations Clark, using the 6-inch-diameter circumferentor, took the magnetic bearing of the sun. Additionally, an assistant recorded the chronometer's time for each observation.

Clark, in his journal for November 24 recorded the three altitudes, bearings and times. He failed, however, to record whether Lewis observed the sun's upper limb, lower limb or its center, and Lewis left no data regarding this observation for Magnetic Declination. From the sun's altitude at the time of observation and evaluations made to determine Local Time for the Lunar Distance observations (see Local Time at Station Camp), the limb observed most likely was the upper. The sun's true altitude at the time of observation, however, was calculated for the upper limb, the center and lower limb to evaluate the differences.

After averaging the sun's altitude from the three observations, the sun's true altitude was calculated for the upper limb, center and lower limb using:

  1. a sextant's index error of +8°45' and,
  2. a longitude of 123°45' W to adjust the sun's semidiameter (for upper and lower limbs) from the 1805 Nautical Almanac.

The Local Apparent Time of the Magnetic Declination observation then was calculated from the sun's true altitude(s) using:

  1. the latitude of Station Camp recalculated from the Meridian Observation of November 16 and,
  2. a longitude of 123°4' W to adjust the sun's declination from the 1805 Nautical Almanac.
  3. Patterson's Problem 3, Form III (1803).

Table 7. Magnetic Declination

Assuming the Sun's Upper Limb was observed:

Recalculated latitude: 46˚15'47.3" N
True altitude of sun’s center 11˚13'02.4"
Sun’s declination at observation: 20˚35'25.6" S
Calculated Apparent Time of the observation: 8:55:32.2
Calculated azimuth of the sun: 136˚32'27"
Average bearing of the sun, S63˚E = azimuth of: 117º
Magnetic Declination from observation average: 019˚32'27" E =  19½˚ East

Assuming the Sun's Center was observed:

Calculated Apparent Time of the observation: 8:57:49.4
Calculated azimuth of the sun: 137˚00'51" =  20˚ East

Assuming the Sun's Lower Limb was observed:

Calculated Apparent Time of the observation: 9:00:07.8
Calculated azimuth of the sun: 137˚29'15" = 20½˚ East
Clark, November 24, 1805  (Moulton, volume, p. 84): 16˚ East
1805 calculated from captains’ observation, upper limb: 19½˚ East
19051 22½˚ East
19552 21¾˚ East
20053 17½º East

Clark's Map of Station Camp Area

To see labels, point to the map.

Clark's map of a large bay on the Columbia River

Although this map is labled "ca. November 16-25" in Moulton (6:52), Clark probably drew it sometime during the winter of 1806 from the field notes he made in November (see Moulton, Atlas, Maps 90-93). For one thing, the almost incessant rain and wind they endured from the time they left Station Camp until the captains moved into their quarters at Fort Clatsop on December 23 would have prevented Clark from preparing such a detailed chart, with its water tinted blue. Moreover, "Point William" (today, Tongue Point) did not acquire that name until November 27 (Moulton, 6:90). His "Point Lewis" is now Smith Point, at the west end of Astoria, Oregon, and slightly east of the mouth of Youngs Bay. "Point Distress" is now Point Ellice, Washington. The distance from Station Camp to Cape Disappointment would actually have been 6.9 miles, not 11; his 7-mile estimate of the distance to Point Adams would have been more accurately 3.8 miles.

 

Comparing the bearings shown on that map with bearings from true North derived from modern maps gives the following magnetic declinations for 1805:

Table 8. Magnetic Declinations Derived from Clark's Map

From To Declination
Station Camp Tip of Cape Disappointment 20˚  East
Station Camp Point Adams 2˚ East4
Station Camp Meriwethers Point 19½˚  East
Station Camp Point William 17¾˚ East
Average (disregarding Station Camp to Point Adams) 19˚ East
 

1. 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

2. National Oceanic and Atmospheric Administration, Declination calculator. http://www.ngdc.noaa.gov/seg/geomag/struts/calDeclination

3. Ibid.

4. At a magnetic declination of 19½° East, Clark should have read a bearing of about S21°W, not S41°W, from Station Camp to Point Adams.

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