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Marker detail: Circumferentor & Pocket Chronometer

Caption: Marker detail: Circumferentor & Pocket Chronometer
Additional Description: Circumferentor —
Also called a surveyor's compass. Usually mounted on a staff, a distant feature is sighted in by looking through the slotted metal piece on one side and lining the feature up with the thin wire on the opposite side. Used to measure the direction the expedition traveled and to measure what direction a place or feature was relative to their position. The circumferentor's needle points to magnetic north, not to true north. A distinction between magnetic north and true north was essential so that the rivers and their route could be shown correctly on their final maps.

Pocket Chronometer —
This precision timepiece was used to determine the correct time of a celestial observation, usually made with the sextant. The chronometer's time was checked by special sun observations that gave the true time that the chronometer should show at that locality. The corrected time of the sextant observations, when compared to the time at Greenwich, England, could be used, among other things, to determine longitude and magnetic declination.
Submitted: January 3, 2019, by Cosmos Mariner of Cape Canaveral, Florida.
Database Locator Identification Number: p459321
File Size: 0.724 Megabytes

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