Photograph as originally submitted to this page in the Historical Marker Database www.HMdb.org. Click on photo to resize in browser. Scroll down to see metadata.
Marker detail: Sextant & Octant

Caption: Marker detail: Sextant & Octant
Additional Description: Latitude and Longitude —
Latitude indicates a location in degrees north or south of the equator (0). Longitude is a measure of location in degrees east or west of the Prime Meridian at Greenwich, England (0). When both the latitude and longitude of a place are known, that place can be located precisely on a map.

Sextant (and octant) —
The captains used these precision instruments to determine the angle between two celestial objects such as the moon and the sun or a star. They also used them to measure the angle between the sun and its reflection from a tray of water or mirror because there is no true horizon on land from which to measure its altitude. The measure of the sun's altitude at noon, used in conjunction with tables from their Nautical Almanac, was the simplest and most reliable way of determining latitude.

Mechanics of the Sextant and Octant
The heart of both instruments was their two-mirror system. One mirror (horizon glass) was opposite the eyepiece; it was silvered only on the side nearest the body of the instrument, the other half was clear glass. The other mirror (index glass) was on the instrument's moveable (index) arm. With his eye to the eyepiece, the captain sighted an object such as a star through the clear part of the horizon glass. Next he moved the index arm until the image from another object - say the edge of the moon - was reflected from the index mirror to the mirror part of the horizon glass that was in direct contact with the other object being sighted through the clear glass. The angle between the two objects was read from the instrument's graduated arc and recorded.
Submitted: January 3, 2019, by Cosmos Mariner of Cape Canaveral, Florida.
Database Locator Identification Number: p459318
File Size: 0.621 Megabytes


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