Understanding by Degrees: Determining Latitude (Part 1 of 3)

Diagram of Marine Sextant courtesy of English Wikipedia.
Diagram of Marine Sextant courtesy of English Wikipedia.

Arctic explorers in the 19th century routinely used a sextant for celestial navigation to determine their location at sea and when trekking across glaciers and other terrain on foot or by dog sledge. This first of three posts discusses basic principles of obtaining solar-noon latitude by sextant and inherent errors often present when attempting to sight the low-angled Arctic sun. Latitude findings enabled explorers to determine and document how far north they had traveled, and at times, to document historical most northern points explored in the High Arctic.

The Greeks developed the concept of latitude and longitude grids that systematically divided the earth into a system of horizontal lines of latitude (going east to west) that originated at the equator. One degree of latitude equals ~70 miles and is constant from the equator to the geographic North and South Poles. The equator represents zero degrees latitude.

The primary instrument used in celestial navigation, the sextant, refers to the Latin word for one-sixth of a circle or 60 degrees. It is designed to measure the relative angle between two items, typically the horizon and the sun, moon, and stars. In addition ascertaining the angle of the sun, an accurate celestial sighting required eye-height above the ocean and exact time.

Hovering over the southern horizon, the Arctic summer sun was the explorer’s constant companion. After the June solstice, from the Arctic Circle (N66º 34’ 44”) to the geographic North Pole, the sun provides 24 hours of light (ergo the “midnight sun”). For this reason, explorers relied upon sun sightings to ascertain latitude. At high latitudes, the sun can often appear as a hazy, opalescent disc rather than a distinct point of light. While this procedure appears straightforward, the High Arctic sun appears to circle (in a horizontal ellipse) over the southern horizon, making exact noon sightings taken under hazy skies often problematic.

The ship’s astronomer (or other trained crew) typically made daily noon-sun sightings and computations to confirm location, especially for off-ship explorations. Out of maritime tradition and practice, documenting explored locations was a routine requirement. Noteworthy events and locations visited were typically verified by accompanying team members (assistants) and officially recorded in ship logbooks and/or in personal diaries, providing a written record of daily crew activities, particularly meteorological and other scientific observations. During 19th century Arctic explorations, nothing of consequence aboard or off-ship was performed in isolation, aside from routine assigned duties, such as melting snow for water or stoking fires.

In general practice, instructions using a sextant have varied little over the past 150 years and are described in basic terms. To properly sight the sun, the sextant should be held vertically. A basic sighting tube is preferred since the sun is a relatively larger disc than the point of light of a star or planet.  With the index arm set at zero degrees, the horizon is sighted using the half-horizon mirror. Next, the index arm is carefully moved forward so that sun is visualized. Once the lower limb (or “edge”) of the sun touches the horizon, the degrees of latitude can be read at the graduated arc. At this point, the index arm should be clamped in place and the sun’s position minutely adjusted as needed with the micrometer or vernier dial. The window in the index arm provides the height of the sun above the horizon measured in degrees.

The eye-height of the observer (and thus the sextant) should be measured above sea level, to include the height of the ship’s deck. Typically, when standing on a flat ocean beach, light-of-sight is about 4.7 km (~3 miles) to water horizon. As observer height increases, so does the light-of-sight across a bay, sound, or the ocean. On modern sextants, several filters or shades reduce intense solar light to a visible disc. These filters were not present on 19th century sextants; thus, crew members who frequently made noon-day sun sightings potentially received repeated eye-blinding injuries. After obtaining the height of the sun (in degrees) at local solar noon, with the exact time (as possible), 19th century Arctic explorers used sight reduction tables found in American and/or British Almanacs for latitude (and longitude) calculations.

Prudent mariners and explorers of the era routinely verified calculated positions, especially those that appeared too far north or south of previously confirmed locations. Daily travels on foot or by boat were verified and tentatively confirmed by making calculations of distance, speed, and time. For example, computed distances of 70 miles (one degree of latitude) by foot or by dog sledge over broken sea ice and/or punishing ice hummocks would have been questioned by knowledgeable crew members and distances recalculated for accuracy.

Crossing The Hummocks, "The Open Polar Sea."
Crossing The Hummocks, “The Open Polar Sea.”

Sextants of the 19th century had inherent errors because of temperature extremes, mirror misalignment, and when other components, such as the spotting scope, may be potentially out of adjustment or collimation (optical-mechanical alignment). All of these potential errors can be further complicated by the extreme rigors of Arctic exploration, including the effects of cold and fatigue, while on dog sledges and/or trekking amongst tumultuous ice hummocks or unstable (“rotten”) ice. The environmental hazards experienced then are still arduous and potentially life-threatening to present-day Arctic explorers and mariners.

This first of three blogs was written for and originally posted on the New Bedford Whaling Museum “Arctic Visions” blog. I thank them for the opportunity to post blogs on their site. http://whalingmuseum-arcticvisions.org/blog/

 

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