POSTED BY FRAN HENNESSEY
In the previous post, I outlined the basic concepts for obtaining latitude by sextant sightings at local solar noon, and illustrated how masters and astronomers on 19th century Arctic-exploring ships determined their northern position whether on the open ocean or trekking across glaciers on foot or by dog sledge. When navigating the open ocean, 19th century masters on Arctic-exploring ships also needed to know their longitude, which requires use of both a sextant and an accurate chronometer. This second of three posts discusses basic procedures used to determine longitude.
One fundamental of navigation is understanding time, distance, and speed. They are mathematically interrelated, since the faster one travels, usually more distance is covered or less time is needed to reach a specific destination. On the featureless, often fathomless ocean, none of these components by themselves provides precise location. However when correlated from a specific point, time can be used to help determine location. For this reason, an accurate and practical-sized chronometer became an essential nautical instrument on many 19th century seafaring ships.
Vertical lines of longitude go north and south from the geographic North Pole to the South Pole. By convention, they originate at the Prime Meridian in Greenwich, England as zero degrees longitude. Thus, grid squares, consisting of degrees, minutes, and seconds, were projected onto a three-dimensional sphere (the earth) and in practical format, were converted to flat two-dimensional maps or nautical charts. However, as these longitudinal lines converge on the geographic poles, their distance apart progressively narrows. This near-pole narrowing of longitude was a significant sighting and computational factor 19th century Arctic explorers. Thus understanding was by degrees, minutes, and seconds.
For a 19th century nautical chronometer to operate properly, time was set to correspond exactly with that of the Royal Observatory in Greenwich, or what is known today as Greenwich Mean Time or GMT. Ideally, the chronometer should lose no more than a minute throughout the entire ocean voyage. The sun at noon in Greenwich is 1200 hours GMT, the standard reference for nautical chronometers.
Since the sun advances westward during the day 15 degrees per hour, the ship’s astronomer made sextant sightings at or very close to solar or 12 noon local time. By using daily computation and correction tables, the astronomer placed the ship’s position on a “celestial line of position” or LOP that extended north to south along the route of travel, for example, from Cape Race, Newfoundland to Cape Farewell, Greenland. Typically several sightings were used to calculate and reconfirm longitude (and latitude) at sea.
In addition to inherent errors in early chronometers and sextants, other factors potentially contributed to errors in ascertaining longitude. Human errors included sextant sighting inexperience or haste and mistakes in math calculations. Extrinsic factors included cloudy days and nights, rough seas, and sun-blocking mountainous terrain. Nevertheless, 19th century explorers ventured far north into Arctic ice-cluttered waters and with reasonable accuracy (typically one degree), could determine their position in uncharted waters and across barren terrain.
Frances Hennessey writes informally on 19th century Arctic exploration, concentrating on Isaac I. Hayes, MD. Fran has a small collection of Hayes first editions, two of which are signed by the author. She also tweets @openpolarsea and has a poetry blog: www.oceanic-visions.com