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    Re: CN & global ocean currents
    From: Frank Reed
    Date: 2019 Oct 31, 09:28 -0700

    Igor S., you wrote:
    "Suppose that during the ocean crossing celestial fix systematically deviates from the dead reckoning (DR) position in more or less the same direction. The following conclusion can be drawn - the existence of a global ocean currents [...] The earlier navigators were the first who might have noticed that. I could not find a clear reference on this account. Perhaps someone knows more details about it."

    First, it's interesting to note that the Gulf Stream, one of the most important ocean currents, was discovered and described well before high-quality celestial navigation became available. There's a good account of Ben Franklin's work and his dependence on New England whalers here: https://www.smithsonianmag.com/smart-news/benjamin-franklin-was-first-chart-gulf-stream-180963066/. Franklin's informant, Thomas Folger, was from a great Nantucket family of Quaker whalers (another of the extended family, Mayhew Folger, famously discovered the last surviving Bounty mutineer and the descendants of the others on Pitcairn in 1808). As described in the article, mariners with knowledge of the Gulf Stream would ride it when sailing north and east, while mariners with less knowledge (or skepticism towards American fishermen) would sail against heading it west and south. If your vessel's speed through the water is 6 knots, and you're fighting a 4 knot current, you will be late for dinner! 

    The general concept of comparing the celestial position and the dead reckoning position was well-known, and it's not unusual to see notations in 19th century logbooks about being "ahead of" or "behind" the reckoning. That is, the dead reckoning says you're 500 nautical miles out, but your celestial fix says you're 600 nautical miles out. And these notations are often followed by a comment like "we must be caught in a current". Sometimes you'll also see notations about double-checking the log, line, and hourglasses since the DR is dependent on those tools. They kept these notes primarily to gain commercial advantage in future voyages. Eventually in the middle of the 19th century, this information began to be collected by government agencies and authorities and eventually led to twentieth-century pilot charts. The pilot charts provide general seasonal advice on average conditions. Today, we have near real-time data on the extent and speed of the currents.

    We know today that the great ocean currents are accompanied by huge wheeling eddies that peel off from the main current. During the various Bermuda Races (Newport-Bermuda this year starts on June 19, if I remember correctly), competitors often get detailed updates on weather and currents from the best available satellite data, and they know exactly where they have to sail to get maximum advantage from the correct side of each eddy. Since the Gulf Stream runs generally northeast in this area, the eddies north of the stream rotate counter-clockwise. Those south of the stream rotate clockwise. So if you're south of the stream heading toward Bermuda, you want to try to ride the east side of a clockwise spinning eddy.

    Much of the ocean has insignificant currents. The major ones are near the equator and the family of currents peeling off the east sides of the continents, similar to the Gulf Stream. The latter set includes the Kuroshio off Japan, the Brazil Current, the Agulhas, and the East Australian Current or EAC ("you're ridin' it, dude!").

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

    PS: Modern Celestial Navigation this weekend at Mystic Seaport. Not too late to register!

       
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