Iron men and wooden ships

This pretty picture, from today’s APOD, shows Venus in proximity to the December moon.

Even after the invention of the chronometer, mariners continued to use the competing method of lunar distance to determine longitude. They measured the angular distance between a bright star or planet and the closest limb of the moon to recover standard time (GMT). During the Napoleonic wars, Royal Navy ships would exchange their estimates of longitude and how they were determined. A Venus lunar was particularly valued as a cross check on their chronometers. “Scientific” Captains of that time (c. 1800) also determined time by observing occulations of Jupiter’s moons. The U.S. Navy dropped the lunar method in the early 1900s by removing the how-to from the 1911(?) edition of The American Practical Navigator.

Those were the days. (Particularly if you didn’t have to to the calculations yourself.)







7 thoughts on “Iron men and wooden ships

  1. Those were the days, indeed.  It’s easy to forget nowadays, where $300 will buy you a GPS device that will determine your location to an accuracy of a yard, how ephemeral (literally- “ephemerides” are tables of positions of heavenly bodies) and complex the reckoning of position used to be.

    Occultations were of course valuable to determine exact time, because they didn’t depend upon reckoning of angles (not always easy from a heaving ship’s deck) and specified a pretty precise point in time.  And while occultations of Jupiter’s moons occur every couple of hours, they are of course not as easily observed as an occultation of Venus, which can easily be seen with the naked eye, even with mild fog.

    I hadn’t known about about the lunar reckoning method-  the story of the battle between lunar observation and the chronometer is fascinating, and yet another example of our gradual substitution of technological gadgets (clocks in this case) for observations of nature (measuring angle of moon to some other body).  Thanks, VernR.

  2. I proudly serve in the USN, and I know from a sailor’s point of view how many traditions have been lost. Its sad really, our only mention in training of navigation is a slight mention of various methods in the Blue Jacket’s Manual.

  3. I proudly serve in the USN

    As I did tome time ago.

    A good part of my knowledge about the origins of some of our tradition comes from Patrick O’brian’s opus. Here are two more that relate to navigation.

    In some sailing ships, speed was measured by casting the log from the stern. The log was relatively immobile, and attached by line to a reel. Knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor’s fingers, while another used a 28 second sandglass to time the operation. The knot count would be reported and used in the sailing master’s dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1851.66 m/h. The difference from the accepted value today is a bit less than 0.02%.

    (I had to refresh my memory on the details at ) The Midshipman of the watch would report ships speed to the Captain in knots and fathoms. “…If you please, sir.”

    The quarterdeck watch used a 30 minute glass to time the bell cycle that regulated the watches. The watch would mark each turn of the watch by ringing the ship’s bell, adding one stroke to the previous count. The strokes were paired to make it easier to count half hours. After observing local apparent noon, the Officer of the Deck would seek permission to strike eight bells on time. The ritual completed with the Captain’s reply “make it so.” That reply set the afternoon watch marking the start of the Naval day. I’m not current but the formalitity of making noon so is/was still observed. (The practice of tapping the glass to make the sand run out a little faster was known as flogging the glass. I suspect one didn’t do that when the Captain was on deck.)

    Our sailors who were captured in the Phillipines at the start of WWII, found some scrap metal in their prison camp and were able to maintain the bell cycle throughout their captivity.

  4. I used to do a fair amount of sailing in SF Bay, but the most difficult navigation involved was when it got too dark to see land, and we had to set a course for the big Safeway sign by the marina…

  5. Before computers I could calculate 6 sun observations on my ti-95 procalc with the hour-angle method (longhand no program)to generate a compass bearing in 20 minutes.  I shudder to think of how long it would take witha slide rule or worse a book of tables.

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