Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Attempting to decipher a "time sight"
    From: Frank Reed
    Date: 2016 May 28, 13:57 -0700

    A few thoughts:
    Zone time was never used. Although time zones on land originate in the 1880s, I doubt you'll find them in use at sea with any frequency until a few decades later. They're pointless anyway! Time zones at sea became useful for organizing work schedules, but they're really just a nuisance when it comes to celestial navigation. In the 19th century, even in the 1890s, you could expect local solar time from the most recent noon would be the normal time kept aboard ship when more than a day out of port.

    Sea days / nautical days were over and done with by the 1890s. On the other hand, you can't assume that you know the exact date from the marginal date. It's always possible in an informal document like this that the date has some other meaning, or that it was written down for one block of numbers on the page but not the block that appears closest to it. You can probably safely assume that it's within a day of the date of the observations. I find it's usually best to try to determine the date from the Sun's declination as used. For dates within a month of the solstices, including this page from December 1896, it's somewhat difficult to discriminate the date from the declination, but not impossible. In other months of the year, the Sun's declination change approaches 1 knot --one minute of arc per hour-- which allows solid confirmation of the date and approximate GMT. In an earlier era, any lunar observations would provide an excellent test of the date since the Moon moves so fast and since the lunar distances never repeat (except by chance, at an extremely low frequency).

    It's important to remember that you're looking at scrap paper. Not legal or formal record such as are found in a logbook, and not even homework problems where "showing your work" mattered as much in the 19th century as it does today. These navigators aren't writing their notes for you. They're writing for themselves, and if they understand what they've written, that's good enough. So when it comes to things like deciding whether a time given as, e.g., 11:00 means 11 am or 11 pm and whether it's local time or GMT, you have to investigate and consider how they worked elsewhere in the notebook. That's part of the puzzle. And it's always possible that the usage is inconsistent. It did not need to be consistent.

    These afternoon sights in the North Pacific in December have a possible source of confusion. The observed altitudes are around 20° and also the declination of the Sun is around 20°. So when you see a number marked down in that range, you should consider both possibilities. It might be an altitude. It might be a Dec.

    The altitude correction of 11 or 12 minutes of arc was simply standardized. This was normal practice in the era. Yes, they may have adjusted the value slightly based on local custom, an individual navigator's practice, or some other issue. Speculate as you like. But one other thing to notice: all of the latitudes are worked by subtracting the altitude from 89° 48'. This is what I call the "navigator's right angle" and it incorporates an assumed altitude correction of 12 minutes --always. Where does it come from? It's not hard to see. It's +16' for Sun SD, -3' for dip, and -1' for average refraction. If you're saying to yourself, "wait, wait, that's not the correct height of eye!", well then, you're thinking like a late 20th century navigator. You were raised to follow certain rules with some arbitrariness to them based on the requirements of typical 20th century expectations. But they were raised to follow different rules with different sorts of arbitrariness appropriate to the needs of typical 19th century expectations. In case you're curious, and you want to "fix" their computations, the actual height of eye from the deck of the Charles W. Morgan is about 14 feet in calm water with the vessel normally ballasted. I know this because I tossed a line over the side and measured it while sailing off Cape Cod.

    And again, bear in mind that you're seeing the work of several different people. It appears to have been recorded by one hand (there's no significant difference in handwriting), but as you follow along there are clear indications of work done by John Layton, who was master of the vessel on this voyage, Elizabeth Layton, his wife who helped out considerably especially as John fell ill, and the first mate (can't recall his name right now).

    Doug, in your original write-up you mentioned "haversines". They would not have called these haversines at this time nor would they have been aware of anything that deserved another name. Although from a later navigator's perspective, the calculated quantity was a haversine, this was not a quantity that required any name other than "time" or "hours" and that's how it was listed in the tables. There were two columns labeled A.M. and P.M. that corresponded to the angular entries in the standard table. That's why there's no conversion from hour angle (as an angle) to time (in hours). This was already part of the standard table.

    Time sights aboard the Charles W. Morgan (and many other vessels before and after) were commonly taken around 9:00 am and 3:00 pm. This was part of the daily pattern. Note that they did not obsess over waiting for the Sun to be exactly due east or west as later histories often imply. They used a latitude updated by DR in the calculations, and that was all that was necessary.

    Frank Reed
    Conanicut Island USA

    PS: Speaking of time-keeping and the start of the day (from noon in the old nautical day), President Putin of Russia is apparently right now visiting the autonomous Eastern Orthodox state in Greece known as "Mt. Athos". This is a place nearly as independent as the Vatican and much larger in area, but less famous since there's no single individual like the Pope there and no pretense of a role in international politics. For us, its unique feature is an ancient time-keeping standard. At many of the monasteries of Mt. Athos, the day begins at sunset, and the hours are counted forward from then. This is sometimes known as "byzantine time". It is identical in practice to "Islamic time". Both are derived from the same historical tradition, and of course this is a system which also matches traditional Jewish time-keeping where the Sabbath, for example, begins at sunset. This is a special case of the day beginning at sunset. And just to be clear, this is really the start of a new day at sunset: Friday becomes Saturday at sunset, the calendar date today flips from the 15th to the 16th at sunset (because they still use the Julian calendar, it's May 15 rolling over to May 16, not May 28 to May 29!).

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site