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    Re: Time-sight time conventions
    From: Frank Reed
    Date: 2018 May 22, 10:35 -0400


    Very likely you saw that Admiralty Rule from 1805 in a post from Don Seltzer, while digging around in the archives. Don has posted about this more than once.

    Just bear in mind that this is a decree that applied only in a narrow community -- the British Royal Navy. The "Sea Day" (or as it's known in later historiography, the "nautical day") was a matter of custom and remained in wide use for decades. And you won't learn the limits of this usage from history books. The history of navigation is not found in history books. The history of navigation, like all good history, is found in primary source evidence, the logbooks and notebooks and letters and articles and other contemporary resources. And it's a "messy" history, because this was not a culture governed by decree and statute. In a community that is governed by laws, like the RN, a historian can find an excellent starting point by reading the list of rules and laws. But even in a culture like the RN, it's important to remember that the master of a vessel was a law of his own. Local rules over-rule.

    The fact that you missed out on the "Astronomical Day" after all this time is surprising. I discuss all of this in the workshops you've attended at Mystic Seaport. Of course you're usually multi-tasking in those workshops --getting the most of your time by working on some calculator analysis while you're following the class, too. You get a lot done that way! It's important to note that the Astronomical Day, starting at noon, and the Sea Day, also starting at noon, were not the same. They each overlapped the Civil Day by twelve hours, but they were distinct from each other by one full day. In real navigation, this was never a matter of any real confusion, at least not that I have been able to detect. The reason is simple: if you get it right on day one of any voyage, then you're set straight through the end, especially with respect to the difference between the Astronomical Day and the Civil Day. I don't think practicing navigators even had a name for this. They simply "knew" what to do to get the work right. The Sea Day was a little more problematic (and wisely abandoned as voyages became shorter) since a vessel arriving for a port stay had to switch from Sea Day to Civil Day and back and would then have days of 36 hours or 12 hours duration.

    We still use these sorts of overnight "days" without even thinking about it, especially in weather forecasts. In any common (US) weather forecast, the day ends at the following sunrise. When I look up tomorrow's official US National Weather Service forecast, I find:
     Wednesday: Patchy fog before 10am. Otherwise, cloudy through mid morning, then gradual clearing, with a high near 66. West wind 5 to 8 mph.
     Wednesday Night: Partly cloudy, with a low around 54. Southwest wind around 7 mph becoming north after midnight.
    Notice that "Wednesday Night" runs through the early morning hours after midnight. This is completely normal, and we're all used to it, and it absolutely contradicts the standard definition of the Civil Day. In US meteorology, the day begins at sunrise. Also notice that this is specific to local culture. I understand that in UK meteorology, it is common to use a Meteorological Day that begins at 9:00am standard zone time. And also note that these day definitions impact data. If I have a long-running dataset of high and low daily temperatures, the low temperature, in particular, is affected by the definition of the day.

    When you're doing a "desk" analysis of some old navigation papers, you may be off by a day in these things and that might worry you at first, but you can count on the historical navigator doing it right. Trust the recorded almanac data as your guide. It will almost always distinguish one day from the next. In the time sight from 1864 that I posted a few weeks ago at the beginning of this latest discussion, the navigator had recorded a Declination that told us not only the exact date, but also informed us that the navigator was using the Declination tables for Greenwich Apparent Time rather than Greenwich Mean Time. The difference is only a dozen seconds of arc. It's of no consquence to navigation, but it tells us something about the culture of navigation.

    Frank Reed

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