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----- Original Message -----

From: Frank Reed

To: NavList@fer3.com

Sent: Tuesday, January 31, 2012 12:34 PM

Subject: [NavList] Re: Nocturnals

Sure, Keith, that makes better sense. What you wrote previously was, "the circumpolarity of Polaris was not know prior to Columbus's crossing." That didn't make sense. There had to be something missing. The idea that they didn't know exactly how to correct for its distance from the celestial pole in terms of actual latitude values is more believable.

Keith, in your earlier message you wrote:
"I have previously posted on the use of a nocturnal to find the time of high
water, given the establishment of the port."

This, I think, should be understood as very much a secondary function. It scarcely involves the tides, and you could easily do it without a nocturnal. I've thought of something that might work as a modern analogy. Back in the 1970s, digital watches and electronic calculators became popular. Then some "genius" realized that these two functions could be combined into one item of high fashion: a digital watch with a built-in four-function calculator. In a few centuries, some historians will discuss the purpose of the digital watch. Does it tell time? Or does it perform basic mathematical functions and also tell time? The "Moon scale" (can anyone tell me: is there a better, preferred name for it?) on a nocturnal doesn't predict tides. It performs a simple math calculation that would sometimes be useful in tide calculations. It was a convenience to add that and other scales to these instruments, analogous to calculator functions on a digital watch, and additional scales also apparently made nocturnals more valuable as "objets d'art". But at the most basic level, the nocturnal tells local time.

The rough outline of the complete calculation of the time of high tide with the scale on a nocturnal goes something like this: 1) get the age of the Moon (number of days since New Moon) from some source like a common almanac, nothing to do with a nocturnal, 2) multiply that number of days by 4/5 (to convert from a range of 0-30 to a range of 0-24) and if it's greater than 12, subtract 12 (converting from a 24-hour clock), 3) add some number of hours (often called the "establishment of the port") specific to the location in question, also nothing to do with a nocturnal. Steps 1 and 2 simply provide an approximation for the time of lunar transit which could also be found by simple observation. Step 2, which is the only part where the scale on a nocturnal comes in handy, converts days since New Moon to the number of hours after noon when the Moon will transit. It's simple enough: if it's 15 days since New Moon, then the Moon transits 12 hours after the Sun transits (12 hours after noon, so in other words, midnight). Proportioning a number by 80% was once difficult. It's a useful scale, but that's all it does.

-FER

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