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    Re: Sight reduction method history
    From: Frank Reed
    Date: 2013 Nov 4, 09:21 -0800

    Lu, you wrote:
    "for example, did the masters of the Charles W. Morgan do celestial (if they did it)?"

    They did celestial from the beginning using the standard methods that you would have found on every vessel from the late 18th century through the end of the 19th century. It was latitude by Noon Sun and longitude by "time sights" (and also on the early voyages, occasional lunars). The "time sight" calculation was a straight-forward spherical triangle solution (though few navigators of the period would have known that). In modern intercept navigation, we simulate the sky from a nearby point, calculating an altitude from a DR or an AP to be compared with the corrected observed altitude. In traditional 19th century navigation, the corrected observed altitude was used directly in the computation to determine the Sun's local hour angle, in effect turning the sextant into a sundial.

    "Or the HMS Rose, late of Newport?"

    That I don't know.

    "Or the USS Constitution?"

    Don Seltzer did some research on this topic, and he may have more to say. He reported that they had chronometers from a relatively early date so lunars would have been phased out on those large USN vessels very early. Similarly, in the Royal Navy, it appears that lunars were more or less over and done with by 1820 while they continued in active, declining use aboard American merchant vessels until about 1850. Regardless, in this period, nearly every vessel would have found latitude by Noon Sun, occasionally by meridian altitudes of stars, occasionally by altitudes of Polaris (not necessarily meridian altitude) and with unknown (low) frequency by specialty methods like double altitudes and ex-meridian sights. And local time (a prerequisite for longitude) was invariably found by time sights, almost always of the Sun.

    "Or the steel clipper Balclutha in the National Maritime Park in San Francisco?"

    I exchanged a bunch of emails a couple of years ago with someone working on this very project. According to one logbook from the Balclutha, they were using lines of position from star sights for part of their navigation by this date (1890s... I can't recall the exact dates). Fixes by celestial LOPs did not become common until about this date and the pattern, as with the decline of lunars, appears to have been navies first (with the Royal Navy leading), British merchant vessels next, and American merchant vessels last. Navigators on many American merchant vessels were still doing the "Old Navigation" in the 1940s: latitude by noon Sun, longitude by time sight.

    The actual sight "reduction" or mathematical process was highly standardized. Nearly every case of time sight calculation that I have found in historical logbooks proceeds using the same mathematical recipe (see PS): a few angles are calculated (in particular, a "half sum", which is half the sum of the polar distance, latitude, and corrected altitude), and then four logarithms get looked up and summed, and finally the sum (or half of it) is sought in the tables in a reverse lookup and the time is extracted. This is "sundial time". After correcting for the equation of time, the resulting LMT gets compared directly with GMT yielding the difference in longitude. That's how it was done on the great majority of vessels at sea for over a century. Note that Sumner's Method, though famous in American histories of navigation, became popular much later than those histories usually suggest. Few navigators found any benefit from Sumner lines until about 1900, and even then the name was really just a shorthand (or misnomer) for any celestial line of position. True Sumner lines were really "double time sights" but the intercept method was generally a much shorter calculation and yielded the same lines of position.

    The adoption of the intercept method in navigation, primarily in the early 20th century, led to a real revolution in the process and paperwork of navigation. Numerous methods were developed and discarded in the period from c.1910-1940 to streamline the calculations. One could spend years on the "botany" of these methods and sub-methods and their inter-relationships.

    For a modern navigator, perhaps one of the biggest innovations in sight reduction was the pre-printed "sight reduction form". Historically, the work was just scribbled haphazardly on scrap paper. Turns out filling out forms is a modern addiction!

    -FER
    PS: There are always exceptions. While time sights were almost always worked using a standard widely-known method, other tables and methods has their followings. Earlier this year we discussed Martelli's "mystery tables" which Greg Rudzinski is very fond of. These solved the same "time sight" problem with a little less work but at the cost of carrying special tables. I have not found any cases where they were used in practice, but no doubt many navigators at least tried them out --who could resist something described as "mystery tables"??

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