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    Re: Time Sight
    From: Frank Reed
    Date: 2014 Aug 5, 11:03 -0700

    Richard, you wrote:
    "I have been reading about the "Time Sight" in several navigation books and find all reference work involves the Sun."

    The Sun was far and away the most popular and the most obvious choice for this procedure. If you could get a single, good time sight during the day, then you could carry that local time forward on a common watch. Of course, there's a "dead reckoning" correction for any accumulated motion east or west. This ability to "carry" the local time was irrelevant in the era of chronometers since you could always check the GMT more or less simultaneously with any time sight, but in the earlier period when navigators shot lunars for GMT (or GAT), it was sometimes necessary to get the local time some hours before or after a lunar distance was observed.

    And:
    "I follow how longitude was derived from a single sight by using a DR Latitude. What however was the procedure when working multiple sights as in morning and evening stars, planets or Moon? Did they work up individual longitudes for each sight and then utilize an average Longitude with what I would assume to be a common Latitude?"

    The short answer is that they didn't do that. The idea of the twilight round of sights is primarily a 20th century invention that evolved as a component of the intercept method and especially the three-body fix. But there are no absolutes in history. There was surely some navigator, even a school of navigation, somewhere that practiced time sights on multiple bodies. Since this would have been a rare and local culture of navigation, procedures would not have been standardized. Apply any reasonable rules you like! Yes, you could average the results of multiple sights. Or you could work up all of them individually, even with slightly different latitudes, and then pick one based on an instinct about the relative quality of the sights.

    Back to the main issue, consider a navigator in the 19th century in the tropics. The Sun is available for time sights (and Sumner lines, if the navigator knew about them) for roughly six hours every day from about 7:00 to 10:00am and again from about 2:00pm to 5:00pm. The twilight stars are only available for a total of about forty minutes more (twenty in each twilight). So not much benefit. The only other object available for time sights would have been the Moon in daylight on those days during the month when it was conveniently placed for observation, and if navigators understood it well enough, this might have been useful. But even today, the additional navigational value rarely offsets the nuisance of clearing Moon altitudes. Given the relatively slow speeds of 19th century vessels, rarely exceeding 10 knots, the Sun was sufficient for nearly all navigation. A time sight in the morning or afternoon (or both if manpower, time, and weather permitted) and Noon Sun for latitude were all that they needed.

    -FER

       
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