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    Re: When did "time sights" fade away?
    From: Frank Reed
    Date: 2011 Jul 12, 19:37 -0700

    Gary, you wrote:
    "Of course, flight navigators never used this method since you need sights more often than once or twice a day."

    Indeed. And like so many other things in celestial navigation, the rapid advances in air navigation undoubtedly helped the on-going transition in marine navigation. High speed is by far the biggest problem with the Old Navigation. It's fine, even today, in a slower sailing vessel, but when you're doing 20 knots, let alone 120 knots, there just aren't enough fixes possible. Also, in a certain strict sense, the Old Navigation doesn't give real fixes at all. Every fix is a type of running fix: in the morning or afternoon, you have the observed longitude with an earlier latitude advanced by DR, and at noon you have the observed latitude with the longitude advanced by DR from the morning time sight.

    You also wrote:
    "As long as we are discussing time sights I also have a question. You need to take the observation of the sun when it is on the prime vertical. But, during at least half of the year the sun will never be on the prime vertical. Any body who's declination is opposite to the latitude never crosses the prime vertical and that is the situation for everybody for half of the year. In addition, whenever the sun's declination is the same as the latitude but is greater than the latitude then it also will never be on the prime vertical."

    Time sights should be taken when the Sun is in the east of west, the nearer the prime vertical the better. So while there's greater sensitivity to an error in the DR latitude when the latitude is high north and the Sun is at far south declination (or the other way around) so that the Sun never gets anywhere near the prime vertical, it still works. This might be one of those cases where Captain Lane could have appreciated some mathematics applied to the subject because he certainly believed that it was critical to take the sights right smack on the prime vertical. Many people who learn "about" mathematical tools but don't actually learn the detailed underlying mathematics have a tendency to see some of the rules as black and white without a concept of a continuous variation.

    -FER
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