NavList:

Greg, for latitude by stars, you asked:
"How did they know when they had meridian passage? "

Same as the Sun: they watched for the maximum altitude. With the Sun at noon or any other object transiting the meridian, you start measuring the altitude some time before meridian passage with the local time of this event estimated either by a short calculation or, especially on land, from the previous day's experience and the usual "four minute rule" (if Sirius transited yesterday at 11:53 pm local time, and you have moved only a handful of miles east or west during the day, then it will transit four minutes earlier today). So starting perhaps ten minutes before the estimated meridian passage, you measure and then measure again. And you find that the altitude of the star is increasing. It's not necessary to record each observation if the only goal is to get latitude. Soon enough, you find that the increase in altitude begins to slow down, and eventually it reaches a plateau where no change in altitude is detectable for two or three minutes. That peak altitude is the meridian altitude. This occurs when the object is close to due south (or due north), but it doesn't really matter whether it's exactly on the meridian since the altitude barely changes close to the meridian.

I'm teaching my "Celestial Navigation: 19th Century Methods" class at the end of this month at Mystic Seaport. It seems like it would be right up your alley! :)

-FER

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