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    Re: [Fwd: lunars hard to shoot?]
    From: R. Winchurch
    Date: 2000 Sep 11, 9:48 AM

    Of course I meant longitude.  Thanks for your input.
    Michael Wescott wrote:
    > "R. Winchurch" wrote:
    > > Excuse my ignorance but would someone give me a primer on lunars.
    > > I understand that prior to the effective use of chronometers lunars were the
    > > only method for determining latitude.  As I understand it one measures the
    > > angle between the moon and selected planets (Jupiter and ?) and certain
    > > stars.  How does this translate into latitude?
    > You did mean longitude there, didn't you?
    > Longitude and standard time are complementary aspects of the same problem.
    > If you can determine one you can get the other. One way to do this is with a
    > clock. Until the invention of the chronometer, this was not practical for
    > ship-board use.
    > Another option is to use astronomical events that can be predicted with some
    > degree of accuracy. Use the event to set a clock, and use the clock to take
    > some sights. The astronomical events used were the eclipses of the moons of
    > Jupiter. These could be predicted with considerable accuracy. Unfortunately,
    > this requires a high power telescope and therefore a stable platform to observe
    > the eclipses. Not practical for shipboard use, but it was used on land.
    > Since the moon moves across the background of the stars at about 13 degrees
    > a day or 30' per hour, determining its position can yield a fairly accurate
    > estimation of the current time. It is possible to determine the Moon's position
    > by  measuring the angle between it and other bright stars or planets. From this
    > you can get time and therefore longitude. That's the theory in a nutshell,
    > anyway.
    > In practice, this is practical on ships and even small boats. Joshua Slocum
    > (the first around alone sailor) seems to have preferred Lunars to keeping his
    > Chronometer on time. The drawbacks are in the difficulties involved with taking
    > three "simultaneous" sights (two altitudes and the angle between them) and in
    > the calculations involved to reduce the sights.
    > Had the Lunar method been perfected earlier, it may well have delayed
    > invention of the Chronometer for some time. As it was, understanding of the
    > motion of the Moon sufficient for accurate tables was achieved at roughly the
    > same time as the invention of the Chronometer so the two methods were
    > competitors for the famous prize.
    > --
    >         -Mike Wescott
    >         wescott@conterra.com

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