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Michael,

Of course I meant longitude.  Thanks for your input.


Dick


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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