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"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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