[Paddlewise] Longitude was GPS Errors Summary

Michael Daly wrote: -
>Determining longitude depends on two things finding
>your local time and knowing Greenwich time.
SNIP
Local time can be determined from observing time of
transit of the sun (hence the importance in the days
of sailing ships of measuring noon), the moon, planets
or stars. With a sextant, you can determine this
indirectly without resort to actually observing a
transit. (Transit is when a celestial body passes your
meridian or is at it's highest point in the sky.)
Regardless of how the maths are done, the process is
equivalent to the above.  Hence, the clock is critical.
SNIP
>but I digress.

G'Day Mike,

Wonderful digression, very much enjoyed it, as far as I'm concerned you can
digress as much as you like!

The book "Longitude" was the source of my question on measuring longitude
without a Greenwich time reference. I realise navigators need a means of
measuring time at their location but still wonder if there are ways of
calculating longitude without the need for a time reference at another point
on the earths surface. I'm thinking in particular of the lunar distance
method.

A sub plot in the Longitude book is the dispute between the Astronomer
Royal, Nevil Maskelyne and John Harrison, the inspired clockmaker. Maskelyne
claimed to be able to measure longitude using the "Lunar Distance" method,
which I think precluded the need for measuring the difference in time
between two points on the earth's surface, however the method required such
careful and prolongued calculation as to be considered impractical. Thats if
it worked at all? The Nautical Almanac, which Maskelyne first wrote,
continues to be updated and published to this day, hence my question. If the
lunar distance method did work, albeit clumsily, then I wondered if nowadays
it would be more practical given the availability of more accurate celestial
tables and computers.

By the way Dava Sobel and William Andrews wrote a second version of
Longitude called "The Illustrated Longitude" which includes a parallel story
of the development of 17th century navigational instruments in notes
attached to each picture. William Andrews has also written a book "The Quest
for Longitude". which is top of my Christmas list:~)

All the best, PeterO
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