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Longitude is Time is
Longitude is ......
Longitude by chronometer; how does this
work?
copyright Bob Graham 2002
 Simply
put, it is this:
Local noon is established somewhere in the world by, for
instance, the use of a sextant--not standard time zone "12
o'clock," but the actual time of the sun's transit of the
meridian--NOON.
Establishing Noon requires before and after
observations, so a calculation is required to establish when
noon occurred (past tense) by the chronometer--we cannot
actually experience the present.
"...I have been anxious to improve the nick of
time, and notch it on my stick too; to stand on the
meeting of two eternities, the past and future, which is
precisely the present moment; to toe that line." Thoreau,
Walden
When the time of local noon has been established, it is
compared to the time the chronometer was showing, which was
the time at 0 degrees longitude-Greenwich, England.
If at local noon the chronometer was showing 16hrs 3min
36sec Greenwich time, then we subtract 12 hours (our noon)
which tells us that we are 4hrs 12min 36sec west of
Longitude 0°.
The sun travels 15° west each hour (1/24th of
360°), so we will be somewhere west of the 60th
meridian (4hrs x 15°). The additional minutes and
seconds are both divided by 4; the minutes of time becoming
degrees of longitude, and the seconds of time becoming
minutes of longitude. Our longitude west of the 60th
meridian becomes West 63° 9'..
If at latitude North 40°, there are about 53 miles
per 1° of longitude*, and about 69.2 miles per 1°
of latitude.
*miles in degree of longitude: 69.172
(miles/1° at equator) * COS (latitude).
What if your chronometer stops?
That is what happened to John Charles Frémont in the
winter of 1844 after he had traveled thousands of miles by
horse and mule in harsh climatic conditions. How do you then
find your longitude and re establish Greenwich time?
At Frémont's latitude a clock error of just one
second would result in a longitude error of nearly a quarter
of a mile!
FRÉMONT AND THE DETERMINATION OF COORDINATES,
or
LONGITUDE AND THE BUENAVENTURA
RIVER. A study of the mid-nineteenth
century methods of determining positional coordinates used
by Frémont in his surveys.
Was Frémont really looking for the Buenaventura
River?
Related pages:
See the article DETERMINATION OF LATITUDE BY FRANCIS
DRAKE ON THE COAST OF CALIFORNIA IN 1579
AN EXPERIMENT IN THE DETERMINATION OF LATITUDE: This is
a followup to the proceeding article, in which the
conclusions made therein are put to practical test that may
be repeated by anyone wishing to go to the trouble.
A DAY AT THE COVE: An actual on-site demonstration of
the determination af latitude with an astrolabe at Campbell
Cove before a group of interested spectators.
WATCHING THE HEAVENS CHANGE.
How polaris has moved 2 degrees closer to the celestial
pole during recorded California history, and why John C.
Frémont got up at 3:00 a.m. to sight polaris in
1844--wasn't it there all night long?
Re. the Drake landing site project, see a comparison of the
16th Century TABLES OF SOLAR DECLINATION by Martin
Cortes with those of William Bourne. Pretty neat!
How easy it is today to carry the correct time
about.
A Short Bibliography
of Essential Reading:
Bourne, William, A Regiment For the
Sea (1574), Cambridge, 1963.
Bowditch, Nathanial, The New American Practical
Navigator: Any 19th Century Edition is most usefull.
Sir Francis Drake (Bart.), The World Encompassed,
1628: Any edition, but especially The Argonaut Press,
London, 1926.
Hanna, Warren L., Lost Harbor, University of
California, 1979.
Kelleher, Brian T., Drake's Bay, Day Publishing, San
Jose, 1997.
Wagner, Henry R., Sir Francis Drake's Voyage Around the
World, John Howell, San Francisco, 1926.
Waters, D. W., The Art of Navigation, Yale University
Press, 1958.
Wright, Edward, Certaine Errors in Navigation (1599),
Walter Johnson, Norwood, N.J., 1974.
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