Mike Carrell wrote:
What is most remarkable is that the Chinese maps, and their derivatives,
show accurate longitude centuries before the Harrison chronometers enabled
the determination of longitude at sea. Another method of determining
longitude for *mapping purposes* is based on observation of the eclipses of
the moon, or transits of the moons of Jupiter.
The Chinese assuredly did not have portable watches of any sort, so they
must have used some form of purely celestial navigation to determine
longitude. There were some methods in use long before Harrison's
chronometer. Lunar methods were the most widespread and reliable. I doubt
the moons of Jupiter played a role, even if they could be observed without
a telescope. An eclipse or transit is indeed is a spectacular and handy way
to establish precise longitude. However I think (but I am not sure) that
the main technique was to predict in advance which stars and constellations
would be eclipsed by the moon, and this was the basis for lunar longitude
calculations that could be performed even at sea. That technique was
perfected just about the same time as Harrison's watch, in the late 18th
century. Lunar tables were published until around 1911, as I recall.
I think other method used the time of moonrise, and the angle between the
moon and the sun, measured with an astrolabe. Some of these techniques
called for timekeeping lasting of several hours, accurate to the nearest
minute, for example, to measure the time from sunset to moonrise. This
could be done with things like sand clocks and, in Europe, with primitive
pocket watches after the late 1600s.
Galileo's method using Jovian moons was never practical at sea, but it was
a great success on land, with observatories. Here is a funny quote from D.
Sobel (p. 27) about that: "King Louis XIV of France, confronted with a
revised map of his domain based on accurate longitude measurements,
reportedly complained that he was losing more territory to his astronomers
than to his enemies."
I have no idea what methods the Chinese used to establish longitude.
Clock based celestial navigation was used well into the 20th century, even
on board aircraft. This was described by the late, great Guy Murchie, in
"Song of the Sky." Murchie taught navigation during WWII. Even after the
war, when radio navigation was being established, navigation using a
sextant in an aircraft observation dome was still essential, and it was
seldom easy or accurate. It sometimes resulted in disastrous errors.
Flights went hundreds of miles off course. In 1949, one was lost at sea off
of Ireland because of a spectacular error. WWII radar navigation and
direction finding was sometimes 180 degrees incorrect (for example in the
U.S. attempt to locate the Japanese fleet after the attack on Pearl
Harbor). This led to the loss of many aircraft, which flew off in the
opposite direction, never to be heard from again.
- Jed
