April 11, 2008
These matters interest me because I practice celestial navigation, way finding by the sun, moon, planets and stars. I have myself developed laser leveling for artificial sea horizons and clino sightings to get elevations in place of the old sextant at sea thing and it can be used anywhere.
There is a chap named Harrison who built a clock so accurate, it could be used to measure the difference in time between Greenwich England and any other place in the world to within a few seconds of error. Because of this invention, longitude measurements became part of normal navigation. The reason this works is because time is distance. The earth rotates a full circumference in about 24 hours so any fraction of time is the same fraction of full distance.
Prior to this invention, mariners used latitude only which they got from the sun's angle above the horizon and because they never quite knew how close they were to any shoals or reefs or land masses in an east west direction, they would often run aground or sink at sea after collision with shoals. There was great loss of life and many ships sitting on the bottom because of the element of chance. Without longitude, accurate charts could not be made. Dead reckoning was prone to errors of drift.
The chronometer as it came to be known, is the single most imortant navigational instrument ever invented and was even more valuable after the sextant came to its present glory. Navigators, assuming good visibility, could estimate their precise location to within a fraction of a mile. To use it they had to have information on the latitude and longitude of the sun at all times of the year and this information is now called the ephemerides and is compiled in a nautical or celestial almanac by The Royal Astronomer or presently by the US Navy Astronomical Observatory. Sun's latitude is called its declination and sun's longitude is called its azimuth.
Having said Kudos to Harrison, the Chinese were the first to measure longitude by the transit of a star behind Venus as I recall. They sent ships out to different places with synchronized water clocks as long ago as 1000 AD, and took times on the emergence of the star from behind the Planet and compared notes when they got together again. Since the earth rotates 15 degrees every hour and every degree is 60 nautical miles at the equator, they could tell how far they were east or west by simple arithmetic. This method was tested by an Australian researcher who found that it worked wonderfully. It has obvious drawbacks.
The Chinese were so close to making the discovery that Harrison had and did not even know it. They had accurate water clocks. Had they used the sun at local area high noon over their journey's destination location meridian to record time set on their water clock at high noon sun's meridian transit at a fixed reference departure point like Beijing, they would have done it first. They could also have used sun rise or sun set.
They may have been baffled by the sun's apparent wavering from regularity over the seasons we now know as the equation of time and not known how to account for that. It of course is due to the fact that the sun rotates at different speeds depending on where it is in its annual elliptical orbit around the sun so that the day is sometimes longer than 24 hours and sometimes less.
The Chinese over a thousand years ago knew latitude very well and measured it by the length of a shadow of a standard bar casting different lengths depending on how far north or south one was at a specific time of year.
I myself compute longitude most often from the sun by taking the Greenwich time of suns transit over my meridian and subtracting the time of suns transit over the Greenwich meridian and multiplying this number times the velocity of the suns orbit every hour. It is very accurate with an accurate timepiece and when using a quartz clock adjusted to the UTC signal from shortwave frequency 5000 or 10000 khz. Sometimes I use the tabular method of St. Hillaire found compartmentalized in the Nautical or Air almanac. The almanac has the suns transit time of Greenwich in each daily table as well as its velocity at the bottom of the table.
However, we now rely worldwide, upon the GPS satellite system put aloft by the US Navy which can be as accurate as a few feet around airports where DGPS is used, but ironically, in a pinch, all of this could be estimated by a shadow and dripping water.
© 2008 J. Jyrkkanen
No comments:
Post a Comment