Saturday, January 2, 2010

How to Tell Time

When accurate clocks became available to the navigator, it became essential to determine and track the error of the individual chronometer, so shipboard time could better be determined. Error consists not merely of how fast or slow the timepiece is relative to Greenwich Mean Time, but also of the rate of error, the rate at which the error is increasing or decreasing. By knowing both the error at a known date, and the rate of error, an accurate estimation of the precise time can be made some weeks or months after the last occasion on which the chronometer's time could be compared with actual time. (Actual time was, and is, itself determined by observing the transit of an astronomical body with the observer's meridian, using a special instrument and perhaps performed at a naval observatory.) In the afloat services, the chronometer is never reset except by a shop ashore, when the chronometer is exchanged for servicing. Rather, chronometer time is corrected, when the sight reduction (translation of sextant sight to line of position) is performed, by reference to the chronometer's known error and known rate of error.

The ship's chronometer is itself set to Greenwich Mean Time, that being the time used for all celestial navigation. G.M.T. has apparently been replaced, in one of those highly suspicious modern developments, by "Coordinated Universal Time", which seems to be the same as G.M.T.
Today, while chronometers are still carried, precise time is determined by listening to a "time tick", broadcast by the U.S. Navy from Fort Collins, Colorado on WWV. The time tick is broadcast on 2.5 megahertz, 5 mHz, 10, mHz, 15 mHz, 20 mHz and 25 mHz, the lower frequency broadcasts generally being better received during daylight, the higher at night. (You can hear a sample of the time tick on the WWV web site.) In practice, the navigator sets a stop watch to the time tick, and the stop watch is then used to determine the precise time of each observation.

Not so long ago every major port in the world signaled noon with a "time ball", hoisted on a mast at the port office. At five minutes to noon, the ball was hoisted to half-staff. At one minute to noon, the ball was hoisted to the masthead, and at the instant of noon the ball dropped, perhaps accompanied by a gun. I last saw a time ball in St. George's Bermuda two years ago, and I'm not sure it's still in use - I hope it is.

6-5-4-3-2-1 --- Happy New Year!!!!!!!!
If all this business about time balls seems vaguely familiar, perhaps you have seen a rather elaborate example on New Year's eve. How many people watching that celebration have any clue that they are seeing an important nautical tradition, one that once represented a triumph of technology?

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.

Site Meter