In reply to Rob Seaman, John Cowan wrote:
Seaman: >> [A]s clock
>>time diverges further and further from solar time, more systems in
>>more communities (transportation, GIS, innumerable scientific
>>disciplines, what have you) would be revealed to need remediation.
>
Cowan:
> Can you spell out some of those implications?
>
Professional and amateur astronomers are not the only ones who need good
estimates of UT1. A far more safety-related issue comes from the
continuing need for celestial navigation as a backup for electrically
powered navigation devices.
Small boats, sea water, and electrical systems don't mix very well. The
damp, salty environment all too often leads to failures of a boat's
electrical system. A prudent sailor should not rely for navigation only
on electrically powered systems like GPS or loran. It is still prudent
to cultivate the skill of taking sights with a sextant and calculating
ones position using sight reduction tables. Today a mechanical watch or
chronometer, or a battery-powered wristwatch, can be set to UTC using
radio time signals. Then when power fails, the sailor still has a
reasonably accurate spprodximation to UT1 available.
As long as one's timepiece is set to within 1 s of UT1, the error in
longitude due to chronometer error will be less than 30 seconds of arc
-- at the equator, an east-west error of half a nautical mile. If DUT1
is known to a resolution of 0.1 s (by counting the "double ticks" from
time and frequency stations such as WWV and WWVH), the error in
longitude due to chronometer error can be reduced to 3 seconds of arc:
an error of 1/20 NM at the equator.
But if UTC is allowed to drift away from UT1 by eliminating leap
seconds, celestial navigation fixes will become less and less useful. A
time error of half an hour in UT1 equates about to 450 NM at the equator.
Celestial navigation was the "raison d'etre" for GMT, and it is still a
useful skill. But celestial navigation depends on knowlege of UT1. The
further civil time and UTC are allowed to drift away from UT1, the more
impact there will be on safety of life at sea.
--
James Maynard
Salem, Oregon, USA
Received on Fri Jan 20 2006 - 14:08:57 PST