NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What time is it, really?
From: Gary LaPook
Date: 2008 Jul 17, 23:36 -0700
From: Gary LaPook
Date: 2008 Jul 17, 23:36 -0700
Gary writes:
It's actually 15.041º per hour (15º 2.5') approximately 361º per solar day.
gl
Bill wrote:
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It's actually 15.041º per hour (15º 2.5') approximately 361º per solar day.
gl
Bill wrote:
As understand it, with an earth rotation of 15d per hour, 1 second time equals 0.25 arc minute. It follows that 4 seconds time would equate to 1 arc minute.
Bill askedWhat time is it, really?I believe the musical group Chicago answered that question back in the late '60s... ;-)And does anyone really care? I do.A while ago there was a thread on time and the affect of dropping leap seconds on cel nav.Don't think I was on the list for that thread, but as I understand it leap seconds are added to UTC as needed to keep it within 0.9 seconds of astronomical time. The rule that I remember from back when I was first learning celnav was that your observation time had to be accurate within 4 seconds, otherwise your LOP could be off by up to 1 NM just from that error alone (I interpret that to mean +/- 2 seconds). So I would say that unless you need exceptional accuracy with your celnav sights you're probably OK just ignoring the leap seconds.As understand it, with an earth rotation of 15d per hour, 1 second time equals 0.25 arc minute. It follows that 4 seconds time would equate to 1 arc minute. An arc minute of longitude would be nominally 1 nm at the equator, but less if the vessel's AP is north or south of the equator. Roughly 1' longitude * cos latitude = fraction of a nautical mile (ignoring oblateness). For example, near an elevated pole 360d longitude could be under 1 nautical mile. And why--despite the "former" CTA's cavalier attitude towards chronometers--would I care? With an artificial horizon, my Astra, and a 3.5 scope, I consider an intercept of 0!0 from an average of 5 or more observations from a known GPS position lucky. 0!1 very good. 0!2 average. 0!3 fair, and > 0!3 has me checking IC and sextant calibration. I figure an artificial horizon cuts IE and observation errors in half, so it gives me 0!0 to 0!6 (averaged-observations intercept) as goal to shoot for under ideal conditions. I have never experienced my ideal conditions. They would include a crisp horizon, clear sky, and a relatively stable (or predictable) platform. And of course accurate UT1 time. <g> But if I ever do... Bill B
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