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Re: What time is it, really?
From: Gary LaPook
Date: 2008 Jul 18, 01:28 -0700
From: Gary LaPook
Date: 2008 Jul 18, 01:28 -0700
The earth completes approximately 366 rotations per year, it completes one rotation in 23 hours-56 minutes-3.9 seconds. gl On Jul 18, 12:18 am, "Greg R."wrote: > --- "Gary J. LaPook" wrote: > > > It's actually 15.041� per hour (15� 2.5') approximately 361� per > > solar day. > > Hmmm... not questioning your math, but if the Earth rotates 1� beyond a > complete rotation every day, wouldn't we need to add leap days every > year (i.e. 365� "extra" rotation in a year = 1 extra day + 5� "left > over"), instead of approx. every 4? Seems like it should be closer to > something like 360.25�/day (?). > > Then again, it's late and I'm not thinking clearly on this one.... > > -- > GregR > > --- "Gary J. LaPook" wrote: > > > Gary writes: > > > It's actually 15.041� per hour (15� 2.5') approximately 361� 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 asked > > > >>>What 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. But if I ever do... > > > >Bill B --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---