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Re: What time is it, really?
From: Greg R_
Date: 2008 Jul 18, 01:33 -0700
From: Greg R_
Date: 2008 Jul 18, 01:33 -0700
--- glapook@pacbell.net wrote: > The earth completes approximately 366 rotations per year, it > completes one rotation in 23 hours-56 minutes-3.9 seconds. OK, that's one "extra" day than the annual allotment of 365, so I go back to my original question about why we don't need leap-days every year instead of every 4 years or so (something sticks in my mind about it being 365 1/4 rotations/year(?) - which would jibe with 1 leap-day every 4 years or so). -- GregR --- glapook@pacbell.net wrote: > > 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 -~----------~----~----~----~------~----~------~--~---