NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Figure out LAN?
From: Brad Morris
Date: 2014 Oct 1, 06:47 -0400
From: Brad Morris
Date: 2014 Oct 1, 06:47 -0400
Agreed Paul.
I was much more interested in the offset from the central meridian of the time zone. That can provide up to 30 minutes of error for Sam, should he have been at the far edge of a time zone
I obtained the time of LAN for 75° from the USNO website, not from the NA. The USNO website only gave that time to the nearest one minute. This was my interpretation error. Thank you for the correction.
Brad
On Oct 1, 2014 12:12 AM, "Paul Hirose" <NoReply_Hirose@fer3.com> wrote:
On 2014-09-29 17:41, Brad Morris wrote: > The USNO website provides the time of transit as 12:51 PM. > > My longitude is 72°47'.6. Therefore my delta longitude is 2°12'.4 or 2.21 > degrees. At 4 minutes of time per degree, the offset is 8.82 minutes or 8 > minutes 49.6 seconds. > > 12h51m - 8m49.6s is 12h42m10.4s Brad, the .1s precision of your result is an illusion. Since the transit time at Greenwich is rounded to one minute, local transit time computed from that value cannot be any better. The USNO celestial navigation calculator says GHA is off by 5.8′ at that UT1, which is equivalent to about 23s error. Time of local apparent noon can be calculated from the equation of time. It's a good deal easier to use GHA, but that's been explained already. Let's look at a different method. Sep 28 +9m19.5s equation of time Sep 29 +9m39.5s Those values are valid at noon UT. But transit at your meridian occurs about 5 hours after noon UT. At that time, EoT is somewhere between the tabular values. Specifically, it's (72°47.6′ / 360°) of the way from the first to second value. That fraction is .2022. EoT increases 20.0s between table entries. Therefore, at your transit it's .2022 * 20.0 = 4.0s more than the tabular value for Sep 28. So at 12h local mean time, local apparent time is 12h09m23.5s. Or, noon LAT = 11h50m36.5s LMT. Zone time would be more convenient. Your computation of the offset from zone center, 8m49.6s, is correct. Subtract that from 11h50m36.5s LMT to obtain 11h41m47s standard time. Though not rigorous, the computation gave an excellent result, as confirmed by the USNO calculator. Sun GHA at that time exactly equals your longitude.
