NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Figure out LAN?
From: Stan K
Date: 2014 Sep 27, 17:26 -0400
From: Stan K
Date: 2014 Sep 27, 17:26 -0400
In "real life" there is not much point trying to calculate the time of LAN to the second, since you don't know your longitude accurately, and you just need to know the approximate time so you can get on deck early enough to take a series of sights starting before the predicted (approximate) time to determine the peak, or at least where it levels off. But just to practice a noon sight from a known location, Bill's procedure will give you the time of LAN to the second. It has an advantage over any method that uses the time of the sun's meridian passage from the daily pages of the Almanac, which starts off being rounded to the nearest minute (and is truly only for the Greenwich meridian, though really accurate enough for any zone meridian). The only change to Bill's procedure I might suggest is, in the third step, rather than converting the difference from arc to time, just go into the Sun column of the Increments and Corrections ("backwards") and extract the minutes and seconds that corresponds to this difference. IMHO this is less subject to error.
-----Original Message-----
From: Bill B <NoReply_BillB@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Sat, Sep 27, 2014 3:27 pm
Subject: [NavList] Re: Figure out LAN?
From: Bill B <NoReply_BillB@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Sat, Sep 27, 2014 3:27 pm
Subject: [NavList] Re: Figure out LAN?
On 9/27/2014 1:06 PM, Samuel L wrote: > Greg, > > Thanks. I usually calculate LAN from 12 UT and subtract the 1 hour for DST. Personally I don't bother with the meridian passage time posted in the NA. If I understand it correctly, that is based on the equation of time when the Sun is on the Greenwich meridian, so not spot on if you are a goodly distance away from Greenwich. --Simply find the Sun's GHA nearest to but less than your (west) longitude. --Subtract that from your AP longitude. --Convert that difference from arc to time. --Add that to the whole hour UT used. --Adjust from UT to your time zone. NB Your time zone adjustment will be an hour greater when daylight time ends, assuming you are on DST. Seat of the pants, being mindful that navigators determined their latitude from sextant Sun shots using only declination long before the chronometer was invented. (Also stars, latitude hooks, astrolabes and other instruments.) When the shadow of something vertical gets close to true south but a bit east of south, start observing. And/or... --Calculate Hc at LAN for your AP latitude. --Do a down and dirty Hs calculation from Hc by reversing standard IC, dip and refraction corrections. --Make the occasional observation. --When your Hs gets close to but lower than that, start observing in earnest. (As Frank implied, the Sun may hang at the same altitude for minutes given the precision inherent in cel nav.)
