NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Finding the true horizon
From: Bruce J. Pennino
Date: 2013 Mar 26, 21:10 -0400
From: Bruce J. Pennino
Date: 2013 Mar 26, 21:10 -0400
Bruce
Hello David:
Recently there have been some postings for the
AH. I really recommend the Empire torpedo level and a simple three screw
mirror on a metal plate or firm/sturdy platform. Not so hard to use with some
initial stuggle and learning curve. Can get results within a couple of minutes
of arc.
Bruce
----- Original Message -----From: David CortesSent: Tuesday, March 26, 2013 8:44 PMSubject: [NavList] Re: Finding the true horizon
Thank you, thank you, thank you. For many years I've been away from my sextant because I can't shoot stars method of without a very difficult to use and inaccurate artificial horizon constructed out of a mirror sitting on adjustable screws. Your idea seems to be quite workable. Thanks yet again. David -----Original Message----- From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed Sent: Tuesday, March 26, 2013 4:57 PM To: dcortes---com Subject: [NavList] Re: Finding the true horizon David, you wrote: "So is this of some help to us land-locked navigators who want to practice with our sextants without the hassle of an artificial horizon? " It can be. If you're in a city, you can find the true horizon this way (probably easiest to connect up the perspective lines by taking a photo). Then, if you're lucky, you'll find that you have some horizontal feature, like a building roof line, sufficiently far away that can serve as a surrogate horizon, close to the true horizon that you've determined by crossing perspective lines. If it's a little above or below the true horizon, then you can just work out some effective dip by measuring between them. In fact, you could also do this by direct altitude observations treating dip as an unknown. The distance "sufficiently far" should be far enough away so that a small change in your height of eye does not significantly move the line. If you always take your sights while standing and from a fixed location (e.g., always on the same floor of your home or maybe always from your back deck), then your height of eye will surely vary by less than six inches. A ratio of 3438-to-1 is a minute of arc, so if the surrogate horizon line is 1700 feet away or more, then it will move by less than a minute of arc. If it's over three miles away, then a six inch change in height of eye would affect the location of that surrogate horizon by only a tenth of a minute of arc. Speaking of treating dip as an unknown, for any of you who were following the thread about measuring dip, you can measure it with an ordinary properly adjusted sextant as long as your goal is just collection of data for statistical analysis (as opposed to live navigation). You measure altitudes from a known location and clear them treating dip as unknown (set to zero). Then the error in the sights is equal to your dip. -FER ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ---------------------------------------------------------------- : http://fer3.com/arc/m2.aspx?i=123120: http://fer3.com/arc/m2.aspx?i=123136