NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sunrise - the Positive Side
From: John Huth
Date: 2013 May 8, 08:42 -0400
From: John Huth
Date: 2013 May 8, 08:42 -0400
Jeremy makes a good point - a lot of times distant clouds will obscure the sun within a degree of the horizon, so you have to take azimuths when it's a few degrees above.
On Tue, May 7, 2013 at 2:58 PM, Bill B <billyrem42@earthlink.net> wrote:
On 5/7/2013 2:15 PM, Jeremy C wrote: > FER - "The (presumably) nearest to correct rule would be dip + 34'(for > mean refraction) - 16'(for SD) or in other words dip + 18' (and to be > yet more accurate, +/- a few minutes of arc for non-standard temperature > and pressure). Considering you're shooting from a greater than average > height of eye, do you have a rule of thumb that incorporates a higher > dip value?" Regarding lower-limb altitude for amplitude, thanks for the HC=0 answer. That nugget cleared it up for me. Referencing the NA 0-10 altitude-correction tables seeking the ideal LL altitude for amplitude at STP with no dip correction using inspection and mental interpolation I arrived at approx. 14’ for HC=0. (LL altitude – refraction). I used an average of the averaged seasonal refraction values to arrive at the 14' target. As Frank suggested, a baseline plus dip and pressure/temp correction should be optimal. Figures I ran for a 10 minute period prior to sunset at 45N 86W on 6 May 2013 averaged 0d 9!7 HC change per minute, and 0d 10!5 per minute for azimuth. Clearly LL altitude being off by 5’is not a huge deal in mid latitudes, keeping in mind refraction will amplify the change in HC. As mentioned, the further you are from the equator the more critical timing becomes. In the bigger picture refraction is the fuzzy variable. By my calculations, the average height of the squished sun with a 14’ LL altitude is 27’, so an RCH over 1 SD should work just fine. More on squish later. Bill B
