NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: George Huxtable
Date: 2008 Jul 4, 20:39 +0100
From: George Huxtable
Date: 2008 Jul 4, 20:39 +0100
I asked Dave Walden what he had taken as his standard for refraction, when comparing various formulae, and he has replied- | I used the star link R F R O subroutine by Wallace (head of the UK | Natu Alm Office. | ( It reproduces the critical table in the Naut Alm exactly.) | | | | * P.T.Wallace Starlink 3 June 1997 | * The routine computes the refraction for zenith distances up | * to and a little beyond 90 deg using the method of Hohenkerk | * and Sinclair (NAO Technical Notes 59 and 63, subsequently | adopted | * in the Explanatory Supplement, 1992 edition - see section | 3.281). ================ Fine. The refraction table in the almanac makes a good choice for comparison. But, of course, there's no "absolute truth" in it. It's chosen to be a good fit to observations, in most circumstances. Indeed, the Hohenkerk / Sinclair predictions are based on a rather simple model of the atmosphere, in which the air temperature falls by 6.5�C from sea level to 11km up (that is, by 71.5�C), and then stays constant at that value right up to the limit of the atmosphere, taken to be 80 km. For many purposes, meteorologists now prefer a more complex model. I've implemented that routine on a pocket-calculator program, and also in an Excel spreadsheet, but in a form in which the atmosphere can, if required, be split into further bands, each with a particular lapse rate. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---