A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Sean C
Date: 2015 Aug 5, 19:19 -0700
The altitude correction tables for stars (in the Nautical Almanac, at least) correct for the effect of refraction of the atmosphere only, not the [semi]diameter of the star. As John pointed out, all stars effectively appear as points of light from a cel-nav perspective. (Incidentally, the almanac gives the formula to work out the correction for yourself with a calculator or other suitable method: Refraction = -0.0167°/tan(H+7.32/(H+4.32)); where H = apparent altitude.)
Apparently, the U.S. Air Force Cambridge Research Laboratories' Sacramento Peak Observatory carried out a digital analysis of speckle photographs to determine the angular diameter of Arcturus:
Digital reduction of speckle interferometry data for the K2 III giant a Boo (Arcturus) has been done. Angular diameters of 0''. 019 ± were obtained at 4200 Å for a uniformly illuminated disk, and 0''. 027 ± 0'.' 010 for a highly limb-darkened disk.
Their figures seem very close to Peter's calculation...far too small to be of any consequence to the navigator.