NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Celestial Tools V5.8.0
From: Stan K
Date: 2018 Nov 21, 10:43 -0500
From: Stan K
Date: 2018 Nov 21, 10:43 -0500
Robert Bernecky's recent (September 2018) paper "Phase Correction for Venus" again got me thinking about what I consider possibly the biggest lack in Celestial Tools, which is that it did what all other programs I know of do regarding the hourly GHA and declination of Venus - it does the calculation for the center of disk rather than the center of light. Most users do not care about this, but Celestial Tools is written primarily for the use of USPS students, who need to use the printed Nautical Almanac, which, for most years (see Celestial Tools Help excerpt below), corrects for the phase of Venus in its GHA and Dec values.
Frank Reed suggested an approach which was different than Robert's and seemed like it would work well with Celestial Tools. I incorporated it into V5.8.0. It did not work at first, but, after a few day, the light bulb finally went on. Celestial Tools now has a feature that, to the best of my knowledge, no other similar program has (with the exception of the U.S. Navy STELLA program, which is not available to the general public).
From the Celestial Tools Help:
The planet Venus exhibits phases like the Moon, which can be seen with optical aid. From 1952 to 1984 the additional corrections for Venus
listed in the Nautical Almanac allowed for both parallax and phase. The phase correction was required because the tabulated position was for the
planet's center of disk. Before 1952 and since 1985 the tabulated position of Venus is for its center of light, and the phase correction is incorporated
into the tabulated values of GHA and Dec. When in the "NA vals.of v/d, GHA/Dec" mode, Celestial Tools tries to reproduce the values of the GHA and
Dec of Venus found in the Nautical Almanac. When in the "Accurate vals. of v/d, GHA/Dec" mode, it calculates the position of the center of disk (as do
most sources other than the Nautical Almanac). (Mars also exhibits phases, but the effect is negligible.)
These modes are included in the Sight Reduction tool. I "spec" the accuracy of the phase-corrected values of hourly GHA and Dec of Venus to be within 0.1' of the values tabulated in the printed Nautical Almanac (with an occasional difference of 0.2'). The "spec" for the "Accurate" mode (center of disk) is the same.
The USPS Junior Navigation course uses excerpts from the 1972 Nautical Almanac, so previous versions of Celestial Tools should have been OK. (Oddly, Robert's cover email of September 23 said, "My intent was to duplicate the values in the Nautical Almanac." The example at the end of the paper was for 1977 March 18, which was a year that the tabulated position of Venus was for the center of disk, not the center of light.) The USPS Navigation course uses excerpts from the 2004 Nautical Almanac, so the change in Celestial Tools V5.8.0 is significant.
(BTW, V5.8.0 includes other new features of less interest to celestial navigators.)
Although I'm pretty sure V5.8.0 meets the requirements I set for it, at this time it should only be considered a beta. If you have the time and inclination, and the printed Nautical Almanacs to support this effort, I would appreciate any reports of dates and hours that do not meet the spec.
Attached is a zipped copy of V5.8.0. For the convenience of those whose emails do no allow executable attachments, the extension was changed from exe to exx before zipping. Remember to change it back to exe after unzipping.
Thanks for your help.
Stan
