A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: George Kaplan
Date: 2020 Sep 19, 11:11 -0700
Hi Frank and others. The folks at USNO (I am an independent consultant to them now) are at the front of the line in being dismayed at the loss of the website. That was imposed from above, about a year ago, by the DoD IT people out of security concerns. In the meantime, an entirely new internal network, much more secure than what we had (but harder to use!) has been put in place and we're all getting used to it. However, the schedule for putting the website back online, which I had hoped would be this fall, is still uncertain. The USNO staff has little to say about all this. And, everybody working from home just makes for more challenges.
Anyway, regarding the USNO celestial navigation data page. Yes, it's a bit of a throwback. It actually comes from the Floppy Almanac, which some of you might remember, that was distributed by USNO between the late 1980s and early 1990s. We were actually using the Floppy Almanac code to generate the plain-text table until the website was pulled offline. (The colors were added by a separate program.) As some of you have noted, there are some oddities that are not part of normal navigational practice. For example, the lower limb was the point of reference for all solar system bodies, which is why the semidiameters of the planets were listed. Also, the phase correction (center of light) for Venus was not included, even though it is included in the Nautical Almanac. That can reach about 0.4' near Venus' superior conjunctions. Some of these oddities kept the code simple -- the original Floppy Almanac had to run in 256k of memory on 1980s-era PCs. Anyway, you can blame me for these peccadillos since I wrote that part of the Floppy Almanac code.
Frank is to be commended for his work in reproducing the page. As they say, immitation is the sincerest form of flattery!
Right now the folks in the Astronomical Applications Dept. of USNO are replacing the underlying code for the USNO page so that when the website comes back, it will have a new computational basis. I have made a number of suggestions for improvement, and removing the lower limb assumption (and the SD value) for the planets is one of them. Also adding the phase correction for Venus. Thanks for pointing out that Gacrux is the wrong color -- I'll put that in the list of things to do. Maybe we can add a check box for whether the upper or lower limb of the Sun and/or Moon is to be used.
I'll repeat here something that was in the notes for the USNO page: the altitude corrections are computed for the DR position you enter, not your actual position. So there might be small differences, depending on how far from your DR position you are. In other words, the corrrections you would use for an actual sight might be somewhat different from what the page displays.