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    Stellarium Moon for celestial navigation
    From: Frank Reed
    Date: 2024 Jun 21, 09:55 -0700

    I think I've spotted a fairly significant "anomaly" (a questionable display choice, rather than an actual "bug") in the celestial navigation tool for Stellarium. This plugin reports the "GP" (subSun / subStar) coordinates for any celestial object. Usually the latitude of the subStar point is the declination of the body, and the longitude is the GHA of the body, as it should be. But the coordinates should be the geocentric coordinates. Stellarium can calculate geocentric coordinates (de-select "topocentric" in the "Tools" tab of the "Configuration" dialog), and then it works as expected, of course. That's extra work. Normally, the altitude of nearby objects, the Moon especially, has parallax factored into the apparent RA and Dec, and that gets passed through to the Lat/Lon of the GP. There is a certain sense in which this is "not wrong" but it's so radically "anomalous" and so different from the usual concepts in celestial navigation that it could cause serious problems.

    On 22 June 2024 at 01:00:00 UT (equivalent to 9:00pm Eastern US time tonight), viewed from 40.0°N and 75.0°W, Stellarium shows the Moon a few minutes after it has risen in the southeast. Fine. That looks good. When the celestial navigation tool is active, it displays "GP: GHA/DEC: 13.78859°/--29.06618° " (note that I also have the decimal degrees option active). The presence of five digits past the decimal point instead of three and the double "negative" on the Dec are minor display issues, worth mentioning, but barely. Things get interesting when we move to a new location. In fact, let's "discover" the problem, just the same way that I enountered it. If that GP is correct, then we should be able to move our observation point to those coordinates as lat/lon, and then we should find the Moon exactly at the zenith. Instead, from that location the altitude of the Moon is just a bit more than 89°. That "roughly one degree" difference is just what we should expect from the Moon's parallax when shifting from the horizon to near the zenith. Let's try an intermediate point. At 5.0° N, 45.0° W, Stellarium displays "GP: GHA/DEC: 14.22728°/--28.87258° ". Different GHA/Dec depending on where the observer is on the Earth! In principle, this is not "wrong", but in practice it is alien to celestial navigation. For any instant of UT, there should be only one unique pair of GHA/Dec for any celestial object.

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

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