NavList:

Paul Hirose wrote:
"Then there's StarPilot. It gives 12:15:12 according to one person, who also said, 'StarPilot, I've always thought, is the gold standard for lunars. They use an iterative approach (see Starpath.com).' "

Since this is all she said/he said/they said/who said... there's no way to be sure or even make a likely guess. A three minute difference in calculated Greenwich Time is big and would be equivalent to 1.5 minutes of arc in the measured distance. That's large, and it should not happen. Is it real? That we don't know. But there are a few things that we can say with some confidence about the StarPilot app.

First, StarPilot is not in any meaningful sense "gold standard for lunars". It was originally, and remains to this day, a good, general celestial navigation analysis product created by Luis Soltero, designed for programmable TI calculators with forks for other platforms which directly emulate the original TI-calculator app. As such, the app uses ultra-efficient algorithms, slick tricks, and also questionable simplifications that are no longer necessary (not for decades) on any other platform --programmable calculators have harsh memory and coding constraints. Soltero presented and described his lunar clearing method, which was mostly sound but included some genuine peculiarities, at the first NavList Navigation Weekend that I organized in Mystic, Connecticut back in 2006. His code was a decade old back then, and now it is thirty years old. While he does apparently update the almanac data, the basic tools have not changed.

Separate from any app, the specific lunar example described here is a visual "crowd-pleaser"... A lovely crescent Moon just a bit less than 10° from Spica. That is a pretty sight... And it's the sort of thing that inspires beginning lunarians to make their first little mistake. "Aha!", they say, "This must be a perfect time to shoot a lunar, just like navigators of old!" But it's not. Lunars for the determination of Greenwich Time were never done historically with distances less than 20° because the interpolation process for time develops significant non-linearity for such short lunars, and the clearing math similarly becomes problematic. The altitudes of the Moon and star matter very little when the Moon is "in distance" (LD near 90°) but for short distances, the exact altitudes are critical. Whether these issues arising from a short, albeit "pretty", lunar distance are the source of the discrepancy in the StarPilot result that Paul describes, none of us can say without further detail. The account here is just hearsay. But trying to get Greenwich Time by lunar distances is dangerous with short lunars, and the method used in StarPilot is questionable for these cases as described here. Yet another possible source of error is something as simple as swapping Moon "UL" for Moon "LL" in the clearing process. Little errors on input, like that, do propagate...

Finally, do not trust StarPath on lunars in general. It is a marginal, unimportant topic for them. And note: do not confuse StarPath with StarPilot. 

Frank Reed