NavList:

I mentioned at the outset on this topic that this is something that no one calculates --and thus FUN :). The exact position and availability of the Sun's LL (Lower Limb) is something that only concerns navigation enthusiasts. It's intriguing since just about everything imaginable in a solar eclipse is calculated by somebody! Eclipse calculations are, of course, among the earliest bits of astronomical calculations, dating back millennia. Many aspects of eclipses have well-tested algorithms and solutions.

To compare the mathematical tools and techniques that you're all using, I have a suggestion: for the eclipse in question (this Saturday) and for the suggested location 28.00° N, 90.00° W, what do you find for the usual solar eclipse events of first contact and last contact? These are the moments when the Moon first intrudes --so very slightly at first-- on the Sun's disk, and the corresponding eclipse-ending event when the last little bite from the Moon disappears from the Sun's disk. These events can be calculated by multiple established tools and compared with results from the techniques that you (each of you who contributed here) would calculate. This would separate the issue of ephemeris uncertainty (things like delta-T issues and exact Moon semi-diameter and so on) from the specifics of the lower limb problem.

Also, while I'm thinking of it, how close is good enough? If I tell you that we lose the exact spot at the middle of the LL at some time, like 17:07:46, but you go ahead and shoot that sharp point at the lower edge of the Sun as if it is still the LL thirty seconds later, how wrong would that be? It shouldn't be much, right? How wrong could you be on the time and incur less than 0.1' error in the Sun LL altitude? Or how about 0.5' error?

Frank Reed