NavList:

When we shoot the Moon's altitude, we have to make sure we're using the good limb, the edge of the Moon that is clean and sharp rather than the dark limb or a limb rendered "defective" by partial shading --a limb with mountains and craters. With the Moon, this is a 50:50 proposition. Half of the time, you can shoot the Moon's LL and the other half of the time, you have to shoot the Moon UL, the Upper Limb. This is easy enough to decide visually, and the calculation is relatively simple, too. Anyone want to describe that calculation?

Today I thought of a different sort of defective limb, and in my years of doing astronomical and navigational calculations, I don't think I've ever seen it mentioned. How do we calculate a defective limb of the Sun? Wait, what?! When would the Sun's limb ever be damaged? Can't we always shoot the lower limb, the Sun LL? Maybe an inconvenient cloud?? Sure, but we can almost always wait in that case. Another obvious case is when the Sun is setting. We lose the lower limb as the Sun goes down, but the upper limb is still available (or reverse the order at sunrise). But there's another scenario where the lower limb can be defective. And that's during a solar eclipse...

Thanks to careful planning, on October 14, when I am teaching my "Modern Celestial Navigation: Sextants & Sun Sights", there's a partial solar eclipse across most of North and South America. It's actually an annular eclipse, but this is a subset category within the partial eclipse category, so it's not a big deal compared to the great total solar eclipse coming in April next year. Here's the calculating puzzle: when does the Sun's lower limb become "defective" for any observer? A specific case: suppose you're near 28.00° N, 90.00° W. That's in the greater Mississippi Delta region in the Gulf of Mexico almost due south of New Orleans by 2° of latitude. You've decided to shoot the Sun at local noon. Can you do so? Can you use the Sun's Lower Limb? No? During what hours at that location is the Sun's LL defective? And what calculating procedure would you use to determine reasonably exact times for any given observer's lat & lon??

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA