A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Apr 19, 11:21 -0700
"Turns out that being able to measure time accurately is important not just in human-navigation."
In historical celestial navigation, this is a lot like the "lunch test". When you work out the Sun's hour angle from the meridian, you need to know whether to convert that into hours p.m. or hours a.m. In classes, I explain that this is actually part of the observation. There is no mathematical test for it. You have to ask yourself, "have I had lunch yet?" And like the butterfly's primitive neural network, those simple, primal neurons linked to your stomach will give you that critical component of the observation: hungry... no lunch? it's an a.m. observation... full... ate lunch? then it's p.m. From another perspective, while it might seem like there's a tricky case to decide in a classroom situation, in the real world there's rarely any question because you "know" that you took the sight in a.m. hours or in p.m. hours, and saying that your "stomach tells you so" is merely a way of emphasizing that reality.
By the way, if you're talking about this butterfly story with folks who have read "just enough about navigation to get themselves in trouble," you may find that they think that this means that butterflies have chronometers (!) and they are determining longitude just like old John Harrison. But no, this is just a very basic, crude measure of the elapsed time during the day --like the lunch test! Elapsed time combined with the Sun's apparent bearing in the sky, with constraints on latitude and time of year (which migrating butterflies certainly have), yields azimuth. :)
By the way, for navigators suspicious of the lunch test on real sextant sights, perhaps a bit too whimsical and informal for some tastes, there is another way to decide the case: was the Sun still rising as you made the observation? If rising, then it's a.m.; otherwise p.m. This is relatively easy to detect (and remember) when shooting the Sun with a sextant since the change in altitude is obvious so quickly. For butterfly observers, not so much.