NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Roger W. Sinnott
Date: 2018 Jan 31, 10:05 -0500
Hi Peter,
You are probably right that a light curve, obtained with a photometer centered on a particular spot or crater, would allow a more careful analysis. But I would say the exact shape of the light curve is not really known in advance, although several people have tried to model it; it involves both refraction and scattering at different atmospheric elevations by aerosols for the light rays as they go from the Sun and skim past the Earth on their way to the Moon (but not for the rays returning). Your earlier suggestion to determine a longitude value from both the ingress and egress of a particular crater is good, so they could be averaged.
Many faint stars are occulted during a total eclipse, and by timing them (again, only a telescope is needed) one could obtain a rather accurate longitude (much better than from crater timings or traditional lunar distances). But this involves fairly elaborate calculations.
Roger
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Peter Monta
Sent: Tuesday, January 30, 2018 7:47 PM
To: rsinnott@post.harvard.edu
Subject: [NavList] Re: Lunars-lite
Hi Roger,
For more detail on the subject of crater timings at a lunar eclipse, you should consult the paper I co-wrote with Dave Herald in the Journal of the British Astronomical Association, vol. 124, issue 5, 2014, pages 247-253.
Thanks, that sounds interesting.
But I need to stress one point: No one should try to come up with a clever new way to make crater timings, such as analyzing a video, taking a series of timed digital images, or using some kind of photometer. The response of an electronic device will surely differ from that of the human eye, meaning you can’t compare such results with the gigantic collection of visual timings over the last couple of centuries. (Have you ever noticed that the umbra’s edge appears much sharper to the human eye than it does on most photographs?) In effect, adopting some new technique means tossing out all those prior visual observations and starting over. And who’s to say that some exotic, “even better” timing method won’t come along a decade or two from now?
I see the point about maintaining continuity with the visual observations, but from a navigation perspective, one just wants a precise observable, the more precise the better. I'm willing to bet that photometry would be better (in the sense of lower variance) than eyeball observations with their human "personal equation"---a light curve would allow fitting the known form of the shadow to get a more precise time.
Cheers,
Peter
