NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Moon - Antares
From: Frank Reed
Date: 2009 Jan 23, 21:26 -0800
From: Frank Reed
Date: 2009 Jan 23, 21:26 -0800
Brad, you wrote: "I have experimented with Mr. Reed's online lunars tool and have found that I can obtain what appears to be the correct result for the immersion of Antares for my lat, lon vs that which is illustrated online at the USNO for the June event." It should be accurate for something like that within a few seconds of time, but bear in mind that there is no "limb correction" in the lunar distance calculator (I may add one at some point). The lunar limb has great basins and great mountain ranges along it which can affect the exact timing of occultations. And also: "I cannot get the tool to function for the emmersion of the same event. I suspect that Mr. Reed's program does not function for the non-illuminated limb. That would be an odd lunar, an un-illuminated limb is not used." Yes, that's right. There's no way to treat an emmersion as a lunar distance, but perhaps there could be in some sort of "expert mode". Oh, and please call me "Frank" or "Frank Reed" --no need to be formal with someone like me. And you concluded: "Bowditch's navigator, mid 1800's, offers a method of solving this event and in particular offers it up as a method of correcting your chronometer." This section in Bowditch was really irrelevant to most users. I suspect it was something Bowditch wanted to see included in later editions for his own particular, personal reasons. I've never seen a single case in 19th century logbooks of anything remotely resembling an occultation calculation although it's certainly possible that some land-based users of Bowditch's book tried that procedure. It was probably just too much work for navigators at sea, and anyone with the high competence necessary to work an occultation probably would never be without a good sextant (which would let you do lunars any old time as opposed to occultations which are rather rare events). And you concluded: "If your chronometer was accurate, however, it could be flipped over to determine longitude." Actually, what you get from an occultation with a known GMT is equivalent to a "lunar distance line of position" with the distance equal to zero. It's not necessarily a longitude. It could even be a latitude. Of course, if you assume an exactly known latitude, then any line of position, except one parallel to latitude, crossed with that known latitude will yield a longitude. But this is a modern point of view... From a historical point of view, an occultation, after some reduction work, could give GMT just like a chronometer. That combined with a time sight (an ordinary altitude sight of some body near the prime vertical converted to a value for local time --effectively using your sextant as a sundial) would then give you longitude. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---