NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Precomputed lunar distances
From: Bill B
Date: 2005 Apr 20, 00:25 -0500
From: Bill B
Date: 2005 Apr 20, 00:25 -0500
Frank > Bill you wrote: > "In fact that is not what raised a red flag for me. I had drilled down too > far and done a scatter graph with Excel, so every or hundredth or thousandth > was magnified." Frank responded: > Yeah, I wondered if that was part of the problem. Remember, if your input > data is accurate to the nearest tenth of a minute of arc, generally you should > quote your output data to the nearest tenth as well. Anything beyond that is > just random garbage. Point well taken. If I recall the concept of "significant digits" is centuries old (not that I am up to that math level yet ;-) Determining when to round, and how many places to carry forward when using a handheld calculator or computer application for computations is a work in progress for me. If I recall correctly, Alex informed me that a significant digit can be added in division of a ten figure average--but that does not apply in this case. All I can accurately state; when someone I respect sends me refraction correction figures to 5-or-so places I respond in kind. Much like the story of the daughter that asked her mother why she always cut the end off the roast before putting it in the roasting pan/oven. Mom replied, "That's the way I learned it from my mom." So they phoned the daughter's grandmother and asked her. Grandma replied, "because my pan was always too short for the roast."Bill wrote: > And: > "Regarding my question, "Another hypothetical scenario. If I take the same > two stars, calculate true separation of 34d 27.7', they have identical Hc's > of 1d 36.8', and hypothetical refraction is -88d, what separation might I > expect to measure with a sextant?" > Frank responded: > I didn't respond to this before because I cannot for the life of me figure > out what you're getting at. If you have two stars with an unrefracted distance > of 34 deg 27.7' and you observe them down at 1.5 degrees altitude, then the > measured distance will be very close to 34d 27.7'. What's this "-88d" number? Perhaps I misstated. Their true/calculated, unrefracted altitudes are nominally 1.5d above the terrestrial horizon. (A value I chose as the center of the sun can be almost -50' true/Hc altitude and still have the upper limb visible, and if I recall list postings stars extinguish near the terrestrial horizon, how near I do not recall--so left a little leeway as I did not want to muddy the waters with technicalities.) The theoretical refraction figure of -88d was proposed to test my understanding of movement up the triangle sides, as well as limits of the refraction-separation formula. (I acknowledge this refraction value is *way* outside physical reality as Earthlings experience it -- but it is sometimes useful for me to reduce an argument to the extreme/absurd.) Sorry for any confusion. My assumption was that given the above *theoretical* scenario, the stars would be lifted up and towards the zenith by 88d (up the triangle sides, straight or arcs ;-) and therefore be *observed* as being close to 0d apart. Bill