NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigation exercise
From: Bill B
Date: 2008 May 25, 02:22 -0400
From: Bill B
Date: 2008 May 25, 02:22 -0400
"George Huxtable responded: > The rise, > and the fall of the Sun are indeed quite symmetrical, which is why his > folding-paper trick will work. It comes down as a mirror-image of the way it > went up. It's just that the centre of that symmetry is displaced from Local > Apparent Noon, exactly as I explained, when there is a North-South component > of the vessel's velocity, or a changing declination. Darn it George, it is my turn to be pedantic. If the declination of the body, the sun in this case, changes during the observations it cannot be literally a mirror image (even from the same location). You did cover this under, "... or a changing declination." Common knowledge is--or at least as I understand it--the declination of the Sun is constantly changing at a rapid hourly/daily rate (as opposed to stars). I believe I addressed this in my infamous "sea monkey" post.NavList 2453, 22 March, 2007 "Why in the name of all that's holy do you do that yourselves? There are only two times a year, and two longitudes on earth that the sun will be 0 declination at LAN. It will have a lower declination before LAN and a higher declination after LAN now and vice versa in September, so Hc and the slope/curve will be affected and not symmetrical. I went through the trauma of the sea monkey kit being brine shrimp in my youth. Then, before cel nav, the belief that the sun would rise exactly in the east and set exactly in the west; and the day (sunrise to sunset) would be exactly 12 hours long. I would check out my topo map, find a farm field(s) with equal elevation miles away, adjust my Silva Ranger for declination (variation) and wait with my binoculars for sunrise and sunset. Always disappointed. Now I know unless there is a bizarre and serendipitous shift in refraction both at sunrise and sunset it will not happen. Also that I will not get a pony for my birthday." Glib moments behind, from my beginning days--which included a preoccupation with LAN for reasons already posted--I have been interested in elegant (simple on the water) method(s) of accounting for the vessel's motion. As I now understand my memory is not as acute as I believed it was, and as list turnover results in many new readers, please (George et al) recap the methods of accounting for vessel motion. Or point me to the archive's thread(s). > when there is a North-South component > of the vessel's velocity. What about east/west displacement if trying to determine longitude from LAN? Thanks Bill B. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---