NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Distance in Wikipedia
From: Renee Mattie
Date: 2007 Jul 25, 14:40 -0400
From: Renee Mattie
Date: 2007 Jul 25, 14:40 -0400
George, You remind us of something we really all "know" in a non-practical way, but tend to ignore because we don't typically use the sun to find the time. I have edited my edit (http://en.wikipedia.org/wiki/Lunar_distance_%28navigation%29#Theory) to reflect this fact. I have also made a note on the article's discussion page, because most of the people who read that article will not be reading the NavList (http://en.wikipedia.org/wiki/Talk:Lunar_distance_%28navigation%29#Esoterica ). But I think I am wasting my time on this example. It seems to imply that one can shoot the moon while making noon to required accuracy. I haven't tried it. I haven't even tried to make noon with my sextant. If someone here can replace that example with something conceptually as simple but not quite so misleading, that would be an improvement. Renee -----Original Message----- From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of George Huxtable george-at-huxtable.u-net.com |Renee Mattie on NavList| Sent: Wednesday, July 25, 2007 10:22 AM To: ..................... Subject: [NavList 3022] Re: Lunar Distance in Wikipedia Frank referred to- "this odd idea of the "Sun at | its zenith" when trying to say that it has reached its maximum | altitude. No one trying to communicate navigational or astronomical | concepts should use zenith that way. The zenith is straight up. That's | all. ========================== Response from George. Well, as one notorious for his own pedantry, I am agog. If Frank claims to be able to hold his finger in the dyke, against the tide of change, he will be sadly disappointed. Neologisms abound, even in the technical literature; indeed, especially in the technical literature. There are no rules. Words mean what people understand them to mean, writing them and reading them. Certainly, when unqualified, "the zenith" refers to the observer's zenith; straight up, and nothing else. What makes such a usage as "the Sun at its zenith" acceptable, if undesirable, is this. It's quite clear that it isn't referring to the "observer's zenith". Instead, it's a zenith belonging to the Sun, because it says so. And there can be little doubt that what's being referred to is the highest point in the Sun's path, so there's no chance of confusion. Is there an alternative single word available to say the same thing? "Culmination", perhaps, but to how many people would that have a meaning? But here a bit more pedantry is called for. Neither the Sun's "zenith", nor its culmination, nor its highest point in the sky, is the real moment of apparent noon. Local apparent noon is when the Sun crosses the observer's meridian, so to us in the North, it's exactly South of us. And because the Sun's declination is continually changing (at around 1 knot at the equinoxes, but not at the solstices) the Sun's altitude is changing slightly, even as it crosses the meridian. For example, at the latitude of Greenwich, around the Autumn equinox, the Sun's highest moment occurs something like 40 or 50 seconds before, at noon, it crosses the meridian. In spring, it's the other way. These are non-negligible time errors that have to be allowed for, in many applications. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---