NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Working a lunar
From: Frank Reed
Date: 2007 Sep 24, 00:01 -0400
From: Frank Reed
Date: 2007 Sep 24, 00:01 -0400
George H wrote: "Well, yes, but if it's going to be of use as an emergency tool, where are those precise lunar distance predictions going to come from? That was why I asked Henry for the source of the lunar distances he had used. And if it comes from some internet source, then I ask whether, at sea, in some undefined "emergency situation", such internet data is going to be available. If it's possible to print out in advance such predictions, and take them to sea, then that would indeed qualify as an "emergency tool" Or if Henry can take to sea some computing gadget that predicts such lunar distances on the spot from first principles, as is perfectly feasible, then lunars could be an emergency tool. But not, surely, if on-line web access is called for." First, I'm sure we all agree that the idea that there will be an emergency situation where you would lose GMT with no means of recovery is pretty outlandish! Almost any radio station will eventually give you a usable time signal. But let's suppose it's part of a game. You're sailing without any electronics (except stored away in a sealed case to be used in case of a REAL emergency) and it's your job to sail by traditional means. So you decide to shoot a lunar... Where do you get pre-calculated lunar distances? Well, you can pop on the Internet the day before you set sail and grab eight or ten weeks worth from any of the various sites that have them (Steve's, Ken's, mine, and others) and print them out. If you decide to stop in Hawaii for a month or two on your trek and your pre-printed tables expire, you hit an Internet cafe the night before re-starting your voyage and you print out a fresh set. No problem! But what do you do if the pet goat on your sailboat eats your lunar distance tables?? Is the game over? Well, no (so long as the goat didn't also eat this year's Nautical Almanac). You can calculate the predicted LD for the hour before and the hour after as a standard great circle distance problem using the GHA and Dec from the standard pages of the almanac. There is a slight inaccuracy introduced here, but nothing much to worry about. It's more work, yes, but it's only a game... Another use for lunars today, as I described last year, is to generate a positional fix when the horizon is obscured. Two sets of lunars, involving two separate astronomical bodies, gives you a complete fix in longitude and latitude. This is the sort of game trick that might come in handy if you're trying to win a sailing race using a celestial handicap --or maybe just trying to win a bet (I assume that the rules allow a computing device since this is computationally intensive, and in fact most sailing race rules apparently do not care how you work the sights, electronically or by-hand). -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---