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    Re: Watch into Compass?
    From: Doug Royer
    Date: 2004 Feb 11, 12:32 -0800
    Frank,when you get some time to finish part 2 of your stellar angle paper I will most definitely read it.
    -----Original Message-----
    From: Navigation Mailing List [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Frank Reed
    Sent: Wednesday, February 11, 2004 11:32
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Re: Watch into Compass?

    Dan Allen wrote:
    "One technique that is presented which I remember learning in Boy Scouts is using a watch to determine North or South.  [...] I seem to recall somebody declaring this procedure useless, but I do not remember the details.  Is this okay for emergency navigation?"

    It's not useless, but the way it's usually taught it can go very badly wrong. If the face of the (analog!) watch is held horizontally, then this method makes the mistake of equating local hour angle with azimuth. Whenever and wherever the Sun's maximum altitude for the day is low, this isn't a terrible approximation. So in mid-northern latitudes when you're not close to the June solstice, it's ok. But any other times and places, it IS a terrible approximation. Clearly, in the tropics, this watch method would fail rather spectacularly.

    I think it's obvious that it would be much better to teach people the simple rule that one hour of time is fifteen degrees of local hour angle, and then explain how LHA is related to the position of the celestial pole in the sky. This sort of basic astronomical information is not that much harder to teach than the Boy Scout watch rule --and it works with digital watches, too!

    Analog watches are not quite as rare as sextants today, but they're not all that common either. More people carry cell phones than analog watches by a wide margin. A cell phone isn't going to do you much good in the middle of nowhere, but amazingly the Boy Scout watch compass trick is a widely available software download (it's in Java) for modern cell phones. The instructions have you place the phone on a level surface, point an arrow at the Sun, click "ok", and then an arrow appears that points towards North. It's exactly the same trick inside as the Boy Scout analog watch method, and it has the same serious flaws.

    Here's a puzzle: can you modify the Boy Scout watch method so that it does not confuse azimuth with local hour angle and thus becomes a somewhat more accurate compass? Note that we don't need to make it perfectly accurate since you will have an unavoidable error using any variation on this trick from equation of time and zone offset (things that you could correct for easily if you kow what you're doing but a technique like this is presumably designed for people who do not know what they're doing).

    Note: I've been very busy the past two weeks but hope to be posting again more frequently soon.

    Frank E. Reed
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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