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    Re: watch as compass
    From: Gary LaPook
    Date: 2007 Jul 25, 01:36 -0700

    Gary LaPook adds:
    
    Yup, that's why I never leave home without my astrocompass.
    
    gl
    
    
    
    
    Frank Reed wrote:
    
    >I know at least one other list member who caught the "Man vs Wild"
    >marathon this past weekend on the Discovery Channel, so I thought I
    >would post my little rant on using a watch as a compass... btw, it
    >turns out the show may have to be re-named "Man vs Motel". Read here:
    >http://www.reuters.com/article/televisionNews/idUSN2439321520070724
    >
    >
    >Using a watch as a compass as seen on "Man vs Wild" is a good trick in
    >those latitudes and times of the year when the Sun's maximum altitude
    >during the day is below 45 degrees. Under that condition, the error in
    >direction is rarely greater than ten degrees. But when the Sun's
    >maximum altitude during the day is greater than 45 degrees, the error
    >can become quite large. For example in the last week of June, in 40
    >degrees north latitude, about the latitude of New York City, the
    >maximum error is 35 degrees and the typical error during the day is 20
    >degrees (with an additional error that I'll describe below). Worse
    >yet, if we go to the latitude of Miami, Florida in late June, the
    >maximum error is above 70 degrees and the typical error is around 45
    >degrees. That is, this 'watch as compass' trick will point you in the
    >wrong direction by 45 degrees on average and as much as 70 degrees --
    >you will believe you're facing south when you're actually facing east.
    >Anywhere in the tropics, the error can approach 90 degrees. One saving
    >grace for these large errors is that the error in morning hours will
    >be cancelled by the error in the afternoon, so if you walk all day,
    >you'll zigzag significantly, but you'll end up going generally in the
    >intended direction.
    >
    >The error I'm describing here results from an intrinsic geometric
    >error with this technique. It equates "azimuth" or true compass
    >bearing with "local hour angle" (in simpler terms, it says that the
    >Sun is exactly due east at 6am, exactly due south at noon or due north
    >at noon in the southern hemisphere, exactly due west at 6pm, and
    >moving uniformly in compass bearing for times in between. In general,
    >that just isn't true. When the Sun's path across the sky is low, this
    >is not a bad approximation. But when the Sun's path is high, the
    >approximation is poor and so are the results as outlined above. In
    >addition to this basic geometric problem, the method has another
    >source of error. It depends on the fact that local watch time is
    >nearly equal to local apparent time (Sun time). Because of the tilt of
    >the Earth's axis and the Earth's varying speed in its motion around
    >the Sun, summarized in something known to astronomers (and navigators)
    >as the equation of time, the Sun can be fast or slow by as much as
    >fifteen minutes. Additionally watches are set to "zone time" which
    >will differ from local mean time by 30 or even 45 minutes. Added
    >together, these effects together mean that the time on a watch differs
    >from local apparent time by as much as one hour. This will lead to an
    >additional 15 degree error in compass direction (on top of the errors
    >I've previously mentioned). In short, this method of determining
    >compass direction by the hands of a watch has very low accuracy except
    >in areas well outside the tropics.
    >
    >Ironically, in the episode of "Man vs Wild" where he uses his watch as
    >a compass, Bear Grylls is in the Kimberley Outback in the wet season.
    >At this time of year the Sun passes nearly straight overhead in that
    >latitude. This is the very worst time to use this trick, and it would
    >have led to errors in compass direction approaching 90 degrees. This
    >is just the sort of thing that would get you very lost. It's fairly
    >clear evidence that the show involves a bit of illusion. It's partly
    >staged (it is after all, a tv show).
    >
    >Finally, the Sun can indeed be used as an accurate compass if you know
    >the local time, but you need to know a little basic astronomy to get
    >it right. I won't go into details on this. I just wanted to point out
    >that the watch trick can go very badly wrong in some latitudes at some
    >times of the year. It's "better than nothing" but not much.
    >
    >-FER
    >www.HistoricalAtlas.com/lunars
    >
    >
    >>
    >
    >
    >
    
    
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