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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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