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Watch as compass
From: Frank Reed CT
Date: 2007 Jul 24, 23:25 -0700

```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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