Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    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.
    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:
    >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.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site