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Posted by on Saturday, October 6, 2007 at 2:56am.

My previous posts have dramatically improved my understanding of how a NM is derived but other questions have been raised:

In most books entitled "Pass your Day Skipper" or "Yachtmaster" or similar nautical/navigational publications Latitude is described and illustrated as the angular distance 0 degrees to 90 degrees either N or S of the equator and in the illustrations the angles are drawn using a common centre of the earth. Is it fair to assume this is an appropriate simplification of the more complex reality.

I am having difficulty assimilating what used to be my understanding ie. the simplified version and my new understanding the more complex version.

OR am I just becoming more confused?

Mike

  • Physics/Nautical Mile - , Saturday, October 6, 2007 at 6:39am

    Those drawings are simplified. The only reason that the "new" definition of the minute of latitude being arc disance subtended by one minute from the center of curvature is that satellite mapping has made it possible to accurately map the real Earth Surface. For surface navigation, one common center of the Earth is possible. Few folks navigating are at the S or N pole, and those who do use GPS.

  • Physics/Nautical Mile - , Saturday, October 6, 2007 at 7:36am

    Hello again Bob, Thanks for your reply. The point that has grabbed my attention now is you refer to the "NEW" definition of the minute of latitude. Things are slowly falling into place at this end. Do you happen to know when the "NEW" definition of latitude was introduced?

    Mike

  • Physics/Nautical Mile - , Saturday, October 6, 2007 at 7:51am

    It couln't be done until we knew the shape of the Earth with some precision, in particular the semimajor and semiminor axis. WGS 84 is the system that defined these lengths with some precision. With that, the point of curvature could be determined, and thence the distance subtended.
    http://en.wikipedia.org/wiki/WGS84
    By the way, the international standard for a nautical mile is fixed as the average distance. All this is somewhat like mom's favorite apple pie recipe, it depends on who is eating, when, and what ingredients were available at cooking.

  • Physics/Nautical Mile - , Saturday, October 6, 2007 at 8:41am

    Hello Bob, love your analogy about mom's apple pie recipe.

    Allow me to enter the NM debate at this point.
    Do you happen to have a good explanation re the origin of the k in knots for "nautical miles per hour"?

  • Physics/Nautical Mile - , Saturday, October 6, 2007 at 9:08am

    http://mathforum.org/kb/message.jspa?messageID=1470195&tstart=0

    This is as I learned it many years ago as a seaman.

  • Physics/Nautical Mile - , Saturday, October 6, 2007 at 10:51am

    That makes sense.
    BTW, interesting thread.

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