Kat Murphy

local apparent noon

with one comment

Often understandably confused with Semester at Sea, SEA Semester is a program I participated in last January. I spent six weeks in Woods Hole, MA taking classes, and six weeks sailing aboard a 134 ft steel brigantine sailing vessel. I recently realized that around this time last year I was in Woods Hole studying…and freezing.

I was responsible for three classes: nautical science, oceanography, and maritime studies. Nautical science is both unbelievably frustrating and surprisingly satisfying. It was interesting to learn how difficult it was to navigate without modern day GPS, or even the capability to determine one’s longitude. Many sailors relied heavily upon local apparent noon, or LAN, to determine their line of latitude. Local apparent noon is the time of day when the sun is at its highest altitude, and is measured with a sextant. The angle measured is called Ha. The Ha angle is then used in a series of sight reduction calculations and you are given an Ho angle. This angle is later plugged into a formula used to determine zenith distance, one of the two components needed to determine latitude. Zenith distance is calculated through the formula 90-Ho=ZD, and is a measure of our angular distance from the sun. The other component needed to determine latitude is the sun’s declination. Declination is determined through an almanac. The formula for latitude can now be determined by drawing a simple diagram:

Latitude formula diagrams from Carl Chase's text, "An Introducation to Nautical Science"

Latitude formula diagrams from Carl Chase's text, "An Introduction to Nautical Science"

As you can tell by the diagrams the formula changes based on the season, and has a total of three possibilities:

Latitude= zenith distance + declination

Latitude= zenith distance – declination

Latitude= declination – zenith distance

And this is just the beginning! There are many other lines of position (LOP) that can be determined. The LOP determined through LAN is only one of the many ways to figure out your location. LOPs can be determine at any time using the sun as well as the stars. The trouble is getting a fix (fixed location). Fascinating!

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Written by katmurphy

January 18, 2009 at 5:41 pm

One Response

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  1. Please contact me in regard to celestial navigation. RAMS
    John Greenlaw
    my e-mail address is palmalito@tamapbay.rr.com
    I have a question I would like to discuss with you. I’m a WWII Navigator.
    Thank you and I look forward to hearing from you
    John

    Stephanie Gardner

    November 15, 2011 at 3:27 pm


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