Noon Sights
The Sun is overhead, what will you do for your noon position ?

From: Jacky Wong

To: navigation Mail list

Thanks members. I was suprised when I saw so many response to my quiz and all replys has touch one or two points.

Summerising the response we can see some points :-

i. It is difficult to take a sight with such high altitude because the azimtuh change rapidly. The restricted space in boats added the difficulty.

ii. The result is not a parallel Latitude, it should be a position circle.

iii. No sight reduction is required, ship's position can be fixed and the geographic position of the Sun it useful.

iv. This only occur in low Latitude place, between 23N and 23S.

Below is what I normally do in this spacial occasions - it is not only a principle but things that I had done it myself for many times sucessfully :-

i. We estimate the meridian passage time as usually. The time normally accurate to seconds.

ii. prepare what you need to shot a 'STAR' - papers, pen, stop watch etc...

iii. Start taking a series of Sun sights about ten minutes before the meridian passage and until ten minutes pass the meridian passage. Record each down of course. Try to catch one at meridian passage if you can. I normally take five sights (sometimes seven), two before and two after MP.

iv. You need to swing your self to get the real direction of the SUN. One trick in taking sights is you don't continue adjust your sextant reading to catch the changes. You should set the reading over the existing situation and wait the change to match it - touching the horizon, then press the stop watch.

v. You may need to take a back altitude - over 90 degrees in a reversed direction - if so happen you are facing the wrong side. It is not a problem because the sextant have a 120 degree arc.

Now, the calculation part.

vi. Use an almanac to find out all the GHA and DEC (should be the same for all sights) of the Sun for the times you shot the SUN and convert it to Lat Lon equivanlants.

vii. Reduce your sights to get the true altitude like what you normally do and minus it by 90 degrees to get the true Zenith distance.

viii. Plot the geographic positions of the sun on the chart. Run the position to same time base or to 1200 if you prefer.

ix. use the true zenith distance as the radii and the geographic positions as centres and plot out all position circles.

x. These circles should meet or intercept at a point which is your ship's position.

I agreed this method require practice but it yield a good fix - don't need an AP, calculator or even the nautical tables.

Contributed by:

Jacky Wong, Hong Kong
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