NavList:

GoodDay Mr. van Asten,

The time of sunrise that I'm referring to and that I used in my gyro checks is the Local Mean Time of Sunrise as given in the Nautical Almanac for the three day period that includes the day of the observation.

The calculated true bearing is derivived from what is sometimes called the Bearing Amplitude of the sun. This time of true rising/setting of the sun (or any heavenly body) occurs when the centre of the body is on the observers Celestial Horizon to the east or west of his meridian. What I referred to as the visible time of sunrise is when the upper limb of the sun is just appearing above the horizon. However this is NOT the true time of sunrise. The true time of sunrise is nominally when the sun's lower limb is one half a semi-diameter above the horizon. This difference between visible rise/set and true rise/set is almost entirely due to refraction. The bearing measured at the true rise/set times are used to compare the gyro/compass bearing of the sun with the calculated true bearing or Bearing Amplitude. In fact the calculated time is not that critical (something like the calculated time of MerPass). The time is used just to make sure that you are there before hand and ready to observe the true rise/set phenomenon via the procedure noted above. However as can be seen in the next paragraph, sophisticated software does make a distinction between the visible and true rise/set times.

NAVPAC (currently at version 3.1.9) is the Royal Navy's Astronomical Navigation Package. This software can calculate the rising and setting times of navigational bodies. However before it does this it asks if you want a listing of visible rise and set times (to be used for planning navigation sights such as stars or MerPass) or whether you want the rise and set times for performing a compass check. The results that it prints out are different depending on how you answered that question. For instance on 14 Sep 2011 at Lat N 32 45.0, Long W 015 30.0 the suns visible rise and set times are 05:45 and 18:10 respectively. However the rise and set times to be used for a compass check are 05:49 at 085.6 deg true and 18:06 at 274.0 deg true. The above is from Figures 3-3a and 3-3c from The Admiralty Manual of Navigation Volume 2, 10th Edition, 2011.

I assume that STELLA, the celestial navigational software used by the USN/USCG, has a similar feature. However I don't know this for sure as I don't have access to STELLA as it is only available to authorized DoD users. If any NavList members are lucky enough to get involved with the actual teaching classes of the USCG student sample problems, then, since Frank has said that they use STELLA, that NavList member might gain some appreciation for STELLAs capabilities.

As I said in my original posting, I never used the sun rise/set bearing for anything other than to determine the gyro error. At the time I was basically just following the procedures as then taught in the Navigation Schools. The typical daily procedure was to do: morning stars, sunrise gyro error, morning suns, MerPass, afternoon suns, sunset gyro error and then evening stars. For the morning sun rise, given that one had just slightly earlier in time taken a round of 4 or 5 morning stars that still needed to be worked out, there seemed to be no need to do anything other than a gyro check with the sunrise observation. I wasn't into as much into theory back then as I am now so I didn't question the doctrine at that time.

Now that I no longer have to strictly follow that doctrine, let me wander a bit and ponder a bit about what I could have done with that true bearing of the sun. Picking a Bearing Amplitude purely at random (nudge, nudge, wink, wink) say 067 deg true. Then I would know that I was on a Line of Position that was 157-337 true. However in addition to the usual problem of selecting the best point on the LOP for your most probable position - from an infinity of choices, you would also have the additional problem of selecting where to place the 157-337 line itself - also from an infinity of possibilities. In this situation you can't position the LOP because you don't have an altitude to give you an intercept to select where that 157-337 perpendicular to the true bearing should go. So unless I'm missing something, doing anything with that true bearing associated with the true rise/set times other than determining the gyro/compass error seems, to my simple sailor mind, like a pretty impossible situation.

Further insights into this would be welcomed.

wmh

On 04 May 2013 @01:31 -0700 h.a.c. van Asten wrote:
__________________________< Original Message>___________________________

Nice comment as regards content, was your "true time" the Greenwich apparent , and your "visible time" the GMT watch time ? (I suppose so but am not sure) . You evidently compared succesfully the pelorus observation for azimuth with the gyro indication of compass point and azimuth , does this mean that an observation by pelorus gives you a reasonable guess of your longitude ?

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