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Hi folks, as usual great stuff! I noticed a number of members averaged
the entire moon set but upon my graphing the set it becomes readily
apparent sites 21-01-40 and 21-03-22 are rejected therefore the line
of best fit falls nicely thru the remaining 6 sites and solving no 3,
the 21-00-48 site, yields an agreeable solution a

On Jun 12, 5:53�am, "Andres Ruiz"  wrote:
> The average in this case is incorrect, see the Hs graph:
>
> -----Mensaje original-----
> De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de George Huxtable
> Enviado el: s�bado, 07 de junio de 2008 15:57
> Para: NavList@fer3.com
> Asunto: [NavList 5360] Re: Exercise #12 Daylight Sun/Moon Fix
>
> I suspect that there are many more unposted attempts at Jeremy's exercises,
>
> behind the scenes, than actually turn up on Navlist.
>
> Here's my go at #12.
>
> Date 28 May 08
>
> Start with a position line for the Sun. Without a 2008 almanac, I have to
>
> rely on my pocket calculator. Its Sun predictions should be good, and should
>
> correspond to the Almanac's. Someone please tell me if they don't.
>
> at UT 21h 06m 15s, I get Sun dec +21� 37.5', GHA 137� 13.2', semidiam 15.8'
>
> corrected altitude from Sun LL is obtained from-
>
> �16� 43.1 � � Sun sextant altitude
>
> - � � � 9.9' � �dip from 106 ft. up
>
> - � � � 3.2 � � refraction
>
> + � �15.8 � � semidiameter
>
> + � � �0.1 � � parallax
>
> ======
>
> 16�45.9 corrected altitude.
>
> Without having alt-az tables for that latitude band, I have to calculate the
>
> Sun altitude using a program which gives me great-circle course and distance
>
> in miles from A to B. From an assumed position of N 14� 37.9' and long of E
>
> 145� 18.6, to Sun at N 21� 37.5', W 137� 13.2', I get a course (= Sun
>
> azimuth) of 71.4�, and a great-circle distance of 4394.9 miles,
>
> corresponding to 73�15' zenith distance, or 16� 45' altitude, to compare
>
> with 16� 45.9' corrected altitude. At the moment of the Sun sight, then, the
>
> Sun was actually 0.9' higher in the sky, and therefore 0.9 miles closer to
>
> the Sun's GP, than �was assumed. So it's on a position line, displaced from
>
> that assumed position �by 0.9 miles in the direction of 71.4�, the line
>
> being drawn at right angles to that displacement.
>
> The Sun was on that line at 21h 06m 15s, but with its course due East at
>
> 14.3 knots, then at 21h 00m it was 1.5 miles further West, so next we shift
>
> that position line bodily sideways by 1.5 miles to the West.
>
> Now for the Moon. Averaging the 8 observations, I get the mean altitude of
>
> 72� 26.0', at a mean time of 21h 01m 57s. Hope others agree.
>
> For that moment, my pocket calculator predicts Moon dec = -3� 05.2', GHA
>
> 214�58.6. It's less precise, for the Moon, than it is for the Sun, so I
>
> wonder what others get. In this exercise, the geometry is such that only the
>
> dec matters, not the GHA.
>
> Working from the same Assumed Position as before, and using the same
>
> technique as for the Sun, I get the calculated Moon altitude to be 72� 16.9
>
> at an aziimuth of 179.8�. So the Moon is very nearly due South, and what we
>
> are finding from it is simply our latitude.
>
> We need to compare that altitude with the sextant altitude, after all
>
> corrections have been made, so the next step is to make those corrections.
>
> 72� 26.0' � Moon sextant altitude.
>
> - � � � 9.9' � Dip from 106 ft. up
>
> - � � � 0.3' �refraction
>
> - � � 15.5' �semidiameter Moon (using UL)
>
> + � �17.6' �parallax taking HP = 57.6 �and calculating HPcos alt.
>
> ==========
>
> 72� 17.9' corrected Moon altitude. This is just 1 mile greater than the
>
> altitude we calculated from the AP, so therefore we are just 1 mile closer
>
> to the Moon's position, or 1 arc-minute further South, which puts us on an
>
> E-W position line at N 14� 36.9. In this case, because that position line
>
> runs E-W, and the ship travels due East, the position line doesn't shift: it
>
> was in the same place at 21 h, near as dammit. And a bit of rough sketching
>
> gives a final longitude just 0.1 miles East of our AP, at 145� 19.6.
>
> So we can congratulate the people in Fort Worth for doing a remarkably good
>
> job in the GPS positions they are puttting out.
>
> Jeremy says "the latitude is consistently off", but it's only a mile out
>
> from GPS, according to me, so nothing to be ashamed of there. Indeed, it's
>
> well within the margin of error that can be caused by natural changes in the
>
> refractive part of the dip. Rather more interesting, to me, is the scatter
>
> in those Moon altitudes. I wonder why. Was the sea state good, at that time?
>
> Often, with a high Moon, contrast can be low, especially if the sky is a
>
> touch milky. That gets worse if you use an all-over horizon mirror, rather
>
> than a half-and-half split job. If Jeremy had recorded a number of altitudes
>
> for the Sun, as he did for the Moon, I wonder whether thet, too, might have
>
> shown scatter. What's his experience?
>
> I didn't really need to do all those hand-corrections, but could have left
>
> the lot to my calculator software instead. However, that inflexibly chooses
>
> a height-of-eye appropriate to the "bridge" of my little craft, at 6ft above
>
> sea level, rather than to Jeremy's 106 ft. If I adapt Jeremy's altitudes by
>
> subtracting 7.6' from them first, then it gives the same answer.
>
> George.
>
> contact George Huxtable at geo...@huxtable.u-net.com
>
> or at +44 1865 820222 (from UK, 01865 820222)
>
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>
> ================================
>
> Exercise #12 �Daylight Sun/Moon fix.
>
> This exercise is for our math friends. �It is a series of Moon lines
>
> taken near meridian transit, crossed with an early AM sunline.
>
> Plotting these lines, you will notice an averaged Moon line giving a
>
> decent position with lines within 5 degrees of the horizontal. � The
>
> sun line will be nearly vertical and gives a good Longitude line as a
>
> cross.
>
> In this case, my calculations show that Latitude is consistently off,
>
> but the Longitude is within a couple of tenths. �I am guessing that my
>
> I wasn't seeing the actual limb of the moon and therefore was off with
>
> the sextant observations by better than a minute of arc. �The moon is
>
> a strange mistress to try and shoot with the sextant.
>
> ----------------------------------
>
> UTC date is 28 May 2008. �The 21h 00m UTC �GPS fix was Latitude 14deg
>
> 37.9' North, Longitude 145deg 18.6' East. �The Ship is sailing course
>
> 090 at 14.3 knots. �Height of eye is 106 feet, Temp/Pressure is 84 F
>
> and 1010 MB. �Index error is 0.0. �The following observations were
>
> made (times in UTC):
>
> Sun (LL): Hs 16deg 43.1' @ 21h 06m 15s
>
> Moon (UL) Hs 72deg 25.6' @ 20h 57m 43s
>
> Moon (UL) Hs 72deg 25.8' @ 20h 59m 14s
>
> Moon (UL) Hs 72deg 25.8' @ 21h 00m 48s
>
> Moon (UL) Hs 72deg 27.2' @ 21h 01m 40s
>
> Moon (UL) Hs 72deg 25.8' @ 21h 02m 20s
>
> Moon (UL) Hs 72deg 26.4' @ 21h 03m 22s
>
> Moon (UL) Hs 72deg 26.0' @ 21h 03m 57s
>
> Moon (UL) Hs 72deg 25.2' @ 21h 05m 08s
>
> Determine a fix at 21h 00 UTC (07h 00m Local)
>
> Jeremy
>
> �image001.gif
> 7KViewDownload
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