NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Exercise #12 Daylight Sun/Moon Fix
From: Mike Burkes
Date: 2008 Jun 13, 06:22 -0700
From: Mike Burkes
Date: 2008 Jun 13, 06:22 -0700
Sorry I accidentally hit send. My finishing statement: "yields an agreeable solution and a run-on sentence"! Mike Burkes > 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 > > _________________________________________________________________ Now you can invite friends from Facebook and other groups to join you on Windows Live� Messenger. Add now. https://www.invite2messenger.net/im/?source=TXT_EML_WLH_AddNow_Now --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
