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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sun sights
From: Gary LaPook
Date: 2011 Jan 31, 01:11 0800
From: Gary LaPook
Date: 2011 Jan 31, 01:11 0800
Two obvious errors, you transposed the labels for latitude and longitude and you made a math error, your intercept should be 40.2 towards not 10.2 towards (BTW, intercept is in NM not minutes.) Since you would expect the intercept for the first shot to be about the same as the other shots, there must be an error. Since it is easy, when using an artificial horizon, to mistakenly measure the upper limb of the sun instead of the lower limb, an easy check to make is to subtract twice the semidiameter from Ho and see if you get a more reasonable result. The S.D. was 16.2' so twice that is 32.4' subtracted from the Ho gives a corrected Ho of 33̊ 40.0' with a resulting intercept of 7.8 NM toward which is in line with the other sights. (An easy way to check that you are taking the lower limb is to try it both ways, touching the edges of the sun one way and then the other, the method producing the lower altitude is the correct way to measure the lower limb.) You can also adjust the Hc calculated from the AP to the coordinates of the DR so that you can directly compare your Ho with the Hc at the DR which then shows the accuracy of you sextant work. First we calculate the adjustment for the difference in LHA. The LHA at the DR is 345̊ 45.2', 14.8' less than the LHA you used for your calculation. Now look in H.O. 229 for latitude 37̊, declination 17̊ contrary and LHA 245̊ which is 34̊ 10.7' , 13.8' less than the tabulated Hc that you used. This means that Hc changes 13.8' less for a whole degree change (60') in LHA. So doing the proportion, (13.8 X 14.8 ) / 60 = minus 3.4'. Then we make the adjustment for the minutes of latitude, in this case 10' less than the latitude you used. This adjustment is made based only on this difference and the azimuth. Unfortunately the table to use for computing this adjustment is not included in H.O. 229 though it was in the earlier tables, H.O. 214. The table is available here: http://fer3.com/arc/img/105707.ho214delta%20lat.pdf Referring to this table for azimuth 164̊ and 10' of latitude shows the adjustment to be plus 9.6'. Combining these two adjustments with your Hc makes the adjusted Hc at the DR 33̊ 38.4'. Comparing this with the corrected Ho of 33̊ 40.0' makes the intercept 1.6 NM toward, not too bad! (BTW, to make the degree symbol use NUM LOCK followed by ALT 248 on the number pad, not the top row of keys.) Gl  On Sun, 1/30/11, Hewitt Schlereth <hhew36.com> wrote:
