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» Online Classroom   »   » Public Discussion of Cel Nav   » Altitude Intercept

Author Topic: Altitude Intercept

 - posted July 20, 2009 06:12 PM      Profile for LSzabo           Edit/Delete Post 
I live in New York, on Long Island, and sail the Great South Bay. Because I take most of my sights within the bay I have to apply Dip Short to reduce my sights correctly. I am getting very large differences in my altitude intercept value that I cannot understand. I’d like to share with you a sight reduction I made on Sunday, June 19th. It is a perfect example of my problem.

Date of sight is June 19, 2009. DR Lat is 40d 39.8' N, DR Lon is 73d 20.6' W. Sight is of the Sun, lower limb. Watch time is 11h 46m 48s, with no correction needed. My ZD is 5 hours. My GMT is 16h 43m 48s, which yields a GHA Hr of 58d 26.1' a GHA Min/Sec of 10d 57.0', for a total GHA of 69d 23.1' . My a-Lon is 73d 23.1', which gives me a difference of -4d. LHA = -4d + 360d or 356d.

My Declination is N20d 55.1', corrected by d corr = -.4, or N20d 54.7'. Therefore, my ending LHA = 356d, Declination = N20d, and a-Lat = 41d.

My tabulated Hc is 68d 44' with a "d" of +59 and a Z of 170d. The Dec Min correction is +54.7', giving me an Hc of 69d 38.7'

My Hs was 64d 17.6'. Index corr is -2.5' (on). Dip Short = -3.5'. Ha was calculated to be 64d 11.6'. My altitude corr = +15.5', giving me an Ho of 64d 27.1'. Note: Dip Short was calculated using a height of 6 feet and a distance of 1.1nm. My sextant is an Astra IIIB, with no side error, no perpendicular error, and an Index Error of -2.5' (On).

So, after the sight reduction is completed I end up with an Ho of 64d 27.1' and an Hc of 69d 38.7'. That means my altitude intercept is over 300nm! Obviously there is something very wrong here but I just can't understand what it is. All of my sights are yielding this type of results.

I would greatly appreciate you reviewing the above sight reduction and pointing out what I am doing wrong.

From: Babylon, NY
David Burch

 - posted July 20, 2009 06:35 PM      Profile for David Burch           Edit/Delete Post 
May i ask first about the 3m difference in the two times you mention?
From: Starpath, Seattle, WA
Capt Steve Miller

 - posted July 21, 2009 06:38 AM      Profile for Capt Steve Miller           Edit/Delete Post 
Upon looking at your data, it appears that you are using the wrong zone description. Yes, you are physically in zone +5, but remember you are under Daylight Savings now so your calculations should be using a zone description of +4. I worked your data with the different zone description and a reasonable answer is produced.
From: Starpath

 - posted July 21, 2009 10:21 AM      Profile for LSzabo           Edit/Delete Post 
How embarrasing. I gave you a WT of 11h 46m 48s. It should have been 11h 43m 48s. I just read your post correcting my ZD. I guess I misunderstood how to do that. I will rework the sight. Thank you for your help.
From: Babylon, NY

 - posted December 27, 2009 01:26 PM      Profile for robertl           Edit/Delete Post 
I've jumped across from Radar for a bit of recreation and re-worked this data manually with lookup using NA then direct calculation using law of cosines and then checked again with StarPilot, both giving me an Hc of 67d 13m from GMT 15:43:48 40d 39.8mN and 73d 20.6m W. I concur that Ho is 64d 27.1m. Can someone confirm the Hc for me please. If I am correct then there is something else going on here ...... 166.7nm seems an unreasonable intercept.

 - posted December 27, 2009 02:17 PM      Profile for HHEW           Edit/Delete Post 
Without the date and body, there's not much I can do. It's cool and rainy here, so I'm indoors for the day and will be glad to assist.

 - posted December 27, 2009 03:11 PM      Profile for robertl           Edit/Delete Post 
Thanks. The data is as given in the original post above by LSzabo, with the time and ZD corrected as per the ensuing posts ....

 - posted December 27, 2009 04:25 PM      Profile for HHEW           Edit/Delete Post 
Yup, I didn't twig to that.

Anyway, since the original post is dated July 20, maybe the sight was taken the previous day, not the previous month.

So, let's try a date of July 19 instead of June 19 and see if that makes more sense.


 - posted December 28, 2009 02:03 AM      Profile for robertl           Edit/Delete Post 
HHEW, That appears to be the error. Swapping June for July gives Hc at 64d 21.9' which works for me! Appreciate your input. Robert.

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