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    Re: Difference between HO 249 and USNO website
    From: John Almberg
    Date: 2016 May 17, 17:04 -0400
    Thanks for the tip of using my actual position as DR position. That would make things a bit easier.

    About the refraction/parallax correction: I didn’t include it because 1) I figured it was insignificant compared to my other errors and 2) I wasn’t sure what value to use when using the astrolabe.

    Now that I’m looking at the USNO values, I’m wondering if I could just use the value in A2 for stars and planets. It’s the correct value, i.e. 2.4

    I’m not sure I’m using this site correctly... I’ve posted 2 messages which did not appear, for some reason. Trying the post-by-email route this time. Hopefully it will get through.

    — John




    On May 17, 2016, at 1:53 PM, Gary LaPook <NoReply_LaPook@fer3.com> wrote:

    It appears that your problem is that you used different assumed positions for tach computation. Look at your input data to the USNO website, it is different than the AP shown in block 6 of the 249 reduction. When you do that you find the Hc is 21° 22.6'

    The good news is that you have mastered HO 249. When comparing you measured altitudes .to computed values to see how accurately you have measured the altitude it it best that you use your known location as the assumed position, as you did on the USNO website, so you can then compare that directly with your measurement  which shows that your's was 17', so your LOP will be only 17 NM from your location which is good work with an astrolabe. There are ways to adjust the HO 249 computation so that you can use your actual position as the AP and I have uploaded the tables for this to Navllist in the past. but it is easier to use the USNO site. When using an astrolabe or bubble sextant or artificial horizon you do not apply dip or semi-diameter so the only corrections that apply is refraction and parallax in altitude. Looking at your result from the USNO you see that when you combine these two corredcions they come to - 2.4'. If you apply them then your Ho becomes 21° 27.6' and the intercept increases to 19.4'.So when shooting altitudes for practice you should continue to use your actual position when using the USNO website. If testing your use of HO 249 then use the same AP for the USNO website.

    gl


    From: John Almberg <NoReply_Almberg@fer3.com>
    To: garylapook---.net
    Sent: Tuesday, May 17, 2016 9:53 AM
    Subject: [NavList] Difference between HO 249 and USNO website

    Hi there. I am wondering why the sight reduction results from HO 249 and the USNO website (http://aa.usno.navy.mil/data/docs/celnavtable.php) differ so much. 
    I've attached my own calculations using HO 249, and the results from the website. The Zn value is the same, but the Hc values are significantly different. Is this normal? Or am I doing something wrong (the most likely scenario!)
    I should mention that I took the sight using a homemade 'astrolabe', so in my own calculations I am including no altitude corrections. i.e., no dip and no SD correction.
    Any thoughts, much appreciated.
    -- John


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