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    Re: Time sight primer
    From: Chris Mitchell
    Date: 2019 Sep 25, 16:41 -0700

    Good Day All,

    This is my first post to the website. I have been browsing the site for the past few days and I am impressed with the amount of quality information and knowledge available here. I was intrigued by Frank's post, “time sight primer” from 24-December-2012 and decided to try to work out my longitude and time from a sight that was taken while at sea a few weeks ago. The below information is from a time sight taken at sea on September 3rd, using the Star Pilot software for sight reduction giving an intercept of 0.0', so I thought it would be a good example to try:

    Date: 03-Sept-2019

    GPS: 46 46.7' N 048 02.7' W

    Time: 20:30:30 UTC

    I.E: 1.0' off arc

    H.E: 21.8m

    Sextant Alt of Sun LL: 12 19.7'

    Declination: N 7 25.2'

    True Altitude of Sun LL: 12 24.3'

    Latitude: 46 46.7' secant 0.16442

    Polar distance: 82 34.8' co-secent 0.00365

    Sum: 141 45.8'

    1/2 Sum: 70 52.9' cosine 9.51524

    Latitude: 12 24.3'

    Remainder: 58.28.6' sine 9.93066

    Ht: 9.80698

    Sine related to Ht: 5:19:02 PM

    UTC: 8:30:30

    Difference: 3:11:28

    Longitude: 47 52.0' W

    With an intercept of 0.0' from the sight reduction I thought that by following the examples in an early 1800's version of Bowditch would have resulted in a longitude closer to the GPS position. Since this is my first attempt at this calculation, no doubt I have probably made an error somewhere along the way. Perhaps someone would be so kind to give me some guidance. I look forward to your replies.


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