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    Re: Pub 229
    From: Frank Reed
    Date: 2017 Dec 7, 15:19 -0800

    Chris: 

    When using pub.229 and similar tables like 249, you do not get to pick your own AP. Instead you select an AP that falls on an integer grid of Latitude and LHA values. So when you say that you're using "AP: N 44° 30', W 73° 15' ", that just doesn't fit. That would be a good AP position for other forms of the intercept method, where you're allowed to select any convenient point, but not for 229. Instead, first of all, you have to select an integral latitude. And I think when you actually used the tables, that's what you did. You apparently used N 44° for your AP latitude.

    The "weird" and puzzling part for many users of 229 and 249 is the process of selecting the AP longitude. Your AP longitude has to be chosen according to a simple rule (keeping it very simple for now by staying in the western hemisphere): your minutes of AP longitude should be identical to the minutes of the body's GHA, and your degrees of longitude should match the degrees of your estimated longitude (possibly +/-1 so that the resulting position is as close as possible to your best estimated longitude). Given your body GHA of 75° 18.7', we know that we need an AP longitude ending in 18.7'. Next we need our AP degrees equal to your estimated longitude degrees. You believe you're near 73° 15' W so the degrees have to 73. Putting those two parts together, the AP longitude is 73° 18.7' W. So this is where we plot from! The AP position is 44° N, 73° 18.7' W.  

    Into the tables now! When you enter Pub.229 for that latitude, 44° exactly and LHA 2° exactly (exactly 2° because we have followed this carefully crafted rule for selecting the AP lon), you get your Hc and Zn (I'm assuming you did the table work correctly), which yields an intercept of 29.6 miles and since the observed altitude is lower than the tabulated altitude, we must be farther away from the Sun so those 29.6 miles are measured off away from the Sun's azimuth. This gives you an intercept "dotted line" that's 29.6 miles long pointed just 3.7° to the east of due north, and this line is plotted from that AP position: 44° N, 73° 18.7' W. If you plot that out, and then draw your line of position at the end of the "dotted line" perpendicular to it, you should find it passes very close to your known observing position. It works!

    Yeah, it works, and it's fine for rote computation. But there are lots of other options to consider. And if this "weirdness" of working from a special AP, designed for the convenience of the tables and not the convenience of the navigator, bothers you, then certainly you should switch to different tables. Many people like H.O. 208 and if you're trying to emulate aviation practice from 75 years ago, they're a great choice.

    Frank Reed

       
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