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    Re: 1973 Altitude Correction tables
    From: Frank Reed
    Date: 2018 Mar 30, 09:02 -0700

    The refraction tables in the official Nautical Almanac have changed very slightly since 1973, but it's not meaningful to navigation. You can use those old tables, no problem. One issue with the sun correction tables is that they jump from winter to summer values. This reflects the small change in the Sun's SD from perihelion to aphelion. You can do a little better by taking the SD from the daily pages and adding that to the refraction from the "stars" table (which is pure refraction). And if you want still better accuracy, you add 0.1' for altitudes below 70° to account for the Sun's very small parallax in altitude.

    You say you had a latitude error of a "few n.m." Did you update the Sun's declination correctly for UT? The dec is changing at a rate of about 1 knot right now, north-bound, so if you make a mistake on UT that can add up quickly (e.g. if you calculate UT/GMT from your local zone time and get the daylight time change backwards, that will yield a two-hour difference implying an error in the dec of 2.0'). For "typical" celestial accuracy of about a mile, you only need the UT accurate to the nearest hour, but if you want to get to the calculational limits for the latitude at noon then you should use the UT accurate to the nearest six minutes or so.

    Frank Reed

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