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    Re: Distant Horizon
    From: Frank Reed
    Date: 2023 May 17, 15:19 -0700

    As you say, it sounds like the horizon you're looking at is very close to a surrogate sea horizon, so for those directions where it works, yes, it's like the open sea! As David Pike suggested, your best test here is the ultimate test: experiment. Give it a try. Shoot some altitudes, and see if you can detect any systematic issues. By the way, the "square root" approximation for dip is valid for all heights of eye where any actual horizon is still visible (not clouds, not oxygen levels seen from space... I mean the actual "sea horizon").

    Whether you're taking sights off that little piece of GSL or off an artificial horizon, you should bear in mind that your location is high enough above sea level that you would need to correct refraction in angular altitude for the other kind of altitude: altitude above sea level. Whatever your sea level tables give for refraction has to be reduced by some factor.

    Let's call your altitude 4500 feet. At that height the mean density of the air is lower by about 15%. Take any standard refraction value and multiply it by 0.85. So suppose you're looking at some object low in the sky and your tables tell you that the refraction should be 4.5'. Multiply that by 0.85 and you get 3.8'. That's the refraction correction that you should use. It's reduced by 0.7 minutes of arc. Note that for altitudes above about 35°, you can probably ignore this correction.

    Don't try this correction for altitude above sea level with tables that include multiple components, like the SD+refraction Sun tables in the standard Nautical Almanac or the dip+refraction tables in some of the guidebooks for my workshops. In cases like those, look up the refraction in altitude separately (it's the "Stars" correction in the Nautical Almanac). Multiply that number (alone) by 0.85. Take the difference, like 0.7 in the earlier example, and then subtract just that from the original tabular correction. Remember: refraction always lifts the stars. We subtract to eliminate its effect. And also remember that refraction is greater when there is more air. So near (or below!) sea level is where we will encounter the most refraction. At higher altitudes above sea level, we're still subtracting to eliminate refraction, but we're subtracting a smaller correction.

    Frank Reed

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