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On 2019-05-26 14:16, Peter Hakel wrote:
> nautical refraction formulae and tables are built for sea level, whereas I 
am at an elevation of 7320 feet (2230 m); I don’t recall right now how much 
difference that makes for altitudes (perhaps aircraft pilots on NavList can 
comment), and I believe that it is academic for lunar distances.

2019-05-26 15:25:00 UT1
-106°19.00' +35°53.00'  east lon, north lat

Moon center, refraction in altitude
1.54' sea level
1.16' at 7320 feet

Sun center refraction
1.12' sea level
0.84' at 7320 feet

lunar distance refraction, center to center
2.01' sea level
1.51' at 7320 feet

All refractions from Lunar 4.4, which uses the Cassini refraction model.
(The International Astronomical Union SOFA "Refco" routine agrees within
.01'.) Sea level refractions assume 50% humidity, 10 C (50 F), and 1010
mb (29.83″ Hg).

For conditions at 7320 feet I inferred Peter's location as Los Alamos in
New Mexico and obtained a weather observation at the city airport near
the time of the lunar distance prediction. Altimeter setting was 30.11
inches Hg, temperature +17.2 C, dew point -7.5 C.

For refraction corrections, the air pressure input to the Lunar program
is altimeter setting, not the actual air pressure (often called "station
pressure"). The reason is that here in the US, the "barometric pressure"
given out by news media and the National Weather Service is actually the
altimeter setting, not station pressure. The latter can be precisely
recovered from altimeter setting and the height of the observer. (This
assumes the altimeter settings are practically identical at the airport
and the observer.) In this case, station pressure is 777.4 mb (22.96″ Hg).

The formally correct barometric pressure corrected to sea level is often
available if you know where to look in airport weather observations,
though it's not in the Los Alamos data for some reason. But even when
present, that value is not a good basis for calculating station pressure.

Usually it's sufficient to assume the standard altimeter setting of
1013.25 mb (29.92 inches Hg), since the effect of temperature is much
greater. A 2% change of altimeter setting, say from 30.0 to 29.3, is
rare. The 2% increase in absolute temperature when you go from 60 F to
70 F has the same effect on refraction.

   

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