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    Re: Daytime Venus Lunar
    From: Paul Hirose
    Date: 2012 Aug 15, 23:08 -0700

    Antoine Couëtte wrote:
    > Further to results last obtained on Greg's second Lunar dated Aug 14th, 
    2012, (i.e. the one with Lunar Distance = 2°05'8 See hereunder), I need to 
    take back some comments on the comparison made yesterday between own results 
    and Frank's calculator results obtained on this specific Lunar 
    > 
    > 
    > With NO phase correction, if I enter my value of 2°05'980 into Frank's 
    Calculator, I am getting a grade of Error in Lunar 0', Error in Longitude 
    0°01'5.
    
    My program allows only the center, near limb, or far limb of the target 
    body to be observed. It expects that the user will observe a limb 
    instead of the center if the target planet appears non-circular. In 
    fact, it analyzes the illumination angles and recommends the correct 
    limbs for altitude and lunar distance. That's very nice when the choice 
    is not obvious. But as we shall see, the program needs help if the 
    observer prefers to shoot the center of light.
    
    In this run I use the far limb in order to get the semidiameter of 
    Venus. (If the center of Venus is selected, semidiameter is not needed 
    and not displayed.) Then I will adjust the computed lunar distance to 
    the center of Venus, and to the center of light.
    
    
    time:
    2012-08-13 13:43:56.000 UTC
    2012-08-13 13:45:03.184 TT (Gregorian)
    JD 2456152.5 0.572954 (TT)
    66.777 seconds delta T
    
    estimated position:
    +34°10.400' -119°13.800' north lat, east lon
    2 meters above ellipsoid
    
    atmosphere:
    19° C (66° F) at observer
    1012.5 mb (29.90" Hg) altimeter setting
    1012.3 mb (29.89" Hg) pressure at observer
    
    Moon to Venus separation angle:
        2°21.135' computed, center to center, unrefracted
           0.069' refraction
        2°21.066' center to center, refracted
          15.174' Moon near limb refracted semidiameter
           0.200' target far limb refracted semidiameter
        2°06.092' Moon near limb to Venus far limb
        2°05.800' observed angle
    -  0°00.292' observed - computed
    
    unrefacted separation angle rate:
    -19.3" per minute (topocentric)
    79% of total angular velocity
    
    geocentric coordinates (true equator and equinox,
    DE421 ephemeris, IAU 2006 precession, 2000A nutation):
      6h11m27.228s +20°49'04.1"  Moon RA and dec.
    14.998' apparent semidiameter
      6h23m24.748s +19°55'58.0"  Venus RA and dec.
    0.201' semidiameter
    
      3h16m36.972s  local apparent sidereal time
    
    geocentric separation angle and rate:
        2°56.337' center to center
    -27.77" per minute
    84% of total angular velocity
    
    illumination conditions:
    75.6° 4.5° Sun unrefracted az, el
    149.9° zenith angle, Moon to Sun
    131.8° Moon phase angle
    150.0° zenith angle, Venus to Sun
    91.0° Venus phase angle
    
    zenith angles:
    165.5° Moon to Venus
    346.1° Venus to Moon
    
    recommended limbs:
    Use Moon lower limb.
    Use Venus lower limb.
    Use Moon near limb.
    Use Venus far limb.
    
    
    My programe says (observed - computed) lunar distance = -.292'. But 
    remember, that assumes an observation of the far limb of Venus. 
    Refracted semidiameter = .200', so, if we assume an observation of the 
    center of Venus, (observed - computed) = -.292 + .200 = -.092'.
    
    In reality, the center of light was observed, so the -.092' error 
    requires a phase correction.
    
    > With Phase correction, if I enter my value of 2°05'896 into Frank's 
    Calculator, I am getting a grade of Error in Lunar 0', error in Longitude 
    0°00'5.
    
    I have never applied correction for phase angle, and did not find much 
    information or formulas. A 1976 paper by Safronov begins with some 
    classic formulas, then presents a new model - which is too elaborate for me.
    
    
    http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1978SvA....22...78S&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES
    
    In the Explanatory Supplement to the Astronomical Almanac is a 
    polynomial for Mercury and Venus phase corrections. It was derived 
    during the creation of the DE200 ephemeris. Rewritten for phase angle in 
    degrees (vs. quadrants) and better computational efficiency, it is:
    
    -.03 + (4.33e-3 + (5.80e-5 - 3.57e-7 * i) * i) * i
    
    That is the correction in semidiameters. E.g., at 45° phase angle, the 
    center of light is .25 semidiameters from the center of the body. At 
    phase angle 0 (fully illuminated) the correction is an obviously wrong 
    -.03, but the small error is trivial since Mercury and Venus are not 
    observable at phase angles near 0 or 180.
    
    The 91.0° Venus phase angle and that formula give a correction of .575 * 
    .200' = .115'. My program says the far limb of Venus is the correct one 
    for a limb shot. Therefore, the correction is away from the Moon - but 
    not directly away! As already noted:
    
    150.0° zenith angle, Venus to Sun
    346.1° zenith angle, Venus to Moon
    
    Zenith angle is the "azimuth" from one body to another on the celestial 
    sphere. The zenith is "north". E.g. if the Moon is exactly above Venus, 
    the zenith angle from Venus to the Moon is zero. The angle increases 
    counterclockwise. This is actually the same sense as azimuth, though it 
    seems backward because you're looking out from inside the celestial sphere.
    
    In this case, the direction of the Sun from Venus, with respect to the 
    direction of the Moon, is 150.0 - 346.1 = 163.9°. The correction to the 
    observed lunar distance is the cosine of that angle, times the .115' 
    phase correction computed above. So add the -.111' to the observed lunar 
    distance (center of light) to get the estimated center of Venus observation.
    
    Recall that I found the observed - computed lunar distance = -.092', if 
    computed for the center of Venus. If computed for the center of light, 
    that becomes -.203'.
    
    That assumes the correction affects only the distance, not the angles. 
    But they do change, especially that close to the Moon. Unfortunately, I 
    don't have time to do a better job. I can only hope that the small 
    semidiameter of Venus prevents any serious error. Also, as Safronov says 
    in his paper, there's significant disagreement among the standard formulas.
    
    One last thought: as dear old George used to say, a single observation 
    says little about accuracy. You could have been lucky, or unlucky. Even 
    a run of several observations could have systematic errors in common.
    
    -- 
    I filter out messages with attachments or HTML.
    
    
    
    

       
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