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    Re: Unexpected USNO height correction precepts
    From: Antoine Couëtte
    Date: 2018 Sep 26, 04:00 -0700

    RE : Unexpected-USNO-height-correction-precepts-AndrésRuiz-sep-2018-g42854

    Dear Andrés,

    In further reply to your queries:

    1 - But the question is: why do you need to correct for phase when taking a sight with a marine sextant?

    In the March 18th, 1977 example Venus was observed during daylight and showed as a pinpoint bright spot in the sextants. I cannot remember whether we were using the sextant magnifying lenses oculars or not, but most probably we had to because of the daylight brightness.

    The planet part observed in the sextant was definitely the "center of gravity" of the light, slightly offset from the planet center by the so-called phase correction, and actually 0.31' higher than the Planet Gravity Center on that day. It could definitely not be the Planet Center of Gravity. It could even less be the Planet Lower Limb.

    Therefore if we had had at our hand the coordinates of this "light center of gravity" - which we had not since the French Ephemerides Nautiques did not (and probably still do not) publish GHA and DEC affected by phase angle - we could have computed a slightly more "realistic" intercept.

    We computed our intercept using the planet center coordinates instead - 0.3' from the center of light - and did not make any SD correction, as the standard French Navy intercept computation procedures state that for all planets SD is to be considered as 0.0' . Accordingly the GHA and DEC coordinates we used were only 0.3' off from "reality" resulting in an intercept computation systematic error of 0.3 NM too, due to the relative Venus and Sun local configuration being in the same azimuth.

    Why ignoring the Planets SD's for intercept computation purposes ?

    The separation power of the eyesight sharpness cannot clearly single out either upper or lower limb of a planet.

    Nonetheless their irradiation effect combined with their actual "non zero" apparent SD's can be quite significant especially for Venus and at times for Mars and Jupiter. To attenuate or even eliminate these effects I never hesitate to use shaded oculars to observe them and I generally "shade/dim" them as mush as I can, therefore shooting their brighter spots, hence their ... "centers of light" :-)  ! ,   even at dawn or dusk.

    Hence there is no choice: for practical purposes we are to consider that we can - and do - shoot/observe only the Planet Light gravity centers.

    The brighter the planet environment is - and especially under daylight observations (Venus, and sometimes Jupiter and Mars) - the better this assumption is and works. This also holds true when - as indicated here-above - you adequately get rid of the irradiation and "non zero" apparent SD's effects for dawn and dusk observations.

    To recap: on that day and for this full daylight Venus observation so close from the Sun we necessarily observed the Planet Center of light and not its Gravity Center or even less its lower limb.

    A last note here and for all Planets:

    It seems that there would be absolutely no inconvenience for Nautical Almanacs to publish the Planets Gravity Centers of light (GCL) instead of their Gravity Centers of Mass (GCM) coordinates. It would not appreciably change any of the Mars, Jupiter and Saturn currently published coordinates since their phase effects are so small. Only for Venus and Mercury (which I like to shoot now and then) phase correction can exceed 0.1' .

    Opinions about publishing Planets GCL's instead of Planets GCM's in the Nautical Almanacs ?

    2 - And, what for the moon?

    In the Moon and the Sun case, the eyesight sharpness perfectly sees and singles out either lower or upper limb. Hence all our usual and standard procedures are correct: specifying whether upper or lower limb is observed, and - since this effect is significant for the Moon only - using the augmented SD. And of course their SD's are "huge": about half a degree. For obvious nautical safety reasons we cannot "skip" them without severely degrading/compromising the accuracy of the intercepts computations.

    3 - If you eye is sharp enough and ones can see the spot of light, or the limb it is not necessary, isn't it?

    If I correctly understand your sentence hereabove: yes ! I agree that if we can see the spot of light, the [lower] limb [correction] is definitely not necessary.

    With our current Marine sextants, I would even state that performing a Planet limb correction is detrimental to the computational accuracy of the Planets intercepts.

    Hope that I adequately interpreted your referenced post and that I replied accordingly.

    Best Friendly Navigational Regards,


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