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    Re: Sun and moon at the North Pole.
    From: Brad Morris
    Date: 2019 Jan 30, 13:28 -0500

    The more I think about this, the more I see the impossibility of Mercury being the crescent shown in Hideaway.

    Mercury would only be illuminated that way when the Earth and Mercury are at inferior conjunction. That is, illuminated with the horns parallel to the horizon. But in order to see it, you would have to be significantly out of the plane of the ecliptic

    From any viewing point on the Earth, Mercury will always be surrounded by the apparent disk of the sun when at or near inferior conjunction, blinding the observer.  Let us assume the North Pole.  ArcTan(Earth radius/radius of Earth's Orbit) is approx 9 arcseconds.  But the Sun's SD is 16 arcminutes.   We simply cannot get far enough away from the ecliptic to not be blinded when observing Mercury at inferior conjunction.

    But if we could be in a spaceship, perhaps we could observe the lower crescent.  I'm not exactly sure how many degrees we need, but let us assume 5 degrees. Tan(5)×Earth's orbit = 8.048 million miles or roughly 2000 times the Earth's radius, perpendicularly out of plane of the ecliptic.  [Try that in Stellarium!]  Of course, its going to be mighty difficult to get the sea & ice foreground in the image now.

    I agree that the image of Mercury, with less than 20% phase, exists out there somewhere.  How to distinguish the wheat from chaff is interesting.  I assumed by giving phase information, it should be a photo and not art, but in retrospect I can see that is not true.  

    So many are just plain art that they can be disregarded.  One interesting animation is 
    which thankfully comes without narration.


    On Wed, Jan 30, 2019, 1:33 AM Frank Reed <NoReply_FrankReed@fer3.com wrote:

    You wrote:
    "Attached find a much better candidate image.  It shows the phases of Mercury."

    No. I'm confident that you'll find that this image is output from a simulation, digital artwork, not photography. But that makes the point... It's quite difficult to see Mercury as a thin crescent. Don't get me wrong: it has been done.. somewhere, out there, is a photo of a "slim crescent" Mercury. It's buried deep among a thousand artistic renderings...

    For anyone else reading along, how many of you have you ever seen Mercury through a telescope? Extending that, who among you have seen it at high enough magnification to detect a hint of phase? I have seen Mercury at phases ranging from perhaps 30% to 70%. It's such a tough target that I can claim no more than that. Even Hevelius had to break his telescope to manage more...

    Frank Reed

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