P3D Re: Re Moon size

[Bob Wier <wier@xxxxxxxxxxxxxxxxx>]

>
>Actually I'm coming to accept that there is an illusion, though their
>description still doesn't quite add up. In separate communications it
>seems that what IS accepted as an effect visible to the eye, and due to
>refraction is a slight flattening of the moon or sun near the horizon.
>While this doesn't change it's apparent horizontal diameter, it would
>give the outline of that body the appearance of having a larger disc
>than is actually there. Later in the sky, when the slight flattening is
>gone, the illusion would be gone too. This makes much more sense. Maybe
>their illusion is valid to some degree too.
>-- 

In the mountains where I spend considerable amounts of time,
the skyline is about 30 degrees up from the horizontal (making
moonrise/sunrise about 2 hours after the "stated" ephermis time).
I'd say that even at that sky elevation there is a slight illusion
of enlargement, although perhaps not as strong as that when the
moon rises above the "true" horizon. 

I recall as a young 'un I had a 3.5" reflector telescope which had
a low power eyepiece where the full moon just filled the field
of vision (and was bright enough to cause loss of dark adaptation).
As best as I can recall, the moon pretty much occupied the same angle
of view regardless of the elevation where I observed it (at least after
it had risen a couple of degrees off the horizon).



-_BW
              Bob Wier
     mailto:wier@xxxxxxxxxxxxxxxxx
   10:51 PM Monday, January 17, 2000
        Unix/Internet Administrator
   Rocky Mountain College, Billings MT.
 keeper of the Photo-3d and Overland-Trails
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