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Gary,

Yeah, I think many people thought he was joking. But I have encountered this specific confusion before among people who have learned just enough positional astronomy or celestial navigation to get themselves in trouble. It is NOTHING BUT a confusion.

The Moon's "horizontal parallax" is just a convenient accounting trick. It's a means of pretending that the Moon is "really" infinitely distant on the celestial sphere but a "correction" has to be added to deal with its position in three-dimensional space. This is perfectly legitimate, of course, but it's merely accounting. Whenever one gets confused by concepts involving the Moon's HP, it's worth considering an artificial satellite in low orbit. While it is just barely possibly to pretend that a low orbit satellite is infinitely distant on the celestial sphere for positional calculations and then "correct" it by applying a huge parallax correction, it's much more obvious that the true position in space is really the more fundamental concept. A satellite is just "up there" at some point in space and you can see it if it's above your local geometric horizon (or conceivably a bit below and lifted by refraction). Like a satellite, the Moon has some position in space, which, if we wanted, we could give in Cartesian x,y,z coordinates relative to any chosen axes. Then from our position on Earth in the same coordinate system, we calculate its position in the sky directly. There's no HP then. It's just a straight-forward 3d calculation of the Moon's position relative to the observer's horizon.

The only reason we can't see the Moon rise on each and every day is because atmospheric extinction near the horizon, especially at sea level, is huge. Extinction is the reduction in apparent magnitude due to scattering and absorption by the atmosphere. At the zenith, the extinction is about 0.3 magnitudes under common conditions at sea level. If you were outside the atmosphere, stars would be about that much brighter (not much! ...except for the lack of "twinkling", the stars and the Milky Way in space look almost identical to their appearance from a high mountain and only slightly fainter than they do from sea level). Naturally, lower in the sky, the extinction is greater. At an altitude of 15 degrees, extinction rises to about 1.0 magnitudes. By 2.5 degrees altitude, it's around 5 magnitudes. That means a star like Vega would be visible to the naked eye as a very faint star at that altitude. Right at the horizon, the extinction is around 12 magnitudes which renders all the stars and planets invisible, and even the Full Moon is reduced to magnitude 0. Normally a magnitude 0 object can be seen easily, but when its light is spread out over a region half a degree wide, that's very difficult to see. But it certainly does happen. One of my favorite memories of a moonrise was seeing the Full Moon rising over Lake Michigan from the Chicago lakefront some years ago. I have photos somewhere... It was a strange, distorted, dusky dome climbing up over the horizon. I was not expecting it, and at first I did not realize what it was.

-FER

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