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    Re: Extinction
    From: Frank Reed
    Date: 2014 Feb 4, 13:42 -0800

    Gary, you wrote:
    "Frank has mentioned extinction in the past and that it prevents actually seeing the moon set, all the way down to the horizon."

    I've certainly talked about extinction, and I'm interested in this issue, but actually I think you're remembering comments from Marcel Tschudin. He has, in the past, suggested that moonrise and moonset are not observable phenomena because the Moon gets extincted to the point of invisibility at the horizon. I think he's since decided that this isn't always true. In any case, I have certainly seen the Full Moon rising, and literally right ON the horizon, though it's difficult and any haze will wash it out.

    You wrote:
    " it got too dim to see with my naked eye two minutes before it touched the horizon but I was able to follow it all the way down with my 7 X 50s. This means that it disappeared to my unaided eye at about 22' before it touched the horizon. But what complicates this experiment is that my height of eye was 460 feet above sea level so the dip was 20.8' This means that I was able to follow the moon (it was one day old, a thin crescent) with my unaided eye until it was at about a zero altitude, exactly horizontal."

    That's really impressive for a thin crescent observed with the unaided eye. I'm sure I've never followed such a young moon down to those altitudes. With binoculars it's not so surprising; magnification increases the contrast significantly.

    Even for point sources there are large variations in extinction at low altitudes. There are standard tables for extinction here: http://www.icq.eps.harvard.edu/ICQExtinct.html. Scroll down to the bottom to see tables for "winter" and "summer" conditions. Notice that the extinction at sea level for 90° zenith distance (right on the horizon) is 9.80 magnitudes in the "winter" table and 12.68 in the "summer" table. It's easy to see from these numbers that no true star can be seen right on the horizon with the unaided eye (assuming the tables are reasonably accurate, which they seem to be). Even Venus which can reach magnitude -4.7 at maximum brilliancy when high in the sky would be as faint as a fifth magnitude star when sitting right on the horizon.

    The problem of observing an extended object like the Moon or a nebula is always more complicated than a point source. Small variations in the aerosol (haze) component in the extinction can decrease the contrast almost to zero. But as seen in the tables, if you're at high altitude, things improve dramatically.

    -FER

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