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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Extinction
From: Gary LaPook
Date: 2014 Feb 4, 02:13 -0800
Frank has mentioned extinction in the past and that it prevents actually seeing the moon set, all the way down to the horizon. I realized that I had never actually seen the moon set so I was curious to observe this effect. Last December I calculated the time of moon set and got in my car to drive down to the ocean (about a 40 minute drive) to see it. I got in my car and started driving but then I remembered that there are some offshore islands that might block me seeing the moon set and I had not calculated the blocked azimuths, so I turned around and came home. I cranked up Google Earth to measure the azimuths to the islands (the moon set would have been blocked) and then discovered a wonderous thing. I have lived here for 25 years, 20 SM from the sea and 500 feet above sea level, and I discovered (via Google Earth) that I didn't need to drive to the ocean at all but that I can see all the way to the ocean from a quarter of a mile away from my house! According to GE there was a three degree gap between two hills that allowed seeing the sea. This seemed too good to be true so the next day I went to the spot and set up my aiming circle and measured the azimuth and it turned out to be correct, a gap from 250° to 253° true.  I then calculated that on January 2nd the moon would set in that gap. So, I was out there to watch it, it was a clear day and Frank was right, 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. So, if I had actually been standing up to my ankles in the surf I should have been able to actually see it set with my naked eye. So, I guess I will have to drive down to the sea to repeat this experiment after all.

gl
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