NavList:

Just before local noon yesterday I started shooting Sun altitudes near Beavertail Lighthouse with an Astra IIIB sextant fitted with a 7x scope. I used my smartphone app to compare against so that I could record just the intercepts from my known location (identical to sight errors) for each sight. I shot continuously for about half an hour. Index correction and height of eye were pre-entered. Here are the intercepts:

-0.4
 0.0
-0.1
 0.1
 0.0
 0.4
 0.4
-0.6
 0.1
 0.0
 0.3
 0.2
 0.2
-0.1
 0.2
 0.0
 0.2
-0.2
23
24
24

You can see that I've got 18 sights in row with a nice average near zero and a standard deviation of about a quarter of a minute of arc. Very nice! Then... uh... what the heck?? I jump from an intercept of -0.2' to 23'!! Did I read the sextant wrong? Am I going blind? After the third anomalous sight, I was interrupted by some passers by who wanted to learn about my "strange camera". We had a nice chat about sextants and celestial navigation, and then I showed them the wonders of the MarineTraffic smartphone app by identifying a large ship out on the horizon. They were entertained by both, of course. And then I had to head home and get some work done. Later in the evening. I sat down to contemplate that extraordinary jump in intercepts. So put yourself in my shoes: what would you look at if you saw such a bizarre change in your observations and analysis? What could cause such a huge shift? I did discover the cause later, but I would be curious to hear how others would go about "debugging" this one.

Frank Reed
ReedNavigation.com
Conanicut Island USA