A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Aug 24, 10:00 -0700
Greg, you also wrote:
"then subtract 1 second on the actual observation to allow for the time it takes the eye to go from scope to watch."
Let me describe how I do this, which I think largely eliminates the reaction time / time to go from scope to watch.
When I take a time-sensitive sight (not lunars, not sights near noon), at the instant I like the sight, I call out "zero" and then count off seconds steadily, either to five or to ten. That's what I teach, too. I;m sure you do something similar. With practice it's possible to do this with very high accuracy. Then when I reach five (or ten) I am already looking intently at the displayed time, and I can tell to a fraction of a second what the display is showing when I hit that limit. If I am lucky, and my count is synchronized with the alternations of the digits, I've got the time of the sight without any error for reaction time or "transit time" from scope to watch. And that's why I want my watch to display the time (really ought to be UT1) to the nearest second and not to the last second that "ticked" (see my previous post).
To test your accuracy counting off seconds, try it with your eyes closed for fifteen seconds using your watch itself as the starting point. Looking at your watch, as soon as it reads some even number, like 30 seconds, call out "zero" and immediately close your eyes, then when you get to 12 or 13, open your eyes. Continue to 15 and note the error. You can train yourself to count. A bit of music with the right tempo can be very effective. All that "one mississippi, two mississippi" stuff they teach kids is very easy to fudge and pace at almost any rate, but a musical beat of some pop song that's familiar to you will hold your counting to a rock-solid pace. Even a 5% change in the tempo of a musical beat is easily detected by our brains.
So what about DUT? Checking here http://maia.usno.navy.mil/ser7/ser7.dat, the value today is about a quarter of a second. But somebody do the signs for me (having trouble thinking this week...): if I am listening to UTC time ticks on the shortwave, and I want my watch to show UT1 to the nearest second, do I offset it 0.25 seconds or 0.75 seconds (half a second for this setting issue and a quarter second for DUT, but which way?).