A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Ed Popko
Date: 2018 Oct 22, 07:31 -0700
In earlier posts about this watch, I included examples that hopefully clarified its use. In retrospect, most of these examples were 'safe' ones. Here I want to point out some subtle cases where calculations with this watch can be confusing - gotchas!
Repeating the obvious, this watch has a 12 hour face and is set to GMT with a second-setting feature that makes it possible to syncronize the second hand with the zero mark on the inner dial. The purpose of the watch is to make it easier and less error prone to find component apparent time angles that sum to the sun's Greenwich Hour Angle. Despite many claims, this watch does not compute longitude by itself unless you happen to be somewhere on the Greenwich meridian.
Here are some 'Gotchas' that result from the way the Equation of Time affects the final calculation the Greenwich Hour Angle.
Gotcha Type 1 - Potential effect of EoT when GMT is very near or on the hour
When GMT is very near or on the hour, the addition of EoT (signed +/-) can shift the apparent time main dial multiplier one hour plus or minus. This has a huge impact on the final Greenwich Hour Angle calculation. Because of the small size of the watch face, these cases are not so easily read. It's best to do a quick estimate of the expected apparent time thus you will know for sure which bezel 15 degree multiplier should be used. Example 2 shows that the right bezel choice when EoT shifts the hour multipler one hour less, Example 8 shows it is one hour more.
Gotcha Type 2 - Special EoT case at noon
Somewhat related to Type 1, the addition of EoT to 12:00 GMT can cause shift the interpretation of apparent time from AM to PM or vice versa. See Example 6 for a PM->AM shift and Example 9 for AM->PM shift.
Despite these Gotchas, the Lindbergh-Weems watch is a lovely invention - if only you could afford one.