A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David C
Date: 2020 Oct 10, 19:41 -0700
In old textbooks there is a big emphasis on the times equations. Without having studied them in detail and learned them by rote they can be confusing. Here is an example.
A week or so ago I took some PM sights and the next day some AM sights. I decided to work a sight from each day as a time sight, reduce them with Martelli which I was studying, and plot the LOP using the two point Sumner. I would use a plane chart. Easy I though. I could start a thread called "mix and match" - at each point in the process I could chose a different path. For no particular reason I worked the PM sight first. The LOP looked sensible and passed close to my GNSS position.
When I worked the AM sight things turned to custard. The resulting longitude did not make sense. I finally realised that the culprit was
ATS = HATS + 12
ATS Apparant time at ship HATS hour angle true sun.
Martelli gives the ATS and uses AM/PM notation. In the afternoon the "PM" takes care of the 12 hour factor. Conversion from ATS to t is a simple matter of using the time to arc table in Norie. However in the forenoon "AM" does not include 12 so the simple table lookup does not work.
I hope that makes sense. I had just resolved the matter when I read Frank's post but will not know for certain until that I am correct until I plot the second LOP and confirm that it passes near my GNSS position