A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert Pearson
Date: 2018 Mar 3, 10:34 -0800
It's a time diagram. Circle is the celestial equator. Vertical line is upper branch of observer's meridian as viewed from south pole. It's used to plot the various hour angles and show the relationship with observer's meridian. See page 235 & figure 1326b in 2017 Bowditch for description.