Celestial Slide Rule The celestial slide rule will significantly reduce the effort and time for determining values used in celestial navigation. This quick and easy device is also a great complement to your navigation computer. The slide rule has GHL (Greenwich Hour Angle), LHA (Hour Angle), t (Meridian Angle), SHA (Sidereal Hour Angle) and observers meridian. When two or more of the values are known, the rest can be determined by a single alignment. Diameter: 7″. Wt. 2 oz. 7543 Weems & Plath Celestial Slide Rule