A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Jim Rives
Date: 2021 Feb 15, 08:59 -0800
This question might be a symptom of cabin fever.... I was flying in the slow plane in the Google Earth flight sim over Beijing recently and noted the striking Forbidden City laid out below. Out of curiosity I measured out the outer perimeter of the walls in GE and noted the true directions of the walls. I simply assumed that the walls were aligned to the true cardinal points. However, in each case, the direction I computed in GE resulted in an error in a westerly direction of an average of 2.4 degrees. For example, the north-south walls per measurement on GE was 357.6 instead of the 360.0 as today's True North. Now, the Chinese were nothing if not knowledgable, skilled, and precise.... obviously. I doubt they would have intentionally offset the walls from the cardinal points, and they certainly were capable of precise surveying. I just wonder if this is a difference that could be explained by the precession of the earth in the past 600 years. The construction began in 1406. I have seen wiki articles, and have done a quick search of NavList archives, but haven't found what I need to confirm my hunch. Is there a table or formula (easy one, please) that would show the angular movement of the north true pole over time from, say, Beijing's perspective?