The Off-cardinal Alignment of Chiang Mai’s City Plan in Relation to the Orion’s Belt

Abstract

Chiang Mai, the largest city in northern Thailand, was once the capital of the ancient Lanna kingdom, founded by King Mangrai on 19 April 1296 (Gregorian).  The historic city plan, which may have been influenced by Vaastu Shastra (an ancient Hindu knowledge of architecture), features almost a square shape with each side length ranging from 1.52 -1.57 kilometers, bordered by four walls facing the cardinal directions. However, a careful examination reveals that the east-west orientation of the city plan is tilted southeastward with an azimuth of 92.5°, prompting further investigation as to which method was used in the city’s orientation. Historically, two ways to lay out the directions were via the gnomon, such as the shortest shadow and the Indian circle methods, and via fixed stars. In this work, we carried out year-long measurements of the shortest shadow and the Indian circle methods and discovered that neither resulted in an error greater than 0.7°, allowing us to consider the orientation method as being associated with specific fixed stars.  A theodolite and a GPS were used to identify the orientation axis along the northern city wall, calibrated using the positional astronomy program Stellarium. Using precession-corrected Stellarium, the ancient sky can be simulated to uncover the aligned star at the same azimuth as Mintaka (δ Ori) in the Orion’s Belt at a height of 4° and possibly Alnilam (ε Ori) at a height of 1.5° (under an apparent sky condition) during the period between 1292 and 1296, which is close to when King Mangrai founded the city.