Astronomic Meridian is a north-south line used in surveying that points to “true north,” found by observing the stars or the sun rather than by using a compass. To find it, surveyors traditionally sighted the North Star (Polaris) or the sun at a precisely recorded time and calculated the direction of true north from that observation. Many older land surveys, including the original government surveys that divided much of the United States into townships and sections, used the astronomic meridian as their starting reference for measuring the direction of property lines. It is different from a magnetic meridian, which follows a compass needle and shifts over time, and from a grid meridian, which is based on the lines of a flat map system. It also differs very slightly from the geodetic meridian used in modern GPS-based surveying, because local differences in the Earth’s gravity can tilt the plumb line a tiny amount. Because all north-south lines draw closer together as they approach the North Pole, the direction of a long boundary line measured from the astronomic meridian can change gradually along its length, which surveyors need to keep in mind when retracing old property boundaries.
Astronomic Meridian
