Two markers were placed on the Capitol grounds in July 1875 by Lt. Bailey of the U.S. Lake Survey, which was the first U.S. Army Corps of Engineers organization in the Great Lakes area. This was done at the request of Governor Baldwin who, in 1872, had appealed to the federal government for help in completing the surveying of Michigan.
The astronomical post is a dressed stone and measures about five feet high, although about three and a half feet of it is buried in the ground. It is a reference stone, used to measure longitude and latitude. Such stones were placed in conspicuous places where they were unlikely to be disturbed, such as the grounds of city halls or, in this case, the state Capitol.
The post's location was determined exactly, using both astronomical and radio observations. Then its location was recorded. This allowed others to use the stone to locate their precise position on the globe, using a transit set up over cross hairs which had been incised on the stone's surface. These cross hairs can still be seen today.
For many years, the purpose of the stone was forgotten and it was called a "mystery stone." Some thought it might be a carriage stone, used to allow passengers to mount of alight from carriages. Research during the Capitol's restoration uncovered its true role as a datum - a fixed reference point - which assisted in the early efforts to survey the interior of Michigan.
A companion to the Astronomical Post is a smaller stone, called an azimuth. It is set flush to the ground and extends for unknown distance underground. It also has cross hairs incised on its surface. This second stone is due north of the Astronomical Post, and somewhat northwest of the Austin Blair statue, near the Women's National Farm and Garden Association Tree. The purpose of this stone was to allow a north-south line to be easily established without the use of a transit or other surveying equipment. One has only to stretch a line between the points of intersection in the cross hairs of the two stones. Once a north-south line was established, then a right angle produced an east-west line, and locations could be measured with reference to these two lines.