A meridian stone (also called meridian marker) is a stone or pillar that serves as a reference point for calibrating astronomical instruments at an observatory or other scientific facility. It is placed at a specific location on the observatory grounds and is carefully aligned with the local meridian, which is the north-south line passing through the point of observation. This particular one is placed exactly 4,030 meters north of the old observatory in Uppsala and was used to calibrate instruments at the observatory. The meridian is an essential reference for astronomers and surveyors, as it helps them determine the exact time and make precise celestial observations. By aligning instruments with the meridian stone, astronomers can accurately measure the altitude and azimuth of celestial objects as they cross the local meridian, which aids in various astronomical calculations and observations.