Noon line
A midday line (lat. Meridianus ) is the shadow cast by a vertical rod or gnomon at the highest daytime sunshine on the ground, which runs in a north-south direction. It is more precise to show the connection between the endpoints of this shadow (midday points) that move over the course of a year with the highest point of the sun .
Noon lines have long been used as astronomical measuring instruments, such as the line set up in 1665 by the astronomer Giovanni Domenico Cassini in the Basilica of San Petronio in Bologna . It was permanently marketed with a brass strip on the church floor according to the highest position of the sun on the days of the solar year .
Such noon lines coincide with the local longitude or the earth meridian of the location. Their length depends on the one hand on the height of the gnomon or on the length of the staff, on the other hand on the latitude : the closer the location is to the equator, the higher the position of the sun at noon and the shorter the shadow and its seasonal change. The end points of the line indicate the respective midday point at the summer and winter solstice ( solstitium ).
In addition to determining the geographical latitude, an important use of the noon lines was the midday calibration of the local wheel clocks . Therefore, in addition to the horizontal noon lines at ground level, there are also vertical measuring lines.