Two sun-dials, horizontal and vertical, and a universal gnomon are assembled on the astronomical ground of the Smolyan Planetarium.

The horizontal sun-dial is 2 m in diameter. It is made of ferro-concrete and its entire surface is mosaic. The lines for reading the time are wavy, having been calculated from the time equation to correct for differences.The gnomon which supplies the shade on the face is made of brass and the angle between it and the face is 41,o5, corresponding to the latitude of Smolyan.

The vertical sun-dial is assembled to the southern facade of the Planetarium. But the wall is oriented away from the exact East - West direction and is at a slight angle to it. When the sun-dial was mounted, its lines were made to include a correction for the wall’s orientation.The time equation is shown graphically under the sun-dial on a brass plate; otherwise the lines on the face would be more complex as a result of correcting for the deviation of the wall.

Both sun-dials show the official (zonal) time, but when they were made the correction from the meridian was brought in for that purpose. That is, the difference between zonal and local time is incorporated in them.

The universal gnomon is assembled on a levelled concrete pedestal. A scale of the altitude is lined on it in the form of concentric circles whose centre is the gnomon base. Radial lines are traced on this horizontal plane for more quckly locating the azimuth of the Sun. The gnomon is made of brass and is 25 cm in length.