The Sphere or clock dial is the central part of the Orloj and represents the astronomical phenomena such as sunrise and sunset, ancient Czech and present day time, movements of the Sun and the Moon and other relative celestial configurations.
As the development of clockwork mechanisms progressed, soon clocks could run with more precision than sun dials and were based on the central local sun position. Astronomers then came to worry about time precision by synchronizing the precision of clocks according to star configurations and their movement around the Earth.
The initial clock was installed in 1410 by clockmaker Mikulas of Kadan with the astronomer and professor of mathematics at Prague Charles University - Jan (Ondrejuv) Sindel (see below about him). - originally accredited with the Orloj concept actually only did some repairs in 1490 and in this second phase is reputed to have added the calendar dial under the astronomical dial.
According the legend - the councilors blinded Hanus so that he could not recreate another similar clock in any other City.
A further legend gives the Skeleton magical power of foretelling the future and says that if the clock is left damaged for a long time, hard times will result for the Czech nation.
The original lone tower housing the present day Orloj was built in 1381.
In times when clock works were imprecise in maintaining their continuous precision, it was necessary to keep correcting them, comparing clocks with other clocks and the best way to do this was according to the then sun clock measurements - the sun dials.