Horologium Nocturnum

The sun is our main natural clock. Due to the Earth's rotation the sun appears to rise over the horizon then marks the noon and finally goes down in the west. After twilight and through the night, we can see the stars moving in similar trails to the sun. Due the earth’s translation the stars are displaced  one degree per night westward, so they do not give us the time directly. The Horologium Nocturnum or Nocturnal allows us to correct this daily accumulative difference and obtain the time for any moment of the night and every epoch of the year. In the northern hemisphere the north pole star Polaris is used with another star to form the hand of our night clock; the second star can be Beta of the Lesser Bear,  named Kochab, or also both Dubhe and Merak, Alpha and Beta of the Great Bear respectively. In the southern hemisphere there isn’t a bright enough star close to the celestial pole as Polaris in the North. Sigma Octantis, the naked eye star nearest to the pole is quite dim, at only 5.5 visual magnitude. Its nearness to the visibility limit of the human eye makes it not useful for this purpose. However the Southern Cross, situated at about 25º from the Celestial South Pole can help us, due both to its visibility and easy recognizability. By setting the different scales in the disks of the Horologium and turning the hand until the holes match this bright constellation, we find the equivalent solar time.