Polaris (The North Star) is located in the constellation of Ursa Minor and can be used to either measure time, or tell the time within 30 minutes of accuracy.
Polaris lies just opposite The Plough (Big Dipper) in the constellation of Ursa Major with a line drawn from the pointer stars (Dubhe and Merak) in the Big Dipper and five times their distance from each other then leading to Polaris. It is located 434 light-years from Earth and is a yellow supergiant 2,500 times brighter than the Sun.
Ursa Major along with neighboring Cassiopeia never set below the horizon and completes a whole counter-clockwise (east to west) rotation around Polaris every 24 hours. Therefore, pointer star Merak for example, would revolve all the way around Polaris and return to its original place within a 24 hour period.
Now imagine Polaris as the center of a celestial 24 hour clock (24 separate hours) and a line drawn from Polaris to Merak as an hour hand with each 15 degree rotation by Merak equaling 1 hour of time passing (360 degrees/24hrs).
To tell when say four hours have elapsed draw a new hour line running 60 degrees counter-clockwise from your original hour hand and note where it would intercept the horizon. When the line eventually reaches that point four hours have elapsed.
Telling The Time
Midnight (2400): Big Dipper directly over Polaris
6AM (0600): Big Dipper is to left (west) of Polaris.
Noon (1200): Big Dipper directly below Polaris
6PM (1800): Big Dipper is to right (east) of Polaris.
Complicating matters, however, is the fact that our celestial clock runs fast by 24 hours each year because the Earth takes 365 days to travel around the Sun but a circle only has 360 degrees. In order to compensate for this, we have to thus adjust the 24-hour (midnight) mark counter-clockwise on our celestial clock by around two hours per month to give an accurate reading of the time.
To be extra accurate subtract 30 minutes for each week or 4 minutes per remaining day to give a reading to within half an hour of accuracy. However, don’t forget to add one hour to the time if DST is in effect.