The Earth is our reference frame in observing the universe. To measure the Earth's rotation with respect to stars, we refer to celestial meridians , imaginary north-south lines passing from observer's position. As the Earth rotates, each star crosses the meridian once in every sidereal day. Some stars whose number and location depending on the position of the observer on the Earth neither rise nor set everyday. These are called circumpolar stars.
Circumpolar stars appear to circle around a point called north celestial pole.
There is a similar south celestial pole in southern celestial hemisphere. Motions of the Earth and the Moon Earth's rotation forms the basis for our timekeeping system. The length of the day is a natural unit of time on which all the living beings have adapted. Subdivisions of the day such as hours, minutes and seconds are based on the numbering system developed thousands of years ago. Sidereal time at any instant equals the right ascension of the star that is at upper transit at that instant.