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Basic Great Circles
All celestial spheres contain "great circles," which feature a plane, intersecting the center of the sphere at various locations. Great circles can take place at multiple locations throughout the sphere. One of the most basic sets is the "celestial pole"---or northern and southern-most points on the sphere.
Great circles also feature another mapping marker that people are familiar with---equators. A celestial sphere that is mapped with a great circle plane that runs at a perpendicular angle to the celestial poles is known as the "celestial equator."
Ecliptic
A slightly more unusual great circle plane that is used in the measurement of celestial spheres, but which the average person will be less familiar with, is the "ecliptic." The ecliptic is an intersecting plane that mimics the earth's orbit compared with the star's location. This added information tells astronomers an exact location of the star compared with the orbit of the earth.
Zenith
As astronomers design coordinates on celestial spheres to allow a measurement compared to the location of the earth, they design hypothetical coordinates over vast distances in space. One of these coordinates is a "zenith," which is a hypothetical coordinate that extends from the center of the earth to the center of the celestial sphere. The exact point where the zenith meets the celestial sphere is known as the "nadir" point.
Astronomical Horizon
The "astronomical horizon" is another great circle intersecting the celestial sphere that is perpendicular to the zenith axis. It contains four points, known as "cardinal points." Each of these points serves as directions, much like compass. The plane created by these various points allows astronomers to perform mapping and measurements, tracking the location of the star relative to the earth as it cycles through its orbit.