So, a set of coordinates is used that is similar to latitude and longitude on the Earth. The Earth rotates around this axis, leaving the poles fixed. The equator is a circle on the Earth's surface that is perpendicular to the axis and equidistant from the poles. It is a great circle because it is centered on the Earth's center, making it as large as possible. Any circle parallel to the equator is called a circle of latitude .
Any semicircle passing through the poles is called a meridian of longitude .
Similarly, the South Celestial Pole is directly above the Earth's South Pole. The star Polaris, also called the North Star , is located very close to the North Celestial Pole. The celestial equator is directly above the Earth's equator. Positions on the celestial sphere can also be measured relative to these markings, although different names are used besides latitude and longitude. Circles of constant declination are all parallel to the celestial equator.