The north celestial pole has a declination of plus 90 degrees. This value represents the maximum positive angular distance perpendicular to the celestial equator. The south celestial pole sits at minus 90 degrees.
The equatorial coordinate system completes one full circuit in approximately 26,000 years. This slow motion known as precession causes the entire system to turn westward about the poles of the ecliptic. A smaller wobbly motion called nutation adds a tiny oscillation to Earth's axis.
The primary direction of the equatorial coordinate system points toward the March equinox. The symbol for the March equinox is often identified by the glyph for Aries, the ram. This point serves as the zero point for measuring right ascension.
Right ascension is measured in sidereal hours, minutes, and seconds instead of degrees. There are 15 degrees in one hour of right ascension and 24 hours of right ascension circle the entire celestial equator. This system was born from the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates.
Right ascension measures the angular distance of an object eastward along the celestial equator from the March equinox. Hour angle measures the angular distance of an object westward along the celestial equator from the observer's meridian. Unlike right ascension, hour angle is always increasing with the rotation of Earth.
Geocentric equatorial rectangular coordinates are used to specify the positions of artificial Earth satellites and space debris. These frames are also known as geocentric equatorial inertial, Earth-centered inertial, and conventional inertial system. The positions of the planets and other Solar System bodies are often specified in these coordinates with a fourth distance coordinate R in units of the astronomical unit.