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. For any position on the surface of the Earth, the point on the celestial sphere that is directly overhead is called the zenith . A star's position along a circle of constant declination is described by a second number called right ascension .
Right ascension corresponds to longitude, but different units are used.
With the two numbers of declination and right ascension, the position of any object in the sky can be precisely described. Although the stars are fixed relative to each other, the Sun moves relative to the stars. Once a year, the Sun traces out a circle on the celestial sphere called the ecliptic . The equinox where the Sun ascends from the southern to the northern hemisphere is called the spring or vernal equinox because the Sun is there on March 21. This position is called the autumnal equinox because the Sun is there on September 23.