Either of the two points (vernal, autumnal) on the celestial sphere where the ecliptic (which is the apparent path of the sun on the sky) intersects the celestial equator. Due to precession, this point moves over time, so positions of stars in catalogues and on atlases are usually referred to a "mean equator and equinox" of a specified standard epoch. For the purposes of the positions of objects dealt with in these ICQ/CBAT/MPC Web pages, the positions are almost always given for "equinox J2000. 0", meaning that the reference system is that at the beginning of the year 2000; prior to 1992, most astronomers were using "equinox B1950. 0". Many older star atlases and catalogues still in use refer to equinox 1950. 0, so observers must be careful when plotting positions (and when reporting positions) to note the proper equinox. (The "B" and "J" preceding the equinox years indicate "Besselian" and "Julian", respectively. See separate definitions for Besselian year and Julian year. ) The differences in an object's position when given in equinoxes 1950. 0 and 2000. 0 amounts to several arc minutes.