Astronautics Primer Contents Geocentric-Equatorial Coordinate System Principle Direction Vernal equinox direction, , or the vector pointing to the first point of Aries. The vernal equinox direction points at the zodiac constellation Aries and is found by drawing a line from the Earth to the Sun on the first day of spring. While this direction may not seem "convenient" to you, it's significant to the astronomers who originally defined the system. Unfortunately, the vernal equinox direction is not perfectly constant. The Earth's orbit precesses around the Sun and the Sun is moving through the galaxy. Therefore, exactly when this direction is defined is extremely important for the definition of the system. We can use two ways of defining these directions. The first is to use the mean or average direction at some point in time. The other is to use the true position at exactly one specific point in time. Various combinations of these definitions using different dates gives us several possibilities for coordinate systems used in STK. J2000 Defines the mean vernal equinox direction and mean Earth rotation axis on January 1 of the year 2000 at approximately 12:00:00.00 GMT. Mean of Date Defines the mean vernal equinox direction and mean Earth rotation axis at the orbit epoch time (the time at which the orbital elements being used are true). Mean of Epoch Defines the mean vernal equinox direction and mean Earth rotation axis at the coordinate epoch time (time at which the coordinate system being used is defined). True of Date Defines the true vernal equinox direction and true Earth rotation axis at exactly the orbit epoch time specified. True of Epoch Defines the true vernal equinox direction and true Earth rotation axis at the coordinate epoch time specified. B1950 Defines the mean vernal equinox direction and mean Earth rotation axis at the beginning of the Besselian year 1950. It corresponds to 31 December 1949 at 22:09:07.20 Greenwich Mean Time (GMT). Mean Equinox, True Equator Defines mean vernal equinox direction and true Earth rotation axis for the orbit epoch time specified. Third Axis The third axis of the geocentric-equatorial coordinate system is found using the right-hand rule.