• Barycenter (質心) is the system’s center of mass.
• Synodic system is a rotating system using the barycenter.
• Ecliptic (黃道) is the plane of the Earth’s mean orbit about the Sun.
• Obliquity of the ecliptic is the angle between the Earth’s mean equator and the ecliptic - about $23.5^{\degree}$.
• The intersection of these two planes are called equinoxes (分點).
• Vernal equinox (春分點)
• Autumnal equinox (秋分點)
• Right ascension (赤經): positive to the east in the plane of the equator from the vernal equinox direction.
• Declination (赤緯): northward from the equator.
• Heliocentric Coordinate System: the origin is at the center of the Sun. Fundamental plane is the ecliptic. X is autumnal equinox; Y is winter solstice. International Celestial Reference System (ICRS) is the current standard.
• Geocentric Celestial Reference Frame (GCRF): standard inertial (fixed with respect to the stars) coordinate system for the Earth. I axis points towards the vernal equinox. Fundamental plane is the equator plane.
  • Sometimes used interchangeably with Earth-Centered Inertial or Conventional Inertial System.
  • Equinox and the equator plane move slightly over time, so inertial can cause confusion.
• International Terrestrial Reference Frame (ITRF): geocentric coordinate system fixed to the rotating Earth. Sometimes we use ECEF.
• Topocentric Horizon Coordinate System (地平坐標系): local horizon forms the fundamental plane using SEZ. S points south. E points east. Z points along the site’s local vertical. This allows us to define azimuth and elevation angles.
• Topocentric Equatorial Coordinate System $I_t J_t K_t$: geocentric equatorial system translated to the origin of the topocentric horizon system.