Equinox and Solstice data from the U.S. Naval Observatory, Washington DC. Cross-Quarter moments are interpolated as the midway points between the Solstices and Equinoxes measured in degrees along the ecliptic. Former NASA scientist Rollin Gillespie uses this spatial method rather than simply splitting in half the time interval between a Solstice and an Equinox.
http://www.archaeoastronomy.com/2012.html