SOLSTICE* (Solsc): either of the two extremes of the Earth’s 23.4° axial-tilt relative to the sunlight
falling on its surface. Only at the Summer Solstice is the Earth’s north pole tilted the full 23.4° towards the Sun...
and only at the Yule Solstice is it tilted the full 23.4° away.
EQUINOX (Eqx): the Spring or Autumn quarter begins for the Northern Hemisphere as the North and South Poles
momentarily are poised on the Earth’s terminator; the boundary-line between night and day. As a consequence, only at
these times of year are the hours of night and day equal.
CROSSQUARTER (XQ): here is defined as the moment in time precisely half-way between an adjacent solstice and
equinox. Not being an actual formal astronomical term, "crossquarter" can be defined in a number of ways (see a more extended
discussion of this issue here).