Solstice is an astronomical term regarding the position of the Sun in relation to the celestial equator. The name is derived from Latin Solstitium (from sol: "sun" and sistere: "stand still"). During the year, the position of the sun seen from earth moves North and South. When it changes direction it stands still momentarily. So Solstices are those moments of the year when the sun reaches its southernmost or northernmost position, at the Celestial Tropic of Capricorn or Tropic of Cancer, respectively.
The Solstice is related to the axial tilt of the planet. A common misconception is that the Solstice occurs at the Solar apsides (aphelion and perihelion) of the planetary orbit. Since the orbital eccentricity of the earth (and most other solar system planets) is close to zero, the orbit is nearly circular. Therefore, the amount of sunlight received for the earth as a whole is nearly the same throughout the year. Seasons are, incidentally, caused by the tilt of the earth, which causes one hemisphere to receive more solar energy each day, at the expense of solar energy received by the other hemisphere. The solstices mark the points of greatest imbalance in energy received by the different hemispheres.
The dates of the Winter solstice and Summer solstice are reversed for the northern and southern hemispheres. The dates of the solstices in the most widely used Gregorian calendar shift in a regular pattern. On the Thelemic Calendar, however, this problem is nonexistent.
Solstice festivals were common (and held primacy) in most cultures of the ancient world.
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