Changing Of The Season - Part Two

One of these days I am going to visit Stonehenge - an ancient astronomical clock dating back to the era transitioning from the Neolithic Era to the Bronze Age.  Astronomical seasons have obviously been around for a long time and are based-upon our observations of the tilt of the earth on its axis as it orbits the sun over the course of a year.

For those of us that live in the northern hemisphere the season called summer begins with the summer solstice when we are tilted in the direction of the sun and on the receiving end of warming sunlight.  Actually, we begin to tilt away from the sun immediately following the summer solstice; but the earth continues to warm as air temperatures lag.  This is called solar irradiance.  (Word of the Day)

For those of us in the northern hemisphere the winter solstice is marked by the day earth's axis is tilted farthest away from the sun resulting in a dearth of sunlight. 

Astronomical fall and spring  are marked by a neutral tilt on earth's axis.  It is in these transition months that the autumnal equinox and vernal equinox occur.  An equinox (Latin word for Equal Night) marks the two times a year when our axis is not tilted toward or away from the sun.  The result is 12 hours of daylight and 12 hours of darkness on those dates.

Ordinarily, the earth transits around the sun over 365 days.  Sometimes it takes 366 days.  As a consequence of this the solstices and equinoxes can vary from one year to the next.  The vernal (spring) equinox is generally around March 20 with the summer solstice landing around June 20-21.  The autumnal (fall) equinox is generally around September 22 and the winter solstice landing around December 21-22.

Remember all of this as there will be an exam.