Equinoxes and Solstices

Thursday, March 20th was the vernal (spring) equinox and the official start of spring - though anyone living in Gunnison or Alaska knows spring doesn't actually start till May.  Or July.  (Promptly followed by fall.)  The vernal equinox is one of two days in the year (the other is around the 22nd of September) when the Sun is up for exactly 12 hours and below the horizon for exactly 12 hours (the word "equinox" means "equal night").  These are also the only two days of the year when the Sun rises exactly due east and sets exactly due west.  Both of these statements are true for all locations on Earth.  

Solstices occur around the 21st of December and June.  On the winter solstice the Sun rises and sets farthest to the south, and is at its lowest point in the sky at noon.  This is the shortest day of the year.  On the summer solstice the Sun rises and sets farthest to the north, and reaches its highest point in the sky for the year.  This is the longest day of the year.

Astronomically speaking, solstices and equinoxes occur because the Earth's axis is tilted by 23.5 degrees with respect to its orbit around the Sun.  The axis points in a constant direction in space - towards the star Polaris (though over thousands of years the direction changes due to precession), so as the Earth orbits the Sun, sometimes the northern hemisphere points towards the Sun and sometimes it points away.  The summer solstice is considered the exact time when north pole is tilted directly towards the Sun, while the winter solstice is when the north pole is tilted directly away (this is a northern hemisphere perspective).  The equinoxes occur halfway between these dates when the axis points neither towards nor away from the Sun.  In terms of the equator, between the vernal and autumnal equinoxes the Sun is north of the equator, between the autumnal and vernal equinoxes the Sun is south of the equator, and on the equinoxes the Sun is directly above the equator.  So we can define the exact moment of the equinox as the moment when the Sun crosses the equator.

Position of Earth relative to the Sun during the solstices and equinoxes.  Figure courtesy of NOAA

For many ancient cultures, the solstices and equinoxes were important times of celebration.  People who could predict when these events would occur attained great social, religious and political power.  In locations such as Chaco Canyon in New Mexico, Chichen Itza in Mexico, and Stonehenge in England structures were built to mark the solstices and equinoxes through distinct solar alignments (an example from Chaco Canyon is shown below) that occur only on those dates.