Image credit: Tunc Tezel/TWAN. "Four Seasons of Lake Venus"

It is that time in our orbit around the Sun when we are closest to our parent star—a result of the slight eccentricity in Earth’s orbit.  But despite being closer to the Sun’s heat, those living north of the equator are having the coldest weather of the year.  In the Southern Hemisphere the situation is reversed, with the year’s warmest weather.  These seasonal changes are caused by the 23½ degree tilt of the Earth, relative to the plane of its orbit around the Sun.

It is time for the December Solstice—the Winter Solstice in the north, the Summer Solstice in the south—which will happen at 05:30 UT on 22 December 2011.  At this moment our Sun reaches its southernmost declination of -23.5 degrees—which is the angle of the North Pole’s tilt.  The Northern Hemisphere experiences its shortest day while the Southern Hemisphere celebrates its longest day of the year.  All places above 66.5 degrees north latitude will be in 24 hours of darkness, while locations below 66.5 degrees south will receive 24 hours of sunlight.

This date is also called the first day of Winter in the north and the first day of Summer in the south.  The name “solstice” comes from the Latin words “sol,” meaning “sun,” and “sistere,” meaning “to cause to stand still.”  Thus, on the December Solstice the sun appears to stand still in its day-by-day progress southward.

Solstices can occur any time from the 20th through the 23rd of December.  The more frequent ones fall on the 21st and the 22nd of the month while December 23rd happened back in 1903 and will not occur again until 2303.  The other least popular solstice date is the 20th of December, and the solstice will not occur on this date again until 2080.

Why the variation in dates?  First, let’s look at the Gregorian calendar, which we now use.  It was initiated by Pope Gregory XII on February 24th, 1582, in an attempt to reform the Julian calendar so as to get Easter to fall in a particular time of year.  The Gregorian calendar has 365 days a year, with 366 days in a leap year.  

The most accurate way to monitor seasonal events is to use the tropical year—which is the length of time the Sun takes to return to the exact position in the season cycle.  The year’s length, based on our perception from Earth, is approximately 365.242199 days.

However, the length of Earth’s orbit around the Sun is impacted by the influence of other planets, Earth’s precession, and the speed variation of Earth as it travels in different parts of its elliptical orbit.  As Earth moves past the other planets, it may experience a tugging and pulling that can create a drag or propel it forward in its orbit.  Also, as the Earth rotates through its day on its axis, it has a slight wobble (procession)—a circular motion of its axis as we see with a spinning top.  This has been proven by the various pole stars documented through time that have been close to Polaris’ current position.   Additionally, we have variations in Earth’s speed in its elliptical orbit, as it goes through its aphelion, perihelion, and equinox positions.

All of these factors combine to make the four seasons of different lengths:  spring lasts 92.8 days; summer, 93.6 days; autumn, 89.8 days; and winter 89.0 days.  The fact that winter is the shortest season can be, at least, some encouragement to those of us entering our coldest time of the year.  To calculate the seasons for yourself, check out   http://www.timeanddate.com/calendar/aboutseasons.html

Wherever you are on 22 December 2011, celebrate your season in the cycle of life with Astronomers Without Borders.  Enjoy your own unique Solstice this year—and why not tell others about the experience?  Being mindfully aware of your place on this moving Earth may bring out the storyteller and poet in you.  We welcome you to share your event reports and poems at the AWB Members’ Blog and AWB Astropoetry Blog.  Send your poems to: [email protected].