Happy Solstice

Imager Credit:  http://www.telegraph.co.uk/news/science

There seems to be some confusion about when the Solstice (winter if you are in the northern hemisphere, summer if you are in the southern hemisphere) is going to take place.  Google says that it will be on December 22nd but quite actually for those of us in North America from the Eastern Time Zone and west to the International Date Line, it will be on December 21st.  So why the difference.  The first thing to realize is that unlike New Years Day celebrations which occur 24 times, one hour apart, the Solstice is a particular astronomical event that takes place at one particular instant in time.  There is only one solstice and it is independent of time zones.  

The December solstice is the globe on the far right.  Note
that the axis (purple lines) points in the same direction (toward
the North Star, Polaris) all through out the orbit around the sun.
Image Credit Wikipedia

Due to the inclination of the Earth's axis, and the fact that the axis of the Earth points at the same direction in the sky, as the Earth orbits the sun through out the year, the apparent motion of the sun follows a path in the sky called the ecliptic.  On a star chart the ecliptic forms a sine wave.  The December Solstice occurs when the apparent motion of the sun reaches the most southern point on that sine wave. At that point the sun is directly over the Tropic of Capricorn.  

As such the solstice doesn't happen on a date per se like new year's day, it happens at a specific moment in time.   That moment of time usually happens on the 21st or 22nd of December, but it can vary out to the 20th to the 23rd although rarely. 

So the question comes to mind, how long does the solstice last?  The ancients would have declared hours if not days.  The apparent motion of the sun as it reaches it most southern point becomes difficult to discern and it appears to stop moving...always a source of concern.  However everything is moving and if you consider the solstice to be defined as when the precise center of the disk of the sun is over the precise line of the Tropic of Capricorn then the theoretical time of transit is an infinitely short instant.  The center of the sun has no dimension nor does imaginary line of the Tropic of Capricorn.  They are mathematical entities of points and lines with no physical dimensions and as such there is no transit time.   

Max Planck 1858 - 1947
The Father of Quantum Mechanics
Image Credit:  Wikipedia

Ahhh but in our universe, there is no infinitely short instant.  If you had a fine enough stopwatch, it would register the Solstice lasting one Jiffy...a slang physics term for Planck's time.  Now if the Tropic of Capricorn was a painted line exactly 6 inches wide, the there would be a transit time.  The exacter center of the sun would pass over the 6 inch line, well actually 3 inch line because we would have to assume that the painted line was centered over the theoretical line which has no dimension. So three inches of the painted line would be south of the actual Tropic.   So in a theoretical sense the moment is infinitely short.  In a real sense it lasts a Jiffy a discreet chunk of time.   


Planck's time which is the amount of time it takes a photon traveling at the speed of light to travel the length of Planck's length.  Plank's length is the shortest possible distance in our Universe.  We would like to think that we could take a ruler and start cutting it in half.  Twelve inches cut into 6, then cut 6 into 3,  1 1/2,  3/4, 3/8, 3/16.........3/  ...that is keep cutting in half forever.  But you can't.  When you get down to Planck's length, a further cut smaller and you are out of the Universe and into the quantum foam. 

So this moment, the Jiffy, is the shortest measurable time in our universe, any shorter and you are again out of the universe and into the quantum foam.  I have no idea how Planck figured this out, or even if he did.  But anyhow the fabric of the universe has discreet chunks, it is not a uniform fabric that gets progressively smaller the finer you look and like wise the clock has specific tics that you can't get any shorter.  So the solstice lasts exactly one Planck's width.  How long is that?   From Wikipedia

Just round that off to 5.4 times ten to the minus forty-fourth power of a second.  No use getting lost in the details. 

So anyhow the Solstice is going to happen at a moment and that moment is tonight on the 21st somewhere around 11:49 PM in EST.  The reason Google says it is on the 22 is that they are using universal coordinated time, which when you are asked what time is it on the planet Earth, this is the time you use.  It is virtually the same thing as the old Greenwich Mean Time except it has atomic clocks applying leap second corrections every now and again.  The term Greenwich Mean Time now only applies to the time zone that surrounds the Prime Meridian going through Greenwich England.  It is no longer the label for the universal standard. 

I don't know if anyone actually knows the precise moment...measured out to some fraction of a second, but according the website TIME AND DATE:

December Solstice in Universal Coordinated Time is on

Tuesday, December 22, 2015 at 04:49 UTC

Yet I have also seen some predictions for 04:48 UTC.  Why the difference,  I have no idea.  In any event I suppose that we could say it lasts 1 minute long.  I would also think from a practical sense that you could say the the solstice lasts as long as a typical sunset into the ocean.  That is allowing for the radius of the sun to pass over the tropic line stop and then reverse direction and pass back.  It should be roughly the amount of time for the diameter of the sun to slip past a horizon. 

Regardless of how long the it takes, it is happening tonight at either 11:48 or 11:49 PM Eastern Standard Time in the US.  So go out and build a bon fire dance and howl at the moon and welcome the first day of winter and the return of Sol. 

For an excellent article see:  The Telegraph, When is the 2015 winter solstice? Everything you need to know about the shortest day of the year