Friday, December 20, 2013

Winter Solstice

Stonehenge is carefully aligned on a sight-line that points to the winter solstice sunset (opposed to New Grange, which points to the winter solstice sunrise, and the Goseck circle, which is aligned to both the sunset and sunrise). It is thought that the winter solstice was actually more important to the people who constructed Stonehenge than the summer solstice. The winter solstice was a time when most cattle were slaughtered (so they would not have to be fed during the winter) and the majority of wine and beer was finally fermented.

   So according to, Stonehenge was built to remind it’s builders when it was time to party.
   The Irish monument Newgrange (Irish: Sí an Bhrú) referred to above also has a connection to the winter solstice (the word "Solstice" comes from the Latin words for "sun" and "to stand still."). It is about a thousand years older than Stonehenge, and about 500 years older then the Great Pyramids of Egypt, being built around 3200BC, making it about 5,000 years old. 
   It is a mound. That’s right, a mound, because that’s what we Irish do... we build mounds. They’re much easier to make than a freaking pyramid, and you don’t have to lug around a bunch of huge stones, just little ones... and some dirt. We’ve got plenty of dirt in Ireland.
   It was built by farmers who also had an interest in being notified when it was time to start partying. About 4 minutes after sunrise on the morning of the day of the winter solstice (today the first light enters about four minutes after sunrise, but calculations based on the precession of the Earth (a change in the orientation of the rotational axis of a rotating body) show that 5,000 years ago first light would have entered exactly at sunrise) the sun’s light would enter the mound through a fixture known as a  roofbox which rests above the main entrance. That light then shines along a 60 foot passageway into three small chambers off a larger central chamber. The light will illuminate the chambers for approximately 17 minutes.  
   Light enters this chamber a couple of days on each side of the winter solstice as the Earth’s orbit around the sun is slow and precise, yet it is only on the solstice that this light will point  to the spot directly above the chamber housing the long lost Ark of the Covenant.
   But don’t open it dear readers... oh no, don’t open it.    
   Anyway, around the globe the solstice has been calculated, observed and celebrated. “The Incans erected a monument to the event called the Intihuatana Stone, or Hitching Post of the Sun, at Machu Picchu and held celebrations. In ancient Pakistan, the Kalash Kafir celebrated Chaomos on the solstice which involved both ritual baths and a torchlight procession. In Rome, the midwinter 
feast of Saturnalia fell at the time of the winter Solstice. The Chinese have celebrated Dongzhi to mark the Winter Solstice since the Han Dynasty.” - Shannon Younger of the Chicago Tribune
   Shannon reminds us that “Pagans celebrated the solstice and Christians saw some significance in the celebration of light and in rebirth, which was marked by the start of a new year. In Scandinavia, The Feast of Juul was a pre-Christian festival celebrated on the Winter Solstice by the lighting of fires. The fires were intended [to] symbolize the heat, light and life-giving properties of the returning sun. The word "Juul" is the source of the word "Yule," which means "Christmas," explains
   Christians sought to replace the existing pagan celebrations with Christmas as they found it appropriate to celebrate Christmas, the birth of Jesus who is known as the “true light of the world” with the celebration of the days becoming longer.”
   In just a few hours, 9:11AM tomorrow, my time, which is the most important time that there is, the Northern Hemisphere of the Earth, the hemisphere I happen to live in, leans away from our Sun the most it will all year, marking the beginning of winter (astronomical winter. Meteorological winter is defined as December 1st through February 28th, the three coldest months of the year. Astronomical winter, however, starts with the winter solstice and is December 21st, 2013 through March 19th, 2014). 
   The Earth does not spin straight up and down like a top, but at an approximate 23.4 degree angle (currently decreasing at a rate of about 47  arcseconds per century) in reference to our orbital plane around the Sun. This is called the Earth’s axial tilt. Because the Earth’s axis is tilted as it is the Northern and Southern Hemispheres alternately receive the Sun’s energy (light and heat) most directly. Currently the Southern Hemisphere is receiving the lion’s share of the sun’s energy, and is experiencing summer. So while it’s cold and snowy in New York City right now, let’s say (overcast at 49 degrees actually), it’s nice and warm and toasty in Auckland, New Zealand, where the Hobbits live (mostly cloudy, but 69 degrees). It’s the axial tilt of our planet that causes seasonal change, not the distance from our Sun as it travels along it’s elliptical orbit 
   The axial tilt also affects the length of the day in the two hemispheres, or I should say the length of the amount of daylight each hemisphere experiences during different times of the year. 
   The length of each day is described as the time from the very moment the upper limb of the Sun's disk appears above the horizon during sunrise to the moment when the upper limb disappears below the horizon during sunset. Due to the Earth’s atmosphere (diffusion and refraction) light from the sun will appear when the sun is slightly below the horizon. The period when it is still somewhat light even though the sun is below the horizon is called twilight, which in the morning is the warning sign for vampires to head on inside... thus we have “The Twilight Saga,” and sparkly vampires. 
   The length of a day varies throughout the year due to that axial tilt and at what latitude (a geographic coordinate that specifies the north-south position of a point on the Earth's surface) you happen to be at. At the winter solstice... tomorrow at 9:11AM my time, the south pole is tilted toward the sun as much as it is ever likely to be, therefore the southern hemisphere has days ranging in duration from just over 12 hours in the northern portion of the Tropic of Capricorn ( the circle of latitude that contains the subsolar point on the December solstice, the southernmost latitude where the sun can be directly overhead. Its northern equivalent is the Tropic of Cancer) to 24 hours in the Antarctic Circle, whereas the northern hemisphere has days ranging in duration from just under 12 hours in the southern portion of the Tropic of Cancer to zero in the Arctic Circle. 
   Conversely, during the summer solstice (occurring June 21st 2014, at 3:51AM my time (PST)),  the north pole, where all of the polar bears are swimming due to global warming, is tilted most  toward the sun, and therefore the northern hemisphere has days ranging in duration from just over 12 hours in the southern portion of the Tropic of Cancer to 24 hours in the Arctic Circle, while the southern hemisphere has days ranging in duration from just under 12 hours in the northern portion of the Tropic of Capricorn to zero in the Antarctic Circle. 
   For example, tomorrow the length of the day here in L.A. (34° 3′ 0″ N, 118° 15′ 0″ W) will be approximately 9 hours, 53 minutes, and three seconds (I’m not going to get into fractions of seconds. I’m sorry, I’m just not going to do it). The length of the day for good old Bilbo in Auckland, New Zealand (36° 50′ 25.5″ S, 174° 44′ 23.53″ E) tomorrow will be 14 hours, 41 minutes, and 32 seconds. Both cities lie at a very close corresponding latitudes so during the next summer solstice the length of the day will be reversed... sort of. 
   Isn’t it wonderful!
   So... tomorrow will be the shortest day of the year in the northern hemisphere, and the longest in the southern. Starting Sunday, December 22, my dear late mother’s birthday, the day length in the north will start getting longer, and longer, and longer, and longer, until the summer solstice, when the process will reverse and the days will start getting shorter again.
   Not to confuse matters but during the autumn equinox (September 23rd, 2014. An equinox occurs twice a year, when the plane of the Earth's equator passes the center of the Sun), the poles are neither tilted toward nor away from the sun, and the duration of a day is generally about 12 hours all over the planet.
   I’m afraid it’s all going to come to an end though.   
   The Earth is constantly losing angular velocity (which every fourth grader knows is defined as the rate of change of angular displacement and is a vector quantity (more precisely, a pseudovector) which specifies the angular speed (rotational speed) of an object and the axis about which the object is rotating)  and rotational energy (the kinetic energy due to the rotation of an object) through a process called tidal acceleration, which leads to a slow lengthening of the day (in this case the time it takes the Earth to rotate on it’s axis). A century ago, the average day was about 1.7 milliseconds shorter than today, while in the late Neoproterozoic about 620 million years ago (when multicellular life first began to form) a day had only about 21.9±0.4 hours. Tidal acceleration also causes the Moon to move further away from the Earth... so one day in the distant future the Earth will stop rotating and the Moon will be far away... that is if the sun doesn’t turn into a Red Giant and swallow us first.
   Now I’m getting depressed.
   I think I’ll lie down for a while.

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