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Date: December 16, 2010

Title: Celebrating the Light of Solstice

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Podcaster: Judith Young

Organization: The University of Massachusestts at Amherst – http://www.umass.edu/sunwheel
The Foundation for Astronomy and Spirituality, Inc. – http://www.astronomyandspirituality.com

Description: The December Solstice happens every year, and our calendars tell us it occurs on Dec. 21. Yet, Solstice means “standstill of the Sun”, and also refers to a period of almost 2 weeks centered on Dec. 21. This podcast describes and explains the characteristics of the Solstice as seen from around the world. In addition, the relation between our knowledge of the tilt of our Earth’s axis of rotation and the Solstices is explained, along with the origin of the seasons. Finally, parallels are noted between the religious holiday celebrations of December and the astronomical characteristics of the December Solstice.

Bio: Judith Young, Ph.D., is a tenured full professor of astronomy at the University of Massachusetts, where she has taught for 25 years. She has authored over 120 scientific publications, and her work with star formation in galaxies is internationally recognized. She has received awards from the American Astronomical Society and American Physical Society for her research, and she has received awards for her teaching and outreach at U.Mass and in the surrounding community. Dr. Young has a not-so-secret-love: inspired by a Sunwheel on former Blackfeet Indian territory in Montana, she built the first original stone circle calendar on a University campus in the world. She hosts solstice and equinox sunrise and sunset gatherings at the U.Mass. Sunwheel every 3 months, and she’s given over 300 talks to 10,000 visitors to the Sunwheel since 1997. She loves teaching people about the sky to help them feel at home in the Universe, and she dreams of inspiring the building of stone circle calendars the world over.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Wayne Robertson, who encourages you to join him in supporting this great podcast.

Transcript:

“Celebrating the Light of Solstice”
Podcast for December 16, 2010
365 Days of Astronomy

Hello everyone — my name is Dr. Judith Young, named Joyous Judy by my daughter, and I’m an astrophysicist and professor of astronomy at the University of Massachusetts at Amherst where I have taught for 25 years. I love teaching, and I especially love getting people outside to pay attention to the sky. To assist in this, I have built a massive astronomically aligned stone circle – a Sunwheel – on the U.Mass. Amherst campus, with 56 tons of granite in fourteen 8’-10’ tall stones in a circle 130’ across, marking the 4 cardinal directions and the rising and setting directions to the Sun on the solstices and equinoxes. I invite the public to attend sunrise and sunset gatherings at the Sunwheel at the beginning of each season, with the goal of bringing greater awareness and understanding of the seasons and of the solstices and equinoxes to everyone. Since 1997, I have taught what I call “Everyday Astronomy” to some 10,000 visitors standing amidst the stone circle, and over 25,000 individuals have explored the Sunwheel on their own.

December 21 is the day of the December Solstice – the beginning of winter in the Northern hemisphere and the beginning of summer in the Southern hemisphere. On this day, the Sun is at its most southerly declination, or latitude on the sky. However, our calendars mislead us into thinking that the Solstice comes and goes in a day. Actually, there is little change in the Sun’s declination for a full week at the time of Solstice, and therefore little change in the direction to look to see the sunrise, the sunset, and the noon-time Sun. The word ‘solstice’ itself is derived from the Latin – ‘sol’ (meaning Sun) and ‘sistere’ (meaning to stand still) – and is based on the little change in the Sun’s apparent position at this time. Thus, the Solstice could also be labeled “the week the Sun stood still.” But of course the Sun is not really standing still – its sunrise and sunset directions are just not changing very much day to day. So while Dec. 16 is 5 days before the official day of the December Solstice, it is within the period of time that can loosely be designated as ‘Solstice’.

What exactly is the December Solstice? Actually, there are several special characteristics of this day for everyone on the Earth. First, the Sun rises in the most southeasterly direction and sets in the most southwesterly direction of the year. This is seen by observers in both the Northern and Southern hemispheres, and only at the December Solstice is this true. Second, at the December Solstice in the Northern hemisphere, the Sun has the lowest noon-time altitude of the year. This means that noon-time shadow lengths are the longest at the December Solstice in the Northern hemisphere. Third, at the December Solstice in the Northern hemisphere, the days are shortest and the nights are longest. And fourth, at the time of the December Solstice, in order to see the Sun directly overhead at local noon, one must stand on the Tropic of Capricorn (latitude 23.5 degrees South of the equator).

Observers in the Southern hemisphere, comprising approximately 10% of Earth’s population, have quite a different experience of the December Solstice when compared to observers in the Northern hemisphere. As seen from the southerly latitudes on Earth, the December Solstice is the time of the longest days, the shortest nights, and the highest noon-time Sun in the sky, like the characteristics of the June Solstice in the Northern hemisphere. Viewing our planet from outer space, what is unique in relation to the rest of the year is that at the December Solstice, the South Pole of the Earth’s axis of rotation it tilted toward the Sun. This means that at the December Solstice the South Pole of Earth is fully illuminated, and the North Pole is in darkness. In fact, the December Solstice represents the midpoint of the 6 months of darkness at the North Pole and the 6 months of daylight at the South Pole that began with the September Equinox.

Have you ever wondered how we know that Earth’s axis of rotation is tilted in space by 23.5 degrees? Of course, if Earth’s axis had no tilt at all, the Sun would always be located directly over the Earth’s equator and we would not experience the seasons as we know them. But we know the Earth’s axis must be tilted because our Sun is not directly overhead at noon as seen from the equator every day – rather, the location where one must stand at noon in order to cast no shadow changes cyclically throughout the year from one Tropic to the other. And, deriving the tilt of Earth’s axis is related to the Solstices – it comes from knowing that at the June Solstice one must stand at the Tropic of Cancer (latitude 23.5 degrees North of the equator), and at the December Solstice one must stand at the Tropic of Capricorn (latitude 23.5 degrees South of the Equator) in order to see the noon-time Sun at the zenith. Thus, we learn that Earth’s axis is tilted by 23.5 degrees because we know the latitudes of the Tropics. In fact, in an astronomical sense, the Tropics on Earth draw their significance from and are noteworthy locations on this planet because of Earth’s tilted axis.

Furthermore, it is Earth’s tilted axis that is the cause of the seasons – of the shorter days, longer nights and the lower noon-time Sun in the sky in the Northern hemisphere which we experience now, and the longer days, shorter nights and the higher noon-time Sun in the sky which we experience in June.

In addition to the seasonal beginnings which it marks, the December Solstice has woven its way into the lives of people on planet Earth through the religious holiday celebrations which occur around this time of year. It is no coincidence that close to the time of the December Solstice many people also celebrate the 12 days of Christmas, or the 8 nights of Chanukah. During the period of time when the majority of the inhabitants of this planet experience the shortest days and longest nights of the year, these religious holidays specifically incorporate light into their celebrations to brighten up this dark time. Furthermore, the extent of the religious holiday celebrations parallels the standstill of the Sun for which the Solstice is named, when there is little change in the Sun’s position at sunrise or sunset from about 5 days before to 5 days after Solstice. Thus, these celebrations of light coincide roughly in both timing and extent with the December Solstice, recalling a time when ancient peoples lived their lives intimately connected with the cycles of nature.

This year Dec. 21 is not only the day of the Solstice, but coincidentally it is also the day of a total eclipse of the Moon, which will be visible in its entirety throughout North America. It is quite possible that eclipses at Solstice were a clue to ancient astronomers, including those at Stonehenge, of the 19-year Metonic eclipse cycle. Then, two years from now, on approximately December 21, 2012, the 5,125-year calendar of the Mayan people, called the Long Count, will complete the current cycle and begin on the next. The Mayan people celebrated each time one of their calendars was being completed and a new one was about to begin, just as we celebrate on New Year’s Eve when our 1-year calendar is completed and a new one is about to start. Thus, the completion of the Long Count at the December Solstice of 2012 will be a cause for great celebration as humanity moves forward into the 21st century.

On December 21 this year, in celebration of the Solstice, I invite the public to join me in witnessing the passing of the seasons by watching the Sun rise and set over the tall standing stones in the U.Mass. Sunwheel. You, too, are invited to witness these events, or you can create your own astronomically aligned stone circle to connect Earth and sky in this beautiful Universe of ours. For more information on programs I lead and on how to build an astronomically aligned stone circle, visit http://www.astronomyandspirituality.com and http://www.umass.edu/sunwheel, and to everyone, a happy December Solstice!

End of podcast:

365 Days of Astronomy
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