Date: December 21, 2009
Title: December Solstice: Celebrating the Light
Podcaster: Judith Young
Description: What is the astronomical meaning of the December Solstice? This podcast describes and explains the characteristics of the Solstice, a word which means ‘the standstill of the Sun’. The December Solstice is discussed in terms of the characteristics as seen from the Northern and Southern hemispheres. Also, the relation between our knowledge of the tilt of 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 on star formation in galaxies is internationally acclaimed. 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. She has a not-so-secret-love: inspired by a Sunwheel on Blackfeet Indian territory in Montana, Dr. Young built the first original stone circle calendar on a University campus in the world. She’s given over 300 talks to over 9,000 visitors at the Sunwheel since 1997, where she hosts solstice and equinox sunrise and sunset gatherings every 3 months. 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 Helene Grogan, in hopes that others will share in all the beauty and wonder the night sky has shown her.
“December Solstice – Celebrating the Light”
Podcast for December 21, 2009
365 Days of Astronomy
Hello everyone – my name is Dr. Judith Young, and I’m a 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 – or Sunwheel – on the U.Mass. Amherst campus, with 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, and since 1997 I have taught what I call “Everyday Astronomy” to over 9,000 people standing amidst the stones. My goal is to bring greater awareness and understanding of the seasons and of the solstices and equinoxes to the general public, and more than 25,000 people have visited the Sunwheel since it was built in 1997.
Today, 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 very little change in the Sun’s declination for a full week at the time of Solstice, and 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 fact of 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.
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, and the longer days, shorter nights and the higher noon-time Sun in the sky in the Southern hemisphere – all of which occur today at the December Solstice. And so, the December Solstice marks the beginning of winter in the Northern hemisphere, and the beginning of summer in the Southern hemisphere.
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 Chanukkah. During the period of time when most 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 was 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.
And finally, over the next few years there are several noteworthy events which will occur near the December Solstice. One year from now, on December 21, 2010, there is a total eclipse of the Moon which will be visible in its entirety throughout North America. Then, 2 years later on 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 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.
Today, in celebration of the December 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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