Date: June 20, 2010
Title: The June Solstice: The Week the Sun Stood Still
Podcaster: Judith Young
Organization: The Foundation for Astronomy and Spirituality, Inc. and the University of Massachusetts at Amherst – http://www.astronomyandspirituality.com & http://www.umass.edu/sunwheel
Description: What is the astronomical meaning of the June Solstice? This podcast describes and explains the characteristics of the Solstice, a word which means ‘the standstill of the Sun’. The June Solstice is first described in terms of the characteristics as seen from the Northern hemisphere, and these properties are contrasted with what is seen from the Southern hemisphere.
Furthermore, the relation between our knowledge of the tilt of Earth’s axis of rotation and the Solstices is explained, as well as the origin of the seasons.
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 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 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’s given over 300 talks to over 10,000 visitors to the Sunwheel, 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 astronomically aligned stone circle the world over.
Today’s sponsor: “Between the Hayabusa homecoming from Itokawa and the Rosetta flyby of asteroid Lutetia, 13 June until 10 July 2010, this episode of “365 Days of Astronomy” is sponsored anonymously and dedicated to the memory of Annie Cameron, designer of the Tryphena Sun Wheel, Great Barrier Island, New Zealand, a project that remains to be started.”
Additional sponsorship for this episode of 365 Days of Astronomy is provided by Chuck McCorvey.
Transcript:
Hello everyone — my name is Dr. Judith Young, named Joyous Judy by my daughter, and I’m an astrophysicist and professor of astronomy and the University of Massachusetts at Amherst where I have taught for over 25 years. I love teaching, and I especially love encouraging people to go outside and pay attention to the sky. To assist in this, I have built a massive astronomically aligned stone circle – a Sunwheel – on the UMass 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 Sunwheel events at sunrise and sunset on the solstices and equinoxes, and since 1997 I have taught what I call “Everyday Astronomy” to some 10,000 people standing amidst the stone circle. 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.
Tomorrow, June 21, is the day of the June Solstice – the beginning of summer in the Northern hemisphere and the beginning of winter in the Southern hemisphere. In the morning, at 11:28 Universal Time, or 7:28 a.m. Eastern Daylight Time, the Sun will be at its most northerly 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 apparent position for a full week at the time of Solstice, in terms of the direction to look to see the sunrise, the sunset, and the noon-time Sun. Astronomically speaking, the Sun’s declination changes very little from about 5 days before to 5 days after Solstice. 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 June Solstice? There are several special characteristics of this day for everyone on the Earth. First, the Sun rises in the most northeasterly direction and sets in the most northwesterly direction of the year. Only at the June Solstice is this true, and it is seen by observers in both the Northern and Southern hemispheres. Second, at the June Solstice in the Northern hemisphere, the Sun has the highest noon-time altitude of the year. This means that noon-time shadow lengths are the shortest at the June Solstice in the Northern hemisphere. Third, at the June Solstice in the Northern hemisphere, the days are longest and the nights are shortest. And fourth, at the time of the June Solstice, in order to see the Sun directly overhead at local noon, one must stand on the Tropic of Cancer (latitude 23.5 degrees North).
So here we have the characteristics of the June Solstice in the Northern hemisphere –
1. the Sun rises most northeasterly,
2. the Sun sets most northwesterly,
3. the Sun has the highest noon-time altitude of the year,
4. the days are longest,
5. the nights are shortest, &
6. any observer at the Tropic of Cancer will have the noon-time Sun directly overhead, and will cast no shadow at local noon.
In contrast, the June Solstice for observers in the Southern hemisphere is different from that in the Northern hemisphere. The Sun still has the most northerly declination, or latitude on the sky, and from the Southern hemisphere the Sun is still seen to rise in the most northeasterly and set in the most northwesterly direction of the year. But as seen from the southerly latitudes on Earth, the June Solstice is the time of the shortest days, the longest nights, and the lowest noon-time Sun in the sky, like the characteristics of the December 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 June Solstice, the North Pole of the Earth’s axis of rotation it tilted toward the Sun. This means that at the June Solstice the North Pole of Earth is fully illuminated, and the South Pole is in darkness. In fact, the June Solstice represents the midpoint of the 6 months of daylight at the North Pole and the 6 months of darkness at the South Pole that began with the March 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. And furthermore, the word “tropic” comes from the Greek tropikos, meaning ‘a turning’. What is turning? Observationally, it is the noon-time altitude of the Sun, going from most northerly at noon on the June Solstice to most southerly at noon on the December solstice, and back – year after year after year.
Finally, it is Earth’s tilted axis that is the cause of the seasons – of the longer days, shorter nights and the higher noon-time Sun in the sky in the Northern hemisphere, and the shorter days, longer nights and the lower noon-time Sun in the sky in the Southern hemisphere – all of which occur on the June Solstice. And so, the June Solstice marks the beginning of summer in the Northern hemisphere, and the beginning of winter in the Southern hemisphere.
Tomorrow, June 21st, I invite the public to join me in witnessing the passing of the seasons at the UMass Sunwheel by watching the Sun rise and set over the tall standing stones for the June Solstice. Given that Solstice means ‘standstill of the Sun’, there is actually a period of over one week during which the Sun can be seen to rise and set over the standing stones in the Sunwheel. You, too, are invited to attend these events, or you can create your own astronomically aligned stone circle to connect yourself with Earth and sky in this beautiful Universe of ours. For more information on programs I lead and on how to build astronomically aligned stone circles, visit http://www.astronomyandspirituality.com and http://www.umass.edu/sunwheel — and to everyone, a happy June Solstice!
End of podcast:
365 Days of Astronomy
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