Date: June 21, 2009
Title: The June Solstice: The Week the Sun Stood Still
Podcaster: Judith Young
Organization: The Foundation for Astronomy and Spirituality, Inc.
http://www.astronomyandspirituality.com
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 discussed 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. The podcast closes with the Emily Dickinson poem “There came a day at summer’s full…”.
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 Barb Geigle, President of the Berks County Amateur Astronomical Society, and dedicated to everyone who encourages girls and women to become interested in astronomy and other sciences.
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 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 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 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.
Today, 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. This morning, at 5:46 Universal Time, or 1:46 a.m. Eastern Daylight Time in the US, the Sun was 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? Actually, 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. This is seen by observers in both the Northern and Southern hemispheres, and only at the June Solstice is this true. 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.
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 – both of which occur today 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.
Today and tomorrow, I invite the public to join me in witnessing the passing of the seasons at the U.Mass. 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 these events (either in person or by viewing photos on the Internet), 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 June Solstice!
End of podcast:
365 Days of Astronomy
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The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.