Date: September 22, 2010
Title: The September Equinox: All Latitudes Are Illuminated
Podcaster: Judith Young
Description: What are the astronomical characteristics of the September Equinox? This podcast covers the uniqueness of the September Equinox in terms of the sunrise and sunset directions, the length of time the Sun is up versus down, and the location on Earth where the noon-time Sun is seen to be directly overhead. Also included are the differences seen in the sky for observers at the North and South poles, and at mid-northern and mid-southern latitudes.
Bio: Judith Young, Ph.D., is a tenured full professor of astronomy at the University of Massachusetts at Amherst, 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’s been nominated for the Distinguished Teaching Award and the Distinguished Outreach Award, but 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 some 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 circles all over the world.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by David Zimmerman… who remembers the kid next door with the telescope and his father’s subscription to Sky and Telescope with its maps of the southern skies and the dark Brazilian skies.
“September Equinox – All Latitudes Are Illuminated”
Podcast – September 22, 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, and simply getting people to pay attention to the sky. 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.
The September Equinox is the official beginning of autumn in the Northern hemisphere and the beginning of spring in the Southern hemisphere. This year, the day of the Equinox is either September 22 or September 23 depending on where you live on planet Earth. The precise instant of the Equinox is 3:09 Universal Time on Sept. 23rd. So if you live in England, the Equinox is early in the morning of Sept. 23, and if you live in the United States the Equinox is late on Sept. 22. Here in Massachusetts, the instant of Equinox is just before midnight on Sept. 22, at 11:09 p.m. Eastern Daylight Time. Astronomically speaking, at that instant, the Sun will be located in the sky exactly on the celestial equator, that is, precisely centered on the imaginary extension of Earth’s equator into space.
What is the Equinox? Well, there are several special characteristics of this day for everyone on the Earth. First, the Sun rises due East and sets due West, and only on the Equinox is this true. Every other day of the year, the Sun rises either South of due East or North of due East, and sets either South of due West or North of due West. You thought that the Sun rises in the East every day? Not due East every day – just on the Equinox. And you thought the Sun sets in the West every day? Not due West every day – just on the Equinox. And on the Equinox, this due East rising and due West setting is best seen if you have no hills, trees, or buildings on the horizon.
Second, on the Equinox the Sun is up for 12 hours and down for 12 hours. This is the basis for the term equinox in the first place, from the Latin and meaning “equal night” – however, the situation is most accurately described as equal Sun up and equal Sun down. Strictly speaking, this 12 hours of Sun up and 12 hours of Sun down is seen when there are no hills on the horizon.
So here we have 4 characteristics of the Equinox –
1. the Sun rising due East,
2. the Sun setting due West,
3. the Sun up for 12 hours, and
4. the Sun down for 12 hours.
And the amazing thing is that everyone on Earth sees this on the Equinox. (Keep in mind that I am excluding for now the special cases of what the Equinox looks like to the penguins and polar bears at the North and South Poles.) So you may be located in Australia, or Ireland, or Ecuador, or Amherst, Massachusetts where I am, and whether you are in the Southern hemisphere or the Northern hemisphere or at the equator, on the Equinox the Sun rises due East and sets due West, with 12 hours of Sun up and 12 hours of Sun down.
And although there is a great deal of similarity worldwide to our experience of the Sun on the Equinox, astronomically speaking the major difference seen from place to place is how high the Sun gets in the sky at noon. For example, in Quito, Ecuador (located on Earth’s equator) on the Equinox the Sun will be directly overhead at local noon and you will cast no shadow. In fact, anyone anywhere on Earth’s equator on the Equinox will have the Sun directly overhead at local noon and will cast no shadow. Here in Amherst, at latitude 42° North, the noontime Equinox Sun has an altitude of 48° above the southern horizon. In Cusco, Peru, at latitude 13° South, the noontime Equinox Sun there has an altitude of 77° above the northern horizon. And still, in each location the Sun rises due East and sets due West, and spends 12 hours up and 12 hours down.
Viewing our planet from outer space, what is unique in relation to every other day of the year is that all latitudes of planet Earth are illuminated by the Sun on the Equinox. That is, our planet is bathed in light from the North Pole to the South Pole. This is only true on the Equinoxes, with the Sun over the equator, so both the North and South Poles get some degree of light, along with every latitude in between. Then as Earth rotates, each location is illuminated by the Sun half of the time (a 12 hour day), and spends the other half of the time turned away from the Sun (a 12 hour night). Except at the Poles.
A special case arises on the Equinox for observers at the North and South Poles. There, at the northern and southern endpoints of the Earth’s imaginary axis of rotation, the Sun is seen to circle around the horizon on the day of the September Equinox, slowly (in about 24 hours) spiraling out of view as seen from the North Pole, and slowly (in about 24 hours) spiraling into view as seen from the South Pole. The September Equinox, then, marks the beginning of 6 months without Sun at the North Pole, and the beginning of 6 months of light at the South Pole.
Finally, the September Equinox in the Northern hemisphere marks the beginning of autumn, when days start becoming shorter than nights and the Sun at noon will be getting lower in the sky. And the September Equinox in the Southern hemisphere marks the beginning of spring, when days start becoming longer than the nights and the Sun at noon will be getting higher in the sky. For observers in both hemispheres, the September Equinox marks the beginning of the 6-month period of time when the Sun is seen rising South of East and setting South of West, which will persist until another Equinox passes next March.
In the Northern hemisphere, perhaps you have already noticed that the day-time temperatures are getting cooler and that the Sun has been setting earlier for the past few weeks. Or in the Southern hemisphere, perhaps you have already noticed the opposite. Wherever you are located on planet Earth, the changes in day length are most pronounced on the Equinox, and they are most noticeable for the 4 weeks around Equinox, from about 2 weeks before to 2 weeks after. During this period of time, the Northern hemisphere moves into autumn with the shorter, cooler days and the changing colors of the trees, and the Southern hemisphere moves into spring with the longer, warmer days and the sprouting of plants. So even though there is a sense of balance about the Equinox, in the Sun rising due East and setting due West, or in the Sun up for 12 hours and down for 12 hours, the Equinox also marks a period of rapid change in the seasons.
On the September Equinox, I invite you to join me in witnessing the passing of the seasons by watching the Sun rise and set. If you live in Massachusetts, please join me among the tall standing stones in the U.Mass. Sunwheel on September 23rd. If you live far away, you can create your own astronomically aligned stone circle – for information on programs I lead and on how to build a Sunwheel, visit http://www.astronomyandspirituality.com and http://www.umass.edu/sunwheel. And may the Equinox remind you and guide you to find balance in your life.
End of podcast:
365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. 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.