Date: March 21, 2009

Title: The March Equinox: All Latitudes Are Illuminated

Podcaster: Judith Young

Organization: The Foundation for Astronomy and Spirituality, Inc. http://www.astronomyandspirituality.com

Description: What are the astronomical characteristics of the March Equinox? This podcast covers the uniqueness of the Equinox in terms of the sunrise and sunset directions, the length of time the Sun is above the horizon and below, 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. Given the commonality of experience for observers all across the globe on the Equinox, in terms of day/night length and sunrise/sunset direction, it is proposed that the Equinox be celebrated as International Unity Day.

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: The National Radio Astronomy Observatory (NRAO). Explore the hidden Universe in radio at www.nrao.edu

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, March 21, is the first full day of spring in the Northern hemisphere and the first full day of fall in the Southern hemisphere.  Yesterday at 11:44 Universal Time, or 7:44 a.m. Eastern Daylight Time in the US, the instant of Equinox occurred.  Astronomically, at that instant, the Sun was located in the sky exactly on the celestial equator, that is, precisely centered on the imaginary extension of Earth’s equator in space.

What is the Equinox?  Well, there are several special characteristics of that day for everyone on the Earth.  First, the Sun rises due East and sets due West, and only on the Equinox is that 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 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 –

the Sun rising due East,

the Sun setting due West,

the Sun up for 12 hours, and

the Sun down for 12 hours.

And the amazing thing is that on the Equinox, everyone on Earth sees this.  (Keep in mind that I am excluding for now the special cases of what the Equinox looks like from the North or 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 you will see the Sun rise due East and set due West, with 12 hours of Sun up and 12 hours of Sun down.

The only difference seen from place to place on the Earth on the day of the Equinox 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 March Equinox, slowly (in about 24 hours) spiraling into view as seen from the North Pole, and slowly (in about 24 hours) disappearing from view as seen from the South Pole.  The March Equinox, then, marks the beginning of 6 months of sunlight at the North Pole, and the beginning of 6 months of darkness at the South Pole.

Finally, the March Equinox in the Northern hemisphere marks the beginning of spring, when days will be getting longer and the Sun at noon will be getting higher in the sky.  And the March Equinox in the Southern hemisphere marks the beginning of fall, when days there will be getting shorter and the Sun at noon will be getting lower in the sky.  For observers in both hemispheres, today, March 21, marks the beginning of the period of time when the Sun is seen rising North of East and setting North of West, as it will until another Equinox passes in September.

Today and yesterday, I invited 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 for the March Equinox.  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 Equinox!

End of podcast:

365 Days of Astronomy
=====================
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.