Thursday, September 26, 2013

Using the UNL Sun's Motions Demonstrator to Understand the Zodiac



I used to tell people that the chart is a map of the sky and now I realize how confusing that is.  An astrology chart is actually a map of a very small part of the sky, specifically the plane of orbit of the planets in our solar system, known as the ecliptic.  Observing the night sky, it is only in the band of the ecliptic that we see lights changing position among the constellations from one night to the next.  The lights we see moving around are our neighbors in the solar system. 

At night, if we in the northern hemisphere face toward the north pole, with our backs to the equator, we see constellations rising slowly from our right side, arcing over Polaris and setting to our left side.  The time each constellation rises gradually changes throughout the year, but the lineup of stars within the constellations remains virtually the same, because all the stars that circle around Polaris are light years away from us.  The Big Dipper Clock by UNL School of Astronomy shows the constellation Cassiopeia circling the North Star (Polaris).

The story is different when we turn our backs to the North Pole and face toward earth’s equator; we see the southern part of the sky where the sun makes its east to west journey every day.  Imagine a continuous blue laser thin light that shines from the earth’s equator out into the sky (the celestial equator), and then imagine a white laser light that marks the plane of orbit of all the planets in our solar system (the ecliptic).  Since our planet is tilted about 23 ½ degrees we see the white belt of the ecliptic crossing the blue belt of our laser light equator at an angle of 23 ½ degrees. 


The above UNL School of Astronomy Sun's Motion Demonstrator shows the motion between the ecliptic and earth’s celestial equator as we rotate on our axis and travel around the sun.  It looks like the default latitude is for Lincoln, Nebraska- about 41 degrees north.  Our latitude here in Raleigh is about 36 degrees north.  Be sure to check all the boxes under General Settings.  Under “animation mode” start out by clicking on ‘step by day’ and then adjust the speed to a leisurely 10 days/sec.  With a little finesse you can click and drag the little or big hand on the clock to adjust the time of day.  I adjust the ‘day of year’ by clicking and dragging the arrow that runs along the bar of months.  Look at the celestial sphere on the left as you move the arrow over Sept, and you can see the yellow circle (for the Sun’s daily path) meet the blue circle of the celestial equator.  You can watch their movement on the globe as you manually move the arrow along the bar of months, or just set it some where and click on the Start Animation box under animation controls.


 When you play with the Demonstrator, keep in mind that all planets move along the same path of the ecliptic, just at varying rates compared to the fairly constant motion of the Sun.  The months along the day of year bar could just as easily be marked off with the signs of the zodiac; Aries would begin at about March 21 and Libra about Sept 21.  All of the planets are located somewhere along that white belt, traveling at different rates around the sun.  An astrology chart is a simplified map of where the planets are along the path of the ecliptic, and which part of the ecliptic is above the horizon, for a certain position on earth, and which is below. 

UNL Sun’s MotionDemonstrator (the above embedded version is a bit fuzzy so I've included a link to the original interactive animation.  Click on the blue text to open a new window at the UNL page.)