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Spinning Tops and the Precession of the Equinoxes

Posted Mon., September 24, 2012

Have you ever watched a spinning top? The intricate motion of this children’s toy can hold one’s interest for some time. At times its motion seems to defy gravity and its movement seems to confound our expectations. We are not as used to watching rotating objects, so our intuition is not as well-developed. However, the rules that govern the motion of a child’s top govern the motion of all rotating object including the earth, the sun, and galaxies.

Precessing TopSaturday was the autumnal equinox – the first day of autumn. My blog the week of the summer solstice discussed why the date and exact time for the beginning of each season is not always the same (let’s say September 22 at noon). The topic for this week’s blog is also related to the timing of the seasons - the gradual shifting of the seasons due to earth’s rotational wobble – also called precession. Precession is a motion familiar to all of us who have watched a spinning top. As a top spins its axis of rotation will trace out a circle – if you imagine the axis of rotation as a line extending out into space, the line will sweep out the surface of a cone. Well, the earth spins (rotates) on its axis once a day. Its axis of rotation is the line connecting the north and south poles passing through the center of the earth. If you extend that line northward it points toward the star Polaris – the North Star, at least it points to Polaris at this time in history.

Celestial Sphere with PrecessionPolaris has not and will not always be the North Star because of the earth’s precession. In fact, Polaris has only been close enough to the North Celestial Pole (the point in space that the extend line through the earth’s north and south poles passing through the center of the earth we referred to above points) to be called the North Star over the past millennium. In another millennium, the North Celestial Pole will have gradually moved in its circular cycle enough that Polaris like all the other stars will not appear stationary. As you might have inferred by the fact that this motion will only be noticeable on the time scale of millennia, earth’s wobble is very slow compared to rate of rotation (one rotation each day) or rate of revolution of the earth around the sun (one revolution each year). The time to make one complete wobble – for the axis to trace out that cone in the sky is just slightly less than 26,000 years. Along the way there will be a few other North Stars beside Polaris but most of the time there will be no nearly stationary bright star in the northern sky. The diagram to the left can be helpful in visualizing how precession changes the North Star.

In addition to the circular drift of the earth’s celestial poles along the precession circle, precession also causes the very gradual shifting of the Ecliptic – the path the sun takes through the sky each year. The constellations along the Ecliptic are the twelve Zodiac constellations – famous for their importance to Astrology, the false belief that the positions of the sun and planets have some predictive power for events on earth. An equinox occurs when the Ecliptic crosses the celestial equator (the projection of earth’s equator on the sky). These occur twice a year once at the beginning of spring (the vernal equinox) and once marking the beginning of autumn. As the earth wobbles, the two points the sun crosses the celestial equator will move from one zodiac constellation to the next, spending a little over two thousand years in each constellation. Today, for instance, the vernal equinox takes place when the sun is in the constellation of Aquarius. This recently changed. Earlier the vernal equinox took place when the sun was in the constellation of Pisces. This inspired the Jefferson Airplane to sing the 1960s anthem about the “dawning of the age of Aquarius”.

All this amounts to some very interesting Astronomy trivia regarding the position of stars and timing of the seasons relative to our annual tour around our sun. However, what is most fascinating to me is that observing a spinning top can lead to understanding the motions of our planet. Rotational motion is essential to understanding not just positional astronomy but many other aspects of the natural world. The earth’s rotation affects the cyclical changing of the North Star and also plays a role in the development of hurricanes, the driving of the jet stream, and the direction of ocean currents. The fact that the universe is governed by laws that apply to tops, planets, stars, galaxies, and clusters of galaxies underscores the hope that through careful observation, study, and thought humans can not only admire with awe but also dare to understand the wonders of creation that surround us.

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