Activity: Precession of the Equinoxes

1995 The Regents of the University of California

by Beth Napier


As explained in the Introduction section of this lesson plan, Precession causes the Earth to wobble on its axis with a period of 26,000 years.

The Earth's rotation on its axis has caused the Earth's shape to diverge from a sphere, and has caused the Earth's equatorial regions to bulge out, in the same way that a skater's skirt spreads outward as she spins rapidly on the ice. Because the Earth's equator is tilted with respect to the orbital plane of the Earth around the Sun, the so-called Ecliptic plane, the Earth's equatorial bulge is also tilted with respect of the plane along which the Sun and Moon travel. The Moon and the Sun exert a gravitational "tug" on the Earth's equatorial bulge, trying to pull the Earth's equatorial region to be aligned with the Ecliptic plane. This tug, along with the rotational motion of the Earth on its axis, the revolution of the Earth around the Sun, and the revolution of the Moon about the Earth, cause the Earth to wobble about its axis of rotation, similar to the motion of a spinning top. This motion is called Precession. An extension of the Earth's axis out into space traces out a conical figure with a time cycle or period of 26,000 years. "Figure 3" shows a schematic of the Earth's precession. The Earth's precession implies that although Polaris is currently the star above our North pole, in about 13,000 years Vega will become our North star; only, after yet another 13,000 years, will our North pole will once again point towards Polaris.

Precession causes the position of the Equinoxes against the background stars to gradually change, with a cycle of 26,000 years. This position change, called the Precession of the Equinoxes imply that the date of the Sun's apparent "entry" into the different constellations of the zodiac is no longer the same as the dates given by traditional astrology. These concepts are easier to demonstrate than to explain. The following simple activity should allow your students to understand these ideas.

Set a toy top or other spinner into motion. Point out to the students that as long as the spinner is moving quickly, the highest point is perpendicular (straight up and down) with respect to the table or surface on which the spinner is moving. However, as the spinner begins to slow down, the highest point begins to wobble, and the high point of the spinner begins to tilt towards the center of the Earth, which is tugging at the top's "equatorial bulge."

This wobble describes a cone-shape in the air that is extremely similar to the shape described by the Earth's axis in space: at one moment, the high point leans more towards one side, and in another moment the point leans towards the opposite side. Have the students estimate how long it takes for the top's high point to go once around, so that they can derive the period of precession of the top, which will be a lot shorter than 26,000 years!

The Earth's axis traces out a conical shape in space in a similar way as the top. At one point in time, our North pole points towards the star Polaris, and 13,000 years later, the North pole points towards the star Vega. This motion of Earth's axis goes back and forth every 13,000 years or so, causing changes in position of the Equinoxes agains the background stars, and therefore causing changes in the the dates of the Sun's apparent "entry" into the constellations of the zodiac.

Many hundreds of years ago, the dates of the Sun's "entry" into the zodiac constellations was recorded by the astrologers of the day. However, modern astronomy has shown that these dates are no longer valid, due to precession. Thus, the dates given by astrology does not match the visual location of the Sun along the Plane of the Ecliptic. The astronomical dates given in "Figure A," "Figure B," and "Figure C," for some of the constellations, as an example, are correct and based on scientific investigation.


Have students re-determine their astrological sign according to the dates of the Sun's actual "entry" into the different zodiac constellations. Then let the students list the differences between their astrological and astronomical signs. Do students agree or disagree with the new attributes assigned to them according to the more accurate dates.

Have students make up a new game based on "Jet-ology" by which their daily lives would be affected by air traffic above their house. Have the students discuss why this new game would be any more or less silly than astrology.

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