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Precession,
The Movie
copyright © 1998-1999 by Richard Nolle
last updated FEB 16, 1999

Precession is the apparent backward motion of the vernal equinox (and the poles) with respect to the so-called 'fixed stars'. You can see it in action from 4,713 BCE (BC) to 10,337 CE (AD) by clicking on the JavaScript or QuickTime movie icon in the selection window at left. And of course there's my own narration to go with the movie, as you'll see if you scroll down through this window. (That way, you can have what you see explained while you see it.) But please do read the following instructions first!

Bear in mind that the QuickTime version is a 2 Megabyte download, so you can't be in a hurry about this. (Load time can be 6-8 minutes with a 28k modem.) If you can't set your browser window to at least 800x600 pixels - or if you don't know what this means - then don't bother selecting the QuickTime option. Naturally, you must have the QuickTime plug-in in order to play this version of the movie. Otherwise you'll be prompted to download and install the plug-in after the movie finishes loading. The latest incarnation of QuickTime (Version 3) is currently a 6.8 Mb free download. (Netscape users beware: QuickTime 3 for Windows is incompatible with your browser's LiveConnect audio control codes, so stick with Version 2. Version 3 works fine with Microsoft's Internet Explorer, however. See the FAQ if you have questions about this.) When you finish watching the QuickTime movie, use your browser's back button to return to the selection window. And unless you also want to view the JavaScript version (same movie, what's the point?), you can then exit this page with a click on your browser's back button.

The JavaScript version, on the other hand, is only a 154 kb download. So it's much faster: less than a minute with a 28k modem. To exit this page after viewing the JavaScript movie, click exit in the movie window (below the celestial sphere) and then click on your browser's back key; or click here. (Which method works depends on what browser you're running: Microsoft and Netscape have different implementations of JavaScript.) If your monitor is large enough and your resolution high enough, you can also try clicking on the back button (left arrow) you'll see in the frame below the movie. (Lower resolution smaller screens won't display the frame.) But here again, this method depends on which flavor of JavaScript your browser recognizes. One way or another, one of these methods will exit this page for you.

Technicalities aside, either movie is seen from the perspective of a point far above Planet Earth (which is at the center of the sphere you see rotating in the movie). This is of course the celestial sphere. As explained in my previous article on Signs & Constellations, the celestial sphere is an imaginary construct which represents the way the Sun, Moon, planets, stars etc. appear to us here on Earth. Envision a great crystal sphere with Earth at its center and everything else out in space lying on (or projected through) the surface of that sphere, and you've pretty well got the picture.

Our view is centered on the northern vernal equinox, the point occupied by the sun as seen from Earth at the beginning of spring (in the northern hemisphere). The red circle cocked at an angle to the celestial equator in the movie is the ecliptic, the apparent annual path the Sun takes through the heavens as seen from Earth. The ecliptic circle is inclined to the equator because Earth's poles are tilted in relation to the path our home planet follows in its orbit around the Sun. Earth's north pole is marked HNP in the movie: it's the axis about which our planet rotates to produce the daily cycle of light and darkness. (The pole of Earth's orbital revolution around the sun is marked ENP.)

Along the red circle of the ecliptic are the constellations of the zodiac. (Their names are abbreviated in the movie.) Selected other constellations are also shown: these are the northern polar star groups. They include the Little Dipper (the Little Bear, Ursa Minor). This constellation includes Polaris, the familiar north star of our era. But as you can see by running the movie, other constellations contain the pole star in other epochs. Just as the equinox moves through the ecliptic (zodiacal) constellations over time, so do the poles point to different circumpolar star groups from one epoch to the next. And so we have different pole stars down through the ages, as well as different constellations to serve as a backdrop for the vernal equinox.

By running and re-running the movie, you can see the backward motion of the vernal equinox through the constellations over time. Starting out at 4,713 BCE in Taurus, the equinox regresses into Aries, then Pisces (its current location), then Aquarius - yes Virginia, that's the so-called Age of Aquarius - then Capricorn . . . and so on to end up in Libra by the year 10,337 CE. At the same time, the pole star is also changing.