Mike's Pepper Garden


Tech Topic: About the Zodiac

The zodiac
First, the zodiac is actually a region of space, not a group of constellations.

Consider the orbit of the earth around the sun. The plane of this orbit is referred to as the ecliptic, or as the plane of the ecliptic.

The Plane of the Ecliptic

The zodiac is the belt of space that lies within 8 or 9 degrees of the ecliptic, i.e. from 8-9 degrees above the ecliptic to 8-9 degrees below the ecliptic (some references specify 8 degrees, some say 9).

The Zodiac

Since the zodiac is defined based on the path of the earth around the sun, it is no surprise that, viewed from earth, the sun appears to progress through this belt of space once over the course of a year. For millenia, it has been common practice to denote the location of the sun along its apparent annual journey by specifying the stars that lay in the same portion of the zodiac. When the sun appeared in the same part of the zodiac as the constellation Leo, it would thus be said that "the sun is in Leo". What this really means is "the earth is at a point in its orbit around the sun where the sun is between the earth and the constellation Leo, and thus it appears that the sun and Leo are in the same part of the sky". Of course, you cannot see the stars while the sun is out, but the Ancients were smart enough to figure out what stars were 'behind' the sun when it was out.

Because the moon and many of the planets stay within the zodiac throughout their travels, this method of denoting the position of a celestial body relative to the earth was usable for them as well. Since the moon completes an orbit of the earth in about 29.5 days, it appears to travel through the zodiac once over the course of a lunar month.

The signs of the zodiac
At some point a few millennia ago, this approach was formalized into a system in which the 360 degrees of the zodiac were divided into twelve equal 30 degree portions, called signs, each of which was then associated with a constellation that lay nearby in the zodiac; one reference associates this system with Ptolemy. This system, sometimes referred to as the tropical zodiac, is still used today for horoscopes and other astrological reckonings. Note that this system does not use the actual constellations themselves; rather, it uses arbitrary 30 degree arcs of space that were associated with nearby constellations.

Precession Precession
Unfortunately, the stars, as viewed from earth, are not fixed in space themselves; rather, they change their apparent position *very* slowly over time. This is because of a phenomena called precession, in which the moon and the sun cause the direction of the earth's rotational axis to drift via their gravitational attractions on the earth's equatorial bulge. The net effect of precession is illustrated to the left; the result is a slow but continuous change in the orientation of the earth. When considered from our viewpoint, this change appears to be a slow but continuous change in the position of the stars. The earth's axis makes one rotation along the path depicted in about 26,000 years.

In fact, precession is only one of a number of astronomical phenomena that contribute to the constant change in the stars' positions as viewed from earth. (One other rather obvious contributor is the physical motion of the stars themselves.) Taken together, the net result of these phenomena is that the constellations are no longer in the positions that they were in when the tropical zodiac was formalized. At this point in time, based on zodiacal ephemeris data from Lunabar 99 V4.0, the constellations have shifted 28 degrees (or almost a full sign) ahead of the signs. Accordingly, when the moon is said to be entering the sign of Aries, it is actually in the modern-day constellation of Pisces.

The constellations of the zodiac
According to Shapiro, there are currently 13 constellations that cross the ecliptic. These are the 12 constellations which are classically associated with the signs of the zodiac, plus Ophiuchus, which falls between Scorpius and Sagittarius. These constellations, taking into account their true widths, currently correspond to the signs of the Zodiac as depicted below. The widths of the constellations are taken from Lunabar (/Reference/Moon's Degree); Shapiro (calculated from Table 1) provides the same values to within a degree, and notes that the widths may vary by a degree from year to year.

When used to denote the positions of celestial bodies, the modern-day constellations of the ecliptic are sometimes referred to as the sidereal zodiac or the astronomical zodiac. On my planting calendars, the position of the moon using this system is referred to as its Zodiacal Constellation (ZC).

Modern Day Zodiacal Correspondences


References:
Site Map What's New Subscribe to Mike's Pepper Garden   Email Mike
Email a link to this page to:


Main | 2005 | 2004 | 2001 | 2000 | 1999 | Notes | Stuff | Science | Links | Refs | Articles | Photos | Map

Last updated 31 January 2015.
(c) 1999-2016 Mike Whittemore
All graphics (c) 1999-2016 Mike Whittemore
Designed for Netscape 4+ and IE 4+ at 640x480 min. resolution with CSS and JavaScript enabled.
Hosted by the The Homestead Collective.