See also: Zodiac Updates and Article: Sign of the times: Astrology story soars like a comet (January 2011)
The Real Constellations of the Zodiac
Dr. Lee T. Shapiro, Director
CB #3480 Morehead Bldg.
University of North Carolina
Chapel Hill, NC 27599-3480
Planetarian, Spring 1977
[see note at bottom]
Sooner or later anyone involved with astronomy has contact (or conflict) with astrology or someone interested in astrology. To a great extent the response from the astronomical and planetarium community has been defensive, denying the validity of astrology when asked. There have been some attempts to go on the offensive, such as the recent publication Objections to Astrology (Prometheus Books, 1975). However, such clearly antiastrology publications are probably only read by those who already are nonbelievers.
To put astrology in the proper perspective, we can do two things to try to remedy the situation. First we must stop spreading astrological information. Yes, I believe all of us have done this even though unintentionally. The prime example and major point of this article is in reference to the band of the zodiac. Consider just the ecliptic. How often have you referred to the twelve constellations that the sun passes through during a year? The number twelve is correct only if one is using astrological constellations. There are thirteen astronomical constellations that cross the ecliptic. Whenever you refer to the zodiac use the number thirteen and name the constellations of the ecliptic (see Table #l). If someone complains that these are not the right constellations, just point out that all constellations are arbitrary and strictly artificial. The ones we use are the official constellations of the International Astronomical Union.
In addition to ceasing our propagation of astrological information, we can also give the public further astronomical information that will hopefully have the effect of raising questions about astrological belief and practice. Table #1 also includes the dates for which the sun is in each constellation of the ecliptic and the number of days it spends in each constellation. If someone asks you what constellation the sun is in on a particular date, use this information and mention that the answer you are giving is the astronomical constellation the sun is in on the date in question. If you cross check between the dates listed here and the standard astrological dates, you will find there are approximately only four dozen dates in common.
Then there is a question of the definition of the zodiac itself. In a quick survey through about ten basic astronomy texts, the definition given was either 8° or 9° on either side of the ecliptic. To be on the conservative side, Table #2 lists those constellations that come within 8° of the ecliptic. (Venus is the only naked eye planet that reaches more than 8° away from the ecliptic. ) You can quickly see that there are not 12 constellations of the zodiac but 24. At the appropriate times you can list the nontraditional constellations in which the sun, moon, or planets can be found. The current American Ephemeris & Nautical Almanac and the Atlas Coeli (or some other suitable stellar atlas with constellation boundaries marked) are all that are needed for producing this type of list.
If you include the orbit of Pluto which is tilted at 17° to the ecliptic, then the zodiac also includes the constellations Bootes, Coma Berenices, Eridanus, and Leo Minor. Using this information, you can have an interesting time incorporating it into shows or creating public displays. If and when astrologers complain, your interest is promoting astronomy, not astrology.
However, there is one caution I would like to mention. If someone asks you whether you believe in astrology, ask them what they mean before you reply. If someone gives me a definition such as "the belief and study of cosmic influences on the earth and its creatures", I can agree that such influences do exist. However, I point out that while it is obvious there are cosmic influences, especially from the sun and the moon, there is no evidence that positions of the heavenly bodies can be used to predict the actions or characteristics of individuals.
Table # 1 Astronomical Constellations of the Ecliptic
Constellation; Dates; # of d
Sagittarius; Dec 18-Jan 18; 32
Capricornus; Jan 19-Feb 15; 28
Aquarius; Feb 16-Mar 11; 24
Pisces; Mar 12-Apr 1;8 38
Aries; Apr 19-May 13; 25
Taurus; May 14-Jun 19; 37
Gemini; Jun 20-Jul 20; 31
Cancer; Jul 21-Aug 9; 20
Leo; Aug 10-Sep 15; 37
Virgo; Sep 16-Oct 30; 45
Libra; Oct 31-Nov 22; 23
Scorpius; Nov 23-Nov 29; 7
Ophiuchus; Nov 30-Dec 17; 18
Dates may fluctuate plus or minus a day from year to year. Table # 2 Astronomical Constellations of the Zodiac
Aquarius, Cetus, Libra, Scorpius, Aries, Corvus, Ophiuchus, Scutum,
Auriga, Crater, Orion, Serpens, Cancer, Gemini ,Pegasus, Sextans,
Canis Minor, Hydra, Pisces, Taurus, Capricornus, Leo, Sagittarius, Virg
Reprinted from the Planetarian
, Vol 6, #1, Spring 1977. Copyright 1977 International Planetarium Society. For permission to reproduce please contact Executive Editor, Sharon Shanks
1999 update: John Mosley of the Griffith Observatory and the Belgian astronomer Jean Meeus investigated this question in 1999 and found that the planets (minus Pluto) actually pass through these 21 constellations (using IAU boundaries):
This is reported in the article "The Real, Real Constellations of the Zodiac."
2011 update from Luc Désamoré: My calculations showed that ... Venus passes (not often) through Auriga, Canis Minor, Serpens and also Aquila. You may see : (in french)
Some dates are :
Canis Minor : 2557-08-14
Auriga : -151-04-15 (152 BC)
Serpens : 4428-02-20
Aquila : 3635-03-01
This has already been discussed and agreed by Jean Meeus and John Mosley in October 2005....
There are in all 25 zodiacal constellations not 21. Venus is the only planet passing through all the 25 constellations.
Société Astronomique de Liège.