Article by John Mosley - 1999
The Real, Real Constellations of the Zodiac
an update of an earlier article in the Planetarian
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In 1977, Lee Shapiro, then Director of the Abrams Planetarium at Michigan State University, published an article in the Planetarian titled "The Real Constellations of the Zodiac." I felt the article was so important and so useful that it is one of the first I posted at the Planetarian web site.
Dr. Shapiro wrote the article to "give the public further astronomical information that will hopefully have the effect of raising questions about astrological belief and practice." To do this, he points out that the sun passes through 13 constellations, rather than the 12 of traditional astrology, and he asks, "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. 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." Dr. Shapiro provides the actual dates when the sun is in each of the 13 constellations.
Dr. Shapiro further points out that the moon and planets stray above and below the ecliptic and pass through additional constellations. These he calls "the astronomical constellations of the zodiac" (as opposed to the 13 "astronomical constellations of the ecliptic"). He defines the "astronomical constellations of the zodiac" as those which lie in part within eight degrees of the ecliptic. They include the approximate limits of the planets except Pluto, which strays farther and which enters yet additional constellations. As Uranus and Neptune stay very near the ecliptic, this 8° limit would include the constellations visited by all the naked eye planets. His list includes 24 constellations, and is printed as Table 2 in his article.
While writing a guide book to accompany a new astronomy program by Sienna Software, I examined this list to verify it, as I wished to include it in the guide. I discovered that it is incorrect. Dr. Shapiro lists all constellations that lie at least in part within 8° of the ecliptic, but he chose this number only as a "reasonable" value. It turns out the correct pattern is not this simple.
I used the desktop planetarium program Starry Night by Sienna Software, and double-checked with Voyager II by Carina Software, to find by inspection those constellations which are visited by the planets. I followed the planets forward and backward in time through several thousand years, and discovered that the planets do not pass through three constellations in Dr. Shapiro's 1977 list: Auriga, Canis Minor, and Serpens. They do pass through the 21 constellations listed below.
As my conclusions were based on visual inspection of the planets' positions with desktop planetarium programs, I sought confirmation from published sources. To my surprise, this problem had not been solved previously, and as far as I am aware, there is no prior published list of the constellations the moon and planets pass through.
Jan Meeus, the well-known Belgian computer of astronomical events, very kindly confirmed this list by performing rigorous calculations. He found the limits of the planets' declinations. I wish to acknowledge my thanks to him for his work.
The sun, moon, and planets are often in Ophiuchus, but they are less often Orion. It is worth pointing out when they are, and the public seems to find this interesting. It is one of those curiosities of the sky that generates a "I didn't know that!" reaction.
Dr. Shapiro was on the mark when he wrote, in 1977, at the conclusion of his article (the italics are mine), "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."
When we present astronomical information to the public, we should be diligent to not buy into astrology. The sun passes through 13 constellations, not 12, and the dates when the sun is within the boundaries of an astronomical constellation do not correspond to the dates when it is within the boundaries of an astrological sign. Use the astronomical information. Likewise, the moon and planets pass through yet additional constellations. When Venus is in Scutum, point that out. It causes people to question their assumptions and beliefs.
The 21 Astronomical Constellations of the Zodiac
Reproduced from the Planetarian, Vol. 28, #4, December 1999. Copyright 1999 International Planetarium Society. For permission to reproduce please contact Executive Editor, Sharon Shanks.
This question has since been investigated rigorously by Jean Meeus as Chapter 58 of More Mathematical Astronomy Morsels, Willmann-Bell, Richmond, Virginia 2002. New work that reveals dates when planets are in additional constellations appears in Le Ciel, Bulletin de la Société Astronomique de Liège, Octobre 2005, Quand les planètes quittent le zodiaque, by Luc Désamoré.