A Sign of the Times

Let's see if I've got the story straight:

Ophiuchus, the Serpent Bearer, is a large but undistinguished constellation well known to stargazers.
Stellarium
Last week Parke Kunkle, who teaches astronomy in Minnesota, noted in the Minneapolis Star-Tribune that the zodiac should actually have 13 (not 12) constellations, the addition being Ophiuchus, the Serpent Bearer. Then he added that, over time, the precessional wobble of Earth's spin axis has shifted the astrological Sun signs a bit. Those who'd long believed they were under the influence of Leo were actually associated with Cancer, and so on. This revelation spread through the Web like wildfire, swamping Kunkle with interview requests from around the world.

It must have been a slow news day.

As pointed out by my S&T colleague Tony Flanders (our go-to guy for matters mythological), the ancient Roman polymath Claudius Ptolemy was perfectly aware of all this when he established both modern astronomy and modern astrology two millennia ago. He was content to have a portion of the constellation Ophiuchus poking over the ecliptic (the Sun's path among the stars) without counting it as a zodiacal constellation. "Needless to say," Flanders adds, "Ptolemy was also very well aware of precession — and very intentionally chose to ignore it."

Now, the point of telling you this isn't to lock horns with astrologers. But if asked they'll openly admit that the zodiacal signs used to derive their horoscopes reflect the Sun's position among the stars as would have been the case long ago, not now. Besides, even before astronomers established modern constellation boundaries in 1930, the Sun never spent exactly one month in each of the 12 wayposts along the zodiac. Sun signs have always been an approximation.

Just to prove that this really is old news, here's a humorous essay by California optician David Hasenauer that Sky & Telescope published in June 1998.


What's Your Sign?

It has happened to most of us. In the course of casual conversation we mention our interest in amateur astronomy, and someone within earshot approaches, smiling, and with great interest starts to discuss … astrology! The person is usually sincere and well meaning, which makes the confusion between the two "astros" especially disheartening! I must admit that in such encounters I used to clarify the difference between astronomy and astrology without too much patience or diplomacy.

Now I embrace the adage "If you can't beat them, join them." I'm not implying that I read my daily horoscope to determine whether to get out of bed in the morning. Rather, I find that any interest in something celestial, even astrology, can be used to introduce people to the joys of astronomy.

These days, when approached by an astrology enthusiast, I ask what his or her birth sign is — which the person is only too happy to share. "But what is your real birth sign?" I follow up. "The traditional ones listed in the newspapers generally aren't correct unless you were born a couple thousand years ago." At this point my unsuspecting student is intrigued, and the door is open to discuss all sorts of astronomical concepts.

For example, people often do not realize that an astrological birth sign means the Sun was "residing" among the stars of a particular constellation when a person was born. I'll often ask someone what month is the best time to view his or her birth-sign constellation in the sky. The usual answer is the month of the person's birthday, which of course is the worst choice! At that time of year the Sun and the given constellation are up together — it's daytime and the stars behind the Sun can't be seen. I explain that the best time to view your birth-sign constellation in the evening sky will be several months before your birthday.

Or I'll mention that every year the Sun drifts through the 12 common zodiacal constellations lying along its path in the sky (the ecliptic). But actually there is a 13th constellation on the ecliptic: Ophiuchus. So it comes as a surprise to most people that if you were born in the first half of December, you're an Ophiuchus.

The reason that a person's "real" birth sign isn't necessarily the one listed in the newspaper is due to the slow, 25,800-year wobble (precession) of Earth's rotation axis. We define one year as the interval from one vernal equinox to the next. The vernal equinox occurs in March when the Sun appears to cross the celestial equator heading north. However, this point shifts among the zodiacal constellations because of precession. Astrologically speaking, this means that for a person born on March 21st some 2,000 years ago (about the time that astrology was systematized), the Sun would have been among the stars of Aries, the Ram. But nowadays the Sun sits in Pisces on that date.

The traditional linking of birthdays and birth signs suggests the Sun spends about 30 days crossing each of the 12 zodiacal constellations. Actually, there is a lot of variation. Each year the Sun spends the most time in Virgo (more than 40 days) but only about a week within the boundary of Scorpius. (So true "Scorpios" are definitely in the minority.) And, as noted, for those born in early December, the Sun spends some time within Ophiuchus, the Serpent Bearer. Although it's one of the largest constellations in area, Ophiuchus is the unrespected Rodney Dangerfield of the zodiac.

Here's a table listing the dates and number of days that the Sun will reside within the boundaries of each constellation during the year. Precession causes these dates to slip about one day later every 70 years.

Sun Signs, Then and Now
ConstellationTraditionalActual (2011)Days
CapricornusDec. 22 – Jan. 21Jan. 20 – Feb. 1628
AquariusJan. 22 – Feb. 21Feb. 17 – Mar. 1224
PiscesFeb. 22 – Mar. 21Mar. 13 – Apr. 1938
AriesMar. 22 – Apr. 21Apr. 20 – May 1425
TaurusApr. 22 – May 21May 15 – June 2138
GeminiMay 22 – June 21June 22 – July 2130
CancerJune 22 – July 21July 22 – Aug. 1121
LeoJuly 22 – Aug. 21Aug. 12 – Sep. 1737
VirgoAug. 22 – Sep. 21Sep. 18 – Oct. 3144
LibraSep. 22 – Oct. 21Nov. 1 – Nov. 2222
ScorpiusOct. 22 – Nov. 21Nov. 23 – Nov. 308
OphiuchusDec. 1 – Dec. 1818
SagittariusNov. 22 – Dec. 21Dec. 19 – Jan. 2033

Armed with this information, you may not succeed in dissuading people from a belief in astrology. But if you build on their interest, you will certainly impart a better understanding of the science behind the pseudoscience.

22 thoughts on “A Sign of the Times

  1. Mike Coren

    “It must have been a slow news day.” Certainly must have been. I wrote a piece about Ophiuchus being the 13th sign of the zodiac in my college astronomy club newsletter in 1985!

  2. Tom Wargin

    I like to go with Guy Ottewell in his “The Astronomical Companion” and the lattitudes and Longitudes from places like Ephemeris.com which consider “Signs” as 30° by about 16° boxes along the ecliptic starting at the Vernal equinox point even though these signs have the same names as twelve of the constellations they are not in the same places as those same named constellations.

  3. Anthony Barreiro

    Interesting article, thanks. A number of people have been asking me about this recently, so it’s good to know the source of this current disruption in the noosphere.
    [Paragraph break] I agree with Tom Wargin that Guy Ottewell’s distinction between astrological signs and astronomical constellations is helpful. Furthermore, it’s useful to distinguish between the tropical zodiac and the sidereal zodiac. Both convey important information. The tropical zodiac is a handy and familiar way to keep track of when the Sun crosses the equator at the equinoxes (Sun entering Aries or Libra) and when the Sun touches the tropics of Cancer and Capricorn at the solstices. This is the fundamental annual cycle for all life on Earth! The sidereal zodiac keeps track of where the Sun is against the background constellations. And, for total wonks, the discrepancy between the tropical and sidereal zodiac keeps track of the 26,000 year long precession cycle.
    [Paragraph break] So I would suggest renaming the columns in the table: “Traditional” should be “Tropical” and “Actual” should be “Sidereal.” Like every frame of reference, either one is more or less useful for any given purpose. (And I appreciate reading an astronomical article that doesn’t spew hysterical contempt for astrology. David Hasenauer is right — interest in astrology can be an entry point to learning about astronomy.)

  4. Anthony Barreiro

    John — Precession is a very slow process, it takes about 2150 years for the Earth’s axis to precess 30 degrees, or one astrological sign. So the table in the article is accurate enough for 1952. On June 9, 1952 the Sun was in the astrological sign Gemini (you’re still a Gemini if you want to be), and the Sun was against the stars of the constellation Taurus.

  5. Charles Isbell

    A real mind-blowing article! Would the Zodiac be “more accurate” if we/they used 13-months (like the Jewish calendar, starting at the ‘first cresent moon’ with, actually, 13/year?!) Or should we stick with our Roman calendar with 30, 31, 28/29 days.
    I was born on (I think) July 4, 1939, under Cancer. Where am I now? How old am I (both+ ways?)
    And I thought learning to count and the proper names of the months ……….?!

  6. Peter Darvas

    Does anyone have a comment on the Zodiac signs having to do more with the 12+ year orbit of Jupiter through the star field? The Sumarians came up with the original symbols ..say 5 thousand years ago .. and probably got the idea from some prior civilization? …

  7. Keith

    The author is more creative and noble than I am. When folks ask for my “sign”, I give them the same answer Bugs Bunny did, “Lobo the Wolf!” Rather than educating them, I prefer to confuse astrologers further. When you tell them “Lobo”, do it with a deep voice so they may think you are a member of some ancient, secret Mayan astrology cult!

  8. Yale

    Another constellation the Ecliptic passes through is Cetus, though because it only clips the middle northwesternmost corner of the constellation’s rectangular-based boundaries, it doesn’t really “count”. In other news, many news sites reported that the star Betelgeuse in Orion could explode as a supernova in 2012 (what a coincidence THAT would be!), but the timing being that exact is quite unlikely even though the volume has shrunk by 40% in 15 years while the actual supernova explosion might not be seen for another million years.

  9. Tom Wargin

    If you go into Ephemeris.com [http://www.ephemeris.com/ephemeris.php] you can give it the exact moment of your bird and birth location and get the astrological sign as well as R.A. and Declination which you can then use to find the astronomical constellation for the Sun, Moon, and the planets

  10. Catherine Kenward

    Interesting commentaries make me want to put in my bit.
    We are running a solar clock (the earth’s relationship to the sun) that naturally divides the year into four quadrants and charts the equinoxes and solstices. That is what the division of the year into twelve signs is based on. It’s a solar calendar./Simultaneously, there’s the Moon’s yearly cycle of 13 months, each 28 days long. You can mark these months off on a regular calendar and observe them also. This hidden month is marked by the “Blue Moon”, that 13th Full Moon that occurs every year. This is where a 13th sign could be, almost asks to be, added. There’s a lot of variability with running a lunar calendar in that “New Year’s” is going to begin with up to a 28 day variation each year. For instance the Chinese and Tibetan calendars celebrate New Years at the New Moon in Aquarius each year. This can occur as early as Jan 21st or as late as Feb 18th. Sometimes you have that sneaky extra lunar month in there and you have two New Moons in Aquarius to chose from. Many different cultures have names that characterize these 13 lunar months./ Then there is the 25,000 (approx) year precessional clock based on the wobble of the earth’s axis. The constellations slowly slide past our (earth’s) equinoxtial markers, putting a different constellation at these points every 2000+ years. So here are three clocks all running simultaneously. What bigger cycles are also in play that are still beyond our scope?/ Even the constellations change as the stars move. If you look at ancient chinese astrology, going back 5000 years or so, some of the constellations have distorted as certain stars have moved. As an astrologer (yes,its true), I posit that the energies we astrologers associate with the stars (ie the signs-I’m not talking about the planets here)are related to a quadrant of the sky not from the constellations we use to mark it.

  11. Sheila Monaghan

    OK, I knew about this and have have been trying to keep it a secret from my family. What I really want to know is what is my and my grandson’s birth stone? (Dec 6 and Dec 18) Inquiring minds want to know especially if you are a Nana who honors her grand kids by wearing a piece of jewelry with their birth stones. I’ve always used the turquoise and Blue topaz of Sagittarius. Love this article.
    Sheila

  12. Tony Flanders

    The fact that Jupiter goes through one zodiac sign per year has always intrigued me, but I suspect it’s coincidence. Far more likely that the choice to divide the ecliptic into 12 pieces derives from the number of months in a year, or from the Sumerian’s numbering system, which was based on 10, 12 and 60. Incidentally, Sumeria almost certainly was the first civilization (society with writing, living in cities) in the world. And the whole astrological/astronomical system is fantastically sophisticated, requiring centuries of recorded observation and lots of abstract thought. So there’s every reason to believe that it originated in Sumeria.

  13. Frank R

    And Kelly, you wrote “let’s see if I’ve got this story straight”… Well, no, probably not. This astronomy teacher, Kunkle, in Minnesota was marginally connected with this story but ENDLESSLY quoted as the source, the creator of the new zodiac, in the social-networky chain letter that ensued. One M. Alex Johnson over at “Technolog” on msnbc.com did a fine job chasing down this craziness (and also putting it in a broader perspective). It started elsewhere… As many people have noted, this “gee astrologers is so stoopid cuz they don’t know about precession” fable has been around for decades. The folks at space.com have done a standard write-up of it for some years, and this year they posted it on New Years Eve. That appears to have been the original recent source of the story, merely highlighted by Kunkle, according to Johnson’s model. That could easily be, and I myself certainly read that version on space.com before the New Year and rolled my eyes (metaphorically) at seeing the same silly old story. This is a re-hash of a re-hash, and it’s complete nonsense. The “signs” of the zodiac are unaffected by precession and quite unrelated to the official (since 1930) borders of the constellations. I should add, however, that I do feel real pity for the folks at space.com who would have killed for this kind of publicity if they could have had it… Such is the insanity of the Internet in 2011.

  14. Pete from Canton

    This deal about the 13th sign and changing the dates of zodiac signs…..About 15 years ago I got in to an argument with someone about this. Their expertise was “ASTROLOGY” …. LOL …. Anyway, since I was the new kid. I didn’t know squat. Even though i had proof. Just goes to show you. People wouldn’t know the truth even if you had proof. Because if you shake their world in a different way. They’re not willing to except it.

  15. Frank R

    Sorry, folks, but it is emphatically bad astronomy to suggest that the signs of the zodiac have been changed by precession. They have not. The signs are no more and no less than thirty degree bands of ecliptic longitude beginning at the equinox –that is, beginning at the “First Point of Aries” (as it has been known for centuries despite the fact that this spot is not in the constellation Aries). The signs (yes, the very same ones referenced in daily newspaper horoscopes) were used by the community of professional astronomers, and the positions of the planets were listed by reference to them, in the world’s best ephemerides and almanacs well into the 19th century, and any beginning student of astronomy in that era could tell you the signs were only NAMED after the constellations, not equivalent to them. The signs have clearly and exactly delimited boundaries which do not change. By contrast, the constellations have been in a state of flux for centuries (find the constellation Quadrans, for which the Quadrantid meteors are named…) and did not have boundaries AT ALL until 1925-1930 when the IAU decided to lay down some formal boundaries for the convenience of astronomers. The idea that the signs should include Ophiuchus is a gross misrepresentation based on a poor understanding of the history of astronomy. Does that mean astrology is right?? Of course not. But spurious brow-beating about precession is not the way to disprove it.

  16. Fred Higgins

    I know it’s a bit ornery, but, whenever asked about my ‘sign’ my response has usually been “No Smoking”. But seriously folks, having been born on December 2, I have always been aware that the Sun was in Opiuchus on that date. Being a lifelong astronomy educator, it has been a natural open to the topic of constellations and precession.

  17. David Fried

    I was a little surprised to see Tony Flanders’ comment that Ptolemy was aware that the sun resides for a while in Ophiuchus. Surprised for two reasons: The modern constellation boundaries were set, as I understand it, by the IAU only in 1920 or so. Did the ancients even think of the constellations as having defined boundaries, and how could they locate them if they did? After all, Andromeda and Perseus traditionally share a star.

    Also, wouldn’t ancient astrologers, like modern ones, have thought of the zodiac as divided into 12 30-degree boxes, as Tom Wargin suggested? Did they ever concern themselves with the actual location of the Sun?

    So if anyone can give a cite or quotation for Ptolemy’s understanding that the sun passes through Oophiuchus, I’d love to see it.

  18. Vladimir from Russia

    Actually, as I can see from my astronomy book, Ptolemy definitely knew about this ancient Ophiuchus, which was included in his famous work Almagest, and its connection to the Ecliptic. I also have difficulties understanding the so called constellations, as one can see them differently: now they are like African nations on the map with arbitrary but rather “streamlined” borders. But at 17-18 cent. charts they were depicted simply with some drawings laid, as I got it, over some basic stars according to a particular “editor’s choice”; sometimes, they were outlined with also rather arbitrary lines (as on children’s pictures), so we can see here the seed of a modern notion of “borders”. But constellations for us are also these geometric figures, with an arbitrary number of lines (see, for example, the difference between the Big Dipper and Ursa Major, which actually are the same, or aren’t they?).I’m not a professional astronomer so view it all also as some interesting field for logicians and mathematicians…))

    As for signs, of course they are not constellations: just 30-degree bands, as Frank R wrote above, and this “speckled band” is perceived as being set just around our Earth, like a band on a girl’s head. But astrology also takes into account those stars far away and their psychic influence, but it’s a bit auxiliary system for the “main” one, though so called sideric astrology also exists (as other systems: heliocentric, etc.). But it’s very numeric: 12 signs can be easily divided into four elements (3 signs in each) or three “crosses”. 12 signs also correspond to 12 houses being united in the notion of “the field”, so any “sign-intruder” could basically destroy the whole beauty of this highly symmetrical canvas…What on earth for, if it decently works without these quite unnecessary…astronomical upgrades))?

  19. terry swann

    OK, sok ye it isn’t just astronomers and astrologers who could use a bit of re-training on constellational nomenclature for how lines are drawn and named. The constellation of
    Taurus, for the most part, will be the home of the sun at the June Solstice. But, Cartographers, you too, goggle (sic) Earth.. Are we going to have the Tropic of Cancer and the Tropic of Capricorn on our maps forever? Well. Like the old adage..Even a stopped clock can be right, twice a day.” Or in this case, twice in 26k years.
    On a similar vain, the boundaries of constellations like Ursa Minor and Octans will certainly be curious to young astronomers (if any, peace prevailing), in a few thousand years. “Teacher?, Why are some constellations bound by weird cone-shaped lines?” Points to ponder..
    Terry, from below the Tropic of Taurus!

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