Let's see if I've got the story straight: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|
|Capricornus||Dec. 22 Jan. 21||Jan. 20 Feb. 16||28|
|Aquarius||Jan. 22 Feb. 21||Feb. 17 Mar. 12||24|
|Pisces||Feb. 22 Mar. 21||Mar. 13 Apr. 19||38|
|Aries||Mar. 22 Apr. 21||Apr. 20 May 14||25|
|Taurus||Apr. 22 May 21||May 15 June 21||38|
|Gemini||May 22 June 21||June 22 July 21||30|
|Cancer||June 22 July 21||July 22 Aug. 11||21|
|Leo||July 22 Aug. 21||Aug. 12 Sep. 17||37|
|Virgo||Aug. 22 Sep. 21||Sep. 18 Oct. 31||44|
|Libra||Sep. 22 Oct. 21||Nov. 1 Nov. 22||22|
|Scorpius||Oct. 22 Nov. 21||Nov. 23 Nov. 30||8|
|Ophiuchus||Dec. 1 Dec. 18||18|
|Sagittarius||Nov. 22 Dec. 21||Dec. 19 Jan. 20||33|
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.