The Magazine About Learning and Teaching Astronomy

The Real Zodiac Odds Are You Aren't The Sign You Think You Are It's a classic bar pickup line...."What sign are you?" (The classic answers include "Maternity Ward" and "No Trespassing"). Chances are whatever sign you say you are, you aren't. First, let's agree on what a "sun sign" is. As you might guess, a sign is a constellation. A sun sign must have something to do with the sun appearing with a constellation. So we should be able to easily agree on the definition of a sun sign: The constellation the sun is in on the day you were born. Agreed? Good. While the newspapers may list 12 equally sized star groupings for horoscope lovers, the astronomical fact is that the Sun passes through 13 constellations and touches a 14th. This automatically means that the sun can not possibly spend 30 days in each constellation! The sizes of the constellations are quite diverse, i.e. the sun does not spend equal amounts of time in each star group but spends as few as seven days to as much as 45 days in a constellation. Furthermore, the start dates of entry into each constellation do not mesh with horoscope dates because the original constellation borders were those of several thousand years ago. The earth's wobble - precession - has caused those borders to shift along the ecliptic, the path of the sun in the stars as seen from earth. Below are the constellations that house the sun, and the dates the Sun enters and leaves each, on average. Because of leap years and the non-integer number of real days in a year, the dates can shift either way by about a day. Dates Sun Is ConstellationIn The Constellation # of Days Sagittarius Dec 18 - Jan 18 32 Capricornus Jan 19 - Feb 15 23 Aquarius Feb 16 - Mar 11 24 Pisces Mar 12 - Apr 18 38 Cetus Mar 14 < 1 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 Did you find your birthdate? Are you aghast with astonishment? The number of days the true astronomical zodiac overlaps with the dates as listed in the newspaper columns totals only 53 days out of 365 days, or only about 1 chance in seven of someday actually having the "sign" they think they do! What about the 13th sign, the one at the bottom of the table? You have better odds at being a Serpent Bearer (that's what Ophiuchus is) than a Scorpion. Only a piece of the latter intersects the ecliptic while the Sun actually spents nearly three times as long traversing one of the good Serpent Holding Doctor's legs. You won't ever see a horoscope for being an Ophiuchan, there is apparently no future in it.... The path of the sun, the ecliptic, does not actually enter the constellation of Cetus but simply touches a point of its boundary between it and Pisces. The Sun, however, is a disk in the sky, not a point, and therefore part of the Sun (up to half the disk) strays into the area of the constellation for not quite a full day. You have to have a whale of a lot of luck to be born under the "sign" of Cetus, the Whale (or Sea Monster, prominent in the legend of Andromeda and Perseus, but that's a story for another night....). This article originally appeared in Hermograph Press' now out-of-print publication The AlterNETive Searcher but has been a feature in many To Teach The Stars Workshops.