Your REAL zodiac sign

The astronomy behind astrology

Do you know that what you consider to be your zodiac sign, might not be correct?
Do you know that since the invention of the zodiac system more than 3000 years ago, the sky has actually changed and the traditional zodiac signs do not correspond to the actual ones?

In this page, I try to give a scientific explanation to these questions and on the way, let you discover your real zodiac sign. I start by introducing some astronomical concepts which I hope will give you some insight about the geometry and the physics of the rotation of the Earth around the Sun and the apparent movement of the objects on the sky.

I like to point out that the main purpose of this page is to show the astronomical facts behind what we call zodiac signs, and I just did it for fun! (after all, being an astronomer I am always asked about other people's horoscopes!). It is not my intention to discuss in depth the validity of astrology or its predictions, however I give my personal opinion as an astronomer, if you're interested.

Introduction

The zodiac and astrology are common concepts among many countries in the Western civilization. Surveys show that around 90% of the population know their zodiac sign, and almost half read their horoscopes regularly. Astrology columns help to sell millions of newspapers and magazines; and are a popular topic in television and radio programs. Although only a small percentage actually admit to take horoscopes seriously, the vast majority of the people do not know the astronomical concepts behind their birth sign.

Astrology originated in Mesopotamia around 3000 years ago. The ancient Babylonians performed methodical observations of the night sky and built great observatories where priests would study the skies, and the celestial bodies that they believed controlled life and events on Earth. In those times, objects in the sky were believed to be affixed to transparent celestial spheres and their motions were thought to be a result of the motion of these spheres as they revolved around the Earth. Groups of bright stars were observed to form prominent patterns in the night sky called constellations, which have been historically ascribed to mythological figures. The early astronomers recognized that constellations appeared and disappeared with the change of the seasons throughout the course of a year. In the same way, the Sun, Moon and planets were observed to move in relation to the fixed background of stars, or constellations. The Earth travels in space as it revolves about the Sun in a planar orbit that is approximately circular. If one drew a line from the center of the Earth through the center of the Sun, that line would "draw" a large plane in the heavens as the Earth orbits the Sun. This large plane is called the ecliptic plane, as is shown in Figure 1.

Figure 1
The ecliptic plane and the ecliptical coordinate system

Figure 2
The equatorial plane, the annual seasons and the equinoxes

Figure 3
The band of the zodiac and the apparent position of the constellations with respect to the Earth-Sun system

Based on their observations of the night sky, ancient astronomers determined that during the daytime, the Sun would appear to "enter" or pass through different constellations throughout the year. Because of their perspective from Earth, they observed that the Sun, the Moon and all the planets visible with the naked eye seemed to pass in the course of a year through a region in the sky occupied by twelve specific constellations. Those constellations are the ones that we would intersect if we extend the ecliptic plane out into space. These twelve constellations were called the Zodiac. Many ancient people believed that a person's behavior, emotions, and fate were heavily influenced by the time of that person's birth i.e. that person's zodiac sign.

The zodiac constellations, as envisioned by ancient astronomers, were ascribed specific patterns that resemble the shapes of animals and human beings. The constellations of the zodiac actually form an imaginary belt in the sky that extends about eight degrees above and below the ecliptic plane as is shown. As we look at the position of the zodiac constellations at any given time of the year, the Sun is between the Earth and one of these constellations, as Figure 3 shows.

Vernal and autumnal equinoxes

We are familiar with the Earth's equator because of our knowledge of geography. If we could extend the earth's equator into space so that it could be viewed against the background of stars, we would be able to see what in astronomy is called the celestial equator. Because the Earth's axis of rotation is tilted with respect to the ecliptic by 23.5∫, the celestial equator and the ecliptic do not lie on the same plane, but cross each other at an angle of 23.5 degrees as is shown in Figure 2. The two points in the sky where these two planes cross are called the equinoxes. We call the vernal equinox the intersection point where the Sun, in its apparent motion against the background stars along the ecliptic, crosses the celestial equator from south to north, usually occurring around March 21^st. Similarly, we call the autumnal equinox the intersection point where the Sun, in its apparent motion against the background stars along the ecliptic, crosses the celestial equator from north to south, usually occurring around September 21^st. The first day of Spring then corresponds to the vernal equinox and the first day of Fall corresponds to the autumnal equinox. During the time of the equinoxes, we on the Earth experience twelve hours of day and twelve hours of night.

The precession of the equinox

Figure 4
The precession of the Earth around its axis

The Earth's rotation on its axis has caused the Earth's shape to diverge from a sphere, and has caused the Earth's equatorial regions to bulge out. Because the Earth's equator is tilted with respect to the orbital plane of the Earth around the Sun, the Earth's equatorial bulge is also tilted with respect to the plane along which the Sun and Moon travel. The Moon and the Sun exert a gravitational drag on the Earth's equatorial bulge, trying to pull the Earth's equatorial region to be aligned with the ecliptic plane. This pull, along with the rotational motion of the Earth on its axis, the revolution of the Earth around the Sun, and the revolution of the Moon about the Earth, cause the Earth to wobble about its axis of rotation, similar to the motion of a spinning top. This motion is called precession. It is the wobbling of the equatorial plane that causes the line of the intersection of the equatorial and ecliptic planes to move. As mentioned above, the intersection of these two planes determine where on the zodiac our spring and fall equinoxes occur. This line of intersection is said to precess or move around the zodiac because of the wobble.

Figure 5
Circular path that the north equatorial pole describes due to the precession of the Earth

Figure 4 shows a schematic of the Earth's precession, this effect gradually changes where on the zodiac the equinox points fall. It takes about 2150 years for the equinox to travel 30∫ or 1/12^th of the ecliptic. This precession means that the spring equinox was just entering Pisces 2000 years ago and it is about to enter the constellation of Aquarius (that is the reason why many astrologers say that we are going to start the Aquarius epoch).

An extension of the Earth's axis out into space traces out a conical figure with a time cycle or period of 26,000 years. The Earth's precession implies that although Polaris is currently the star above our north pole, in about 13,000 years Vega will become our north star; only after yet another 13,000 years, will the north pole will once again point towards Polaris, as shown in Figure 5. Therefore, because of the Earth's precession, the constellation which is behind the Sun nowadays is actually different from the one predicted by astrologers.

Ophiuchus, the 13^th constellation of the zodiac

The constellations of the zodiac at the present

Unlike the zodiac signs in astrology, the astronomical constellations vary widely in size. If we think of the sky as a great sphere, the areas that different constellations cover can be drawn fairly accurately. There are a number of days of the Earth's orbit when the Sun is between our planet and any one of the zodiacal constellations. Since each constellation is of different size and since the ecliptic passes through larger or smaller portions of each constellation, and the speed of the Earth around the Sun varies along its orbit, the Sun is between the Earth and each zodiacal constellation for varying periods. For example, more days (44 days) are spent with the Sun between the Earth and the largest constellation, Virgo, than are spent with the Sun between the Earth and the smallest constellation, Scorpius (7 days).

The boundaries of all the constellations in the sky were set by the International Astronomical Union (IAU) in 1930. This was essentially a mapping exercise to make the work of astronomers more efficient. 2000 years ago, there were 12 constellations in our zodiac. At present our ecliptic passes through the boundaries of 13 constellations, the usual 12 and a new one known as Ophiuchus (or Serpentarius). Ophiuchus is depicted as a man supporting a serpent, the interposition of his body divides the snake into two parts, Serpens Caput and Serpens Cauda. Ophiuchus, is located in a position near the center of the Milky Way galaxy amid clouds of molecular hydrogen and dust. In addition, although not part of the original constellation stars, the so-called Barnard's Star is located within Ophiuchus; this object has the largest known proper motion relative to the Sun. Just as a remark, within a few hundred years the ecliptic will no longer pass through Scorpius but will also include the constellation of Orion.

Most astrologers use a different system to determine the size of our zodiacal constellations. The ecliptic (which is also the name to the apparent path of the Sun on the sky, which creates a circle of 360∫) is simply divided up into twelve equal segments (of 30∫) corresponding to the same amount of months in the calendar, just by convenience, marking the start of the year cycle at the so-called first point of Aries, i.e. the point on the sky where the ecliptic and equatorial planes intersect, i.e. the vernal equinox. This point occurred 2000 years ago in the constellation of Aries, but nowadays this occurs in the constellation of Piscis, making this constellation the start-point of the current zodiac solar system.

Your REAL zodiac sign

The following table provides the dates at which the Sun is located within the boundaries of a specific zodiac constellation as defined in 1930 by the International Astronomical Union, i.e. the periods of the real zodiac signs.

The dates can vary by as much as 2 days from year to year, depending on the cycle of leap years. The solar zodiac column indicates the actual dates when the Sun is located within the boundaries of the named constellation. If you are on the boundary between any two signs and you want to know your real sign (together with your ascendent and descendent signs, defined below), use the free software called Stellarium indicating the date, time and location of birth and it will give you a nice sky chart of your birthday through a nice interface.

Sign	Meaning	Symbols	Astrological (tropical) zodiac	Astronomical (solar) zodiac	Duration
Aries	The Ram		21^st March to 20^th April	19^th April to 14^th May	25 days
Taurus	The Bull		21^st April to 21^st May	15^th May to 21^st June	38 days
Gemini	The Twins		22^nd May to 21^st June	22^nd June to 20^th July	30 days
Cancer	The Crab		22^nd June to 22^nd July	21^st July to 10^th August	21 days
Leo	The Lion		23^rd July to 23^rd August	11^th August to 16^th September	37 days
Virgo	The Virgin		24^th August to 22^nd September	17^th September to 31^st October	44 days
Libra	The Balance		23^rd September to 23^rd October	1^st November to 23^rd November	23 days
Scorpius	The Scorpion		24^th October to 22^nd November	24^th November to 30^th November	7 days
Ophiuchus	The Serpent Bearer		N/A	1^st December to 18^th December	18 days
Sagittarius	The Archer		23^rd November to 22^nd December	19^th December to 19^th January	31 days
Capricornus	The Goat		23^rd December to 20^th January	20^th January to 16^th February	28 days
Aquarius	The Water Carrier		21^st January to 19^th February	17^th February to 11^th March	24 days
Pisces	The Fish		20^th February to 20^th March	12^th March to 18^th April	38 days

The ascendent and descendent signs

The ascendent sign constellation correspond to the position on the sky in which the ecliptic intercepts the horizon on the east at the local time and place of birth. At this point, there is a zodiac sign on the sky which is just rising from the horizon. Due to the Earth's rotation, approximately every 4 minutes any object on the zodiac belt would elevate 1 degree from the horizon; hence the ascendent sign changes during the course of the day. On the other hand, on the west horizon, the constellation which is setting below the horizon at the time of birth constitutes the descendant sign. Therefore, in order to find the ascendant and descendant signs, one needs to know the precise local time and location (geographical latitude, longitude and even altitude) of birth. Just as a remark, the ascendent and descendent signs have no astronomical meaning or importance whatsoever.