How To Find Out My Zodiac Sign

The astronomy backside astrology

Practice you know that what you consider to be your zodiac sign, might non be right?
Do you know that since the invention of the zodiac system more 3000 years ago, the sky has really changed and the traditional zodiac signs do not stand for to the actual ones?

In this folio, I try to give a scientific caption to these questions and on the style, permit y'all discover your real zodiac sign. I first by introducing some astronomical concepts which I hope will give yous some insight about the geometry and the physics of the rotation of the Earth around the Lord's day and the apparent movement of the objects on the heaven.

I like to point out that the main purpose of this page is to show the astronomical facts backside what we call zodiac signs, and I only did it for fun! (after all, being an astronomer I am always asked about other people'southward horoscopes!). It is non my intention to discuss in depth the validity of star divination or its predictions, however I give my personal stance as an astronomer, if you're interested.

Introduction The zodiac and astrology are mutual concepts among many countries in the Western civilization. Surveys show that around ninety% 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 pop topic in television and radio programs. Although simply a small percentage really admit to take horoscopes seriously, the vast majority of the people practice not know the astronomical concepts behind their nativity sign.

Astrology originated in Mesopotamia effectually 3000 years agone. The ancient Babylonians performed methodical observations of the night sky and built bang-up 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 heaven were believed to be affixed to transparent celestial spheres and their motions were thought to be a outcome of the movement of these spheres as they revolved around the Globe. Groups of brilliant stars were observed to grade prominent patterns in the nighttime sky called constellations, which take been historically ascribed to mythological figures. The early astronomers recognized that constellations appeared and disappeared with the alter of the seasons throughout the class of a twelvemonth. In the same way, the Sun, Moon and planets were observed to motion in relation to the fixed groundwork of stars, or constellations. The Earth travels in space as information technology revolves about the Sun in a planar orbit that is approximately circular. If ane drew a line from the center of the Earth through the center of the Sun, that line would "describe" a large plane in the heavens as the World orbits the Sun. This large aeroplane is chosen the ecliptic plane, as is shown in Figure 1.

Effigy one
The ecliptic aeroplane and the ecliptical coordinate system

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

Figure iii
The band of the zodiac and the apparent position of the constellations with respect to the Earth-Lord's day organization

Based on their observations of the night sky, ancient astronomers adamant that during the daytime, the Sunday would appear to "enter" or pass through different constellations throughout the year. Because of their perspective from Earth, they observed that the Lord's day, the Moon and all the planets visible with the naked center seemed to pass in the class of a year through a region in the heaven occupied past twelve specific constellations. Those constellations are the ones that we would intersect if nosotros 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.eastward. that person's zodiac sign.

The zodiac constellations, as envisioned past ancient astronomers, were ascribed specific patterns that resemble the shapes of animals and human being beings. The constellations of the zodiac actually course an imaginary belt in the sky that extends about viii degrees above and below the ecliptic plane as is shown. As we look at the position of the zodiac constellations at any given fourth dimension of the twelvemonth, the Sun is betwixt the Earth and one of these constellations, every bit Effigy three shows.

Vernal and autumnal equinoxes

Nosotros are familiar with the Earth's equator considering of our knowledge of geography. If we could extend the earth's equator into space then that information technology could exist viewed against the background of stars, we would be able to see what in astronomy is called the celestial equator. Because the Earth'southward centrality of rotation is tilted with respect to the ecliptic past 23.5∫, the angelic equator and the ecliptic do not lie on the same airplane, only cantankerous each other at an angle of 23.v degrees as is shown in Figure 2. The 2 points in the sky where these two planes cross are called the equinoxes. Nosotros call the vernal equinox the intersection indicate where the Sun, in its apparent motility against the background stars along the ecliptic, crosses the celestial equator from south to n, usually occurring around March 21^st. Similarly, nosotros call the autumnal equinox the intersection point where the Sunday, in its credible motion against the background stars along the ecliptic, crosses the angelic equator from north to due south, usually occurring effectually September 21^st. The get-go mean solar day of Spring so corresponds to the vernal equinox and the first day of Autumn corresponds to the autumnal equinox. During the time of the equinoxes, we on the Earth feel twelve hours of day and twelve hours of dark.

The precession of the equinox

Effigy 4
The precession of the Earth around its axis

The Earth'southward rotation on its centrality has acquired the Earth'south shape to diverge from a sphere, and has acquired the Earth's equatorial regions to bulge out. Considering the Earth's equator is tilted with respect to the orbital plane of the World around the Sun, the Earth'southward equatorial bulge is also tilted with respect to the plane along which the Sun and Moon travel. The Moon and the Lord's day exert a gravitational elevate on the World'south equatorial bulge, trying to pull the Earth's equatorial region to be aligned with the ecliptic aeroplane. This pull, forth with the rotational motion of the Earth on its axis, the revolution of the Earth around the Dominicus, and the revolution of the Moon near the World, cause the Earth to wobble about its axis of rotation, like 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 in a higher place, the intersection of these two planes determine where on the zodiac our leap and fall equinoxes occur. This line of intersection is said to precess or move effectually the zodiac because of the wobble.

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

Figure iv shows a schematic of the Earth'southward precession, this result gradually changes where on the zodiac the equinox points fall. It takes about 2150 years for the equinox to travel xxx∫ or 1/12^th of the ecliptic. This precession ways 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 nosotros are going to start the Aquarius epoch).

An extension of the Globe's axis out into infinite traces out a conical effigy with a time wheel or period of 26,000 years. The Earth's precession implies that although Polaris is currently the star to a higher place our northward pole, in almost 13,000 years Vega will become our due north star; only later on still another 13,000 years, will the due north pole will once once again betoken towards Polaris, as shown in Figure v. Therefore, because of the World's precession, the constellation which is behind the Sun nowadays is actually dissimilar from the 1 predicted past 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 call up of the sky as a dandy sphere, the areas that different constellations comprehend can exist drawn fairly accurately. There are a number of days of the World'south orbit when the Sun is between our planet and any i of the zodiacal constellations. Since each constellation is of unlike size and since the ecliptic passes through larger or smaller portions of each constellation, and the speed of the Earth around the Lord's day varies along its orbit, the Lord's day is between the World and each zodiacal constellation for varying periods. For example, more days (44 days) are spent with the Dominicus between the Earth and the largest constellation, Virgo, than are spent with the Sunday between the Earth and the smallest constellation, Scorpius (seven days).

The boundaries of all the constellations in the heaven were set by the International Astronomical Union (IAU) in 1930. This was essentially a mapping practise to brand the work of astronomers more efficient. 2000 years agone, there were 12 constellations in our zodiac. At present our ecliptic passes through the boundaries of 13 constellations, the usual 12 and a new 1 known as Ophiuchus (or Serpentarius). Ophiuchus is depicted as a human being supporting a ophidian, the interposition of his body divides the snake into ii parts, Serpens Caput and Serpens Cauda. Ophiuchus, is located in a position near the middle of the Milky Fashion galaxy amid clouds of molecular hydrogen and dust. In add-on, 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 movement relative to the Lord's day. Merely as a remark, within a few hundred years the ecliptic will no longer pass through Scorpius but will likewise include the constellation of Orion.

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

The following table provides the dates at which the Sun is located inside the boundaries of a specific zodiac constellation as defined in 1930 past the International Astronomical Union, i.eastward. the periods of the existent zodiac signs.

The dates can vary by as much as ii days from year to yr, depending on the cycle of leap years. The solar zodiac cavalcade indicates the bodily dates when the Sun is located inside the boundaries of the named constellation. If you lot are on the purlieus between any 2 signs and you want to know your real sign (together with your ascendent and descendent signs, divers beneath), use the gratuitous software called Stellarium indicating the date, time and location of birth and it will give yous a nice heaven chart of your birthday through a overnice interface.

Sign	Significant	Symbols	Astrological (tropical) zodiac	Astronomical (solar) zodiac	Duration
Aries	The Ram		21^st March to 20^thursday Apr	xix^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^thursday August	21 days
Leo	The Lion		23^rd July to 23^rd August	11^th Baronial to 16^th September	37 days
Virgo	The Virgin		24^th Baronial to 22^nd September	17^th September to 31^st October	44 days
Libra	The Residuum		23^rd September to 23^rd October	1^st November to 23^rd November	23 days
Scorpius	The Scorpion		24^thursday October to 22^nd November	24^th November to xxx^th November	seven 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 Dec to 20^thursday January	20^th Jan to 16^th February	28 days
Aquarius	The Water Carrier		21^st Jan to 19^th February	17^th February to xi^thursday March	24 days
Pisces	The Fish		20^thursday February to 20^th March	12^th March to 18^th Apr	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 nascence. At this point, there is a zodiac sign on the sky which is just rising from the horizon. Due to the Globe'due south rotation, approximately every four minutes any object on the zodiac chugalug would elevate 1 degree from the horizon; hence the ascendent sign changes during the course of the mean solar day. On the other hand, on the west horizon, the constellation which is setting below the horizon at the fourth dimension of birth constitutes the descendant sign. Therefore, in gild to find the ascendant and descendant signs, i needs to know the precise local time and location (geographical latitude, longitude and fifty-fifty altitude) of birth. Simply as a remark, the ascendent and descendent signs have no astronomical meaning or importance whatsoever.