The movement of the Sun from below to above the equator sets the astronomical definition of spring in the northern hemisphere, because daytime then becomes longer than nighttime. Around 2000 years ago this equinox, traditionally called the vernal equinox (Latin: ver , "spring") was chosen to be the starting point for measurement through the zodiac. It is called the "zero degree", or "0° Aries" because Aries is the first of the twelve signs of the zodiac.

The angle between the ecliptic and equator is the reason the Earth experiences seasons. When the Sun is on the part of the ecliptic that crossses the equator, day and night are equal all over the world, so these intersection points are called "equinoxes" (Latin: aequus , "equal" + nox , "night").

A sign communicates information. Zodiac signs, sometimes popularly called "star signs", are determined by astronomical principles. The astronomers who originated the zodiac system also worked as astrologers and sought to attach and extract more than purely astronomical information from them; however, zodiac signs never have, and never will be visible - nor are they expected to make a close astronomical aligment with the constellations whose names they share. This would not be possible because constellations vary greatly in size whereas zodiac signs do not.

The sidereal zodiac is the same design as the tropical zodiac in all respects except for its starting point; it also uses an equal division of the zodiac into twelve 30° signs. The alignment with visible constellations is not expected to present an exact correlation with the constellation boundaries, some of these being much larger than others in astronomical reality.

Since the sidereal system no longer aims to consider the impact of seasonal change within its zodiacal meanings, it need not suffer the criticism that is meanings are prejudiced towards a northern hemisphere perspective. It focusses instead on the meaning of constellations and the angular relationship of the stars to each other. In fact, both systems offer an effective tool for the latter analysis, so tropical and sidereal astrologers share many astrological principles, have much to agree about, and often engage in productive discussions without any sense of essential conflict.

The theoretical problem that the tropical zodiac faces, is that its seasonal associations are designed for the experience of life in the northern hemisphere: there is no allowance for how mid-summer in the northern hemisphere is mirrored by mid-winter in the southern hemisphere. One supporting argument is that the tropical zodiac relates to an overall global experience, the greater landmass of the northern hemisphere meaning that, as a whole, the earth greens and grows at the time of the vernal equinox, and experiences a cycle of seasonal changes that accords with the principles of the zodiac.

Reference to the zodiac never faced historical criticism on theological grounds. Even important church leaders who criticised the use of astrology for making fatalistic predictions supported reference to the zodiac as a means to judge how the quality of time at birth leaves its imprints on temperamental disposition and physical health.

It is hard to overstate the massive historical impact this zodiac has had on Arabic and European art, culture and science, with most people expecting the zodiac to set the framework of their working year and planning their activities to co-ordinate with it.

1 The BBC brands its iWonder portal as being ""the BBC's new factual and educational site" at www.bbc.co.uk/learning , published 2004; (visited, 23/03/15).





2 Text from the BBC's promotional link to the page from its iWonder public Facebook page , published 11:14 am, 21/03/15, (visited, 22/03/15).





3 The BBC's editorial guidelines and values are published at www.bbc.co.uk/editorialguidelines/page/guidelines-editorial-values-editorial-values/standard , (visited, 23/03/15). My own previous attempts to inform the BBC of historical, astronomical, and factual innacuracies in reports they offer on the technicalities of astrology has shown these guidelines to be treated as lip service only for any subject that falls outside the mainstream definition of modern science. Instead the BBC appears to adopt a civil duty to pour ridicule on subjects like astrology by any means possible, fair or foul. On the basis that modern science rejects the credibility of astrology, astrology is excluded from the expectation of accurate and balanced reporting of its principles.





4 Geminos, a Greek scientific writer of the first century B.C., provides an early definition of the width of the zodiac as 12° (Intro. to the Phenomena, J. Evans and J. L. Berggren, Princeton Univ. Press, 2006, V.53; p.157). Later astronomers, such as Al Biruni , extended this to 18° (9° either side the ecliptic) to accomodate the greater latitude of Venus.



The principle of the zodiac belt being defined by maximum planetary latitudes, or "wide enough to accomodate the wanderings of the planets" is given in the 2nd century text of Achilleus, (Intro. to the Phenomena of Aratus, Maass, 1898, 53; quoted by Evans and Berggren p.157, n.28).



See also reference 9, for the definition of the zodiac given by the editors of Encyclopaedia Brittanica (2014).





5 The cycle between one New Moon and the next is called the Moon's synodic cycle, the word synodic deriving from a Greek word meaning "union", because this describes successive unions of the Moon with the Sun. The average length of the synodic month is 29.53059 days (or 29 days, 12 hours, 44 minutes and 3 seconds). This is longer than the time it takes for the Moon to make a revolution around the Earth (27.3 days) because as the Moon is travelling around the Earth, the Sun is also moving forward, so the Moon must move this extra distance to catch up with the Sun.



The solar year (successive returns of the Sun to the vernal equinox) contains 12 synodic lunations (354 days) with 11 days left over. Most ancient calendars, and the traditional Muslim calendar, account for this by adding an extra lunar month every three years, and a final extra lunar month every 30 years.





6 The use of a rudimentary form of the zodiac that supported observational astronomy is known to extend back thousands of years before the full development of the mathematical zodiac. The full development of the latter occured sometime around the 6th century B.C. A cuniform tablet dated to 523 B.C. shows that by then astronomers had gained the ability to precisely predict luminary positions and upcoming planetary conjunctions, and (in this tablet at least) were defining planetary positions within the mathematically derived zodiac signs. Because of its clear indications of what are seen as feats of astronomical development, this tablet has been described as "the oldest known document of the scientific astronomy of the Chaldeans" (F. Cumont, Astrology and Religion Among the Greeks and Romans, 1911, Dover, p.8)





7 Because the starting point of the tropical and sidereal frames of measurement roughly coincided 2000 years ago, many ancient astrological sources relate the zodiac's meanings both to the fixed stars and to the seasonal cycle. As the starting points for the two methods of measurement drifted apart over time, astrologers who favoured the tropical system gave less emphasis to the former and astrologers who favoured the sidereal system gave less emphasis to the latter. (For an understanding of what the tropical and sidereal frames of measurement are, see here ).





8 The Almagest, by Ptolemy edited by R. Hutchins, translated by R. Catesby Taliaferro, pp.1-479 in Vol. 16 of Great Books of the Western World. Chicago: Encyclopaedia Brittanica, Inc., 1952; VII.5, p.239.





9 See www.britannica.com/EBchecked/topic/657680/zodiac , last updated: 07/08/2014; (visited 24/03/15).



The definition of the zodiac in astronomy and astrology begins: zodiac, in astronomy and astrology, a belt around the heavens extending 9° on either side of the ecliptic, the plane of the earths orbit and of the suns apparent annual path. The orbits of the moon and of the principal planets also lie entirely within the zodiac. The 12 astrological signs of the zodiac are each considered to occupy 1/12 (or 30°) of its great circle. These signs no longer correspond to the astronomical constellations in which the sun actually appears. The constellations are irregular in size and shape, and the sun regularly passes through one constellation (Ophiuchus) that is not considered a member of the zodiac.





10 BBC Magazine: "Have the Zodiac and star signs changed?", www.bbc.co.uk/news/magazine-12207811 , published 17/01/11 (visited 24/03/2015).





13 Sidereal definitions of the zodiac were also historically used by some Mesopotamian, Hellenistic, Persian, and Arabic astrologers. (See also reference 7).



Amongst the oldest extant texts in any Indo-European language, the four canonical sacred texts of Hinduism, known as the Vedas, give testimony as to how old and sacred the twelve-fold design of the zodiac is:



Twelve spokes, one wheel, navels three.

Who can comprehend this?

On it are placed together

Three hundred and sixty like pegs.

They shake not in the least.

(Rig Veda 1.154.48)



One is the wheel; the bands are twelve;

three are the hubs  who can understand it?

Three hundred spokes and sixty in addition

have been hammered therein and firmly riveted .

(Atharva Veda X, 8)



These Vegas are believed to have been composed around 15001200 B.C.; for discussion of the relevancy of the quoted passage to the use of the zodiac see The Importance of Makar Sankranti in Hindu Calendar Reform, by Patrizia Norelli-Bachelet (2010). My thanks to Lori Tompkins for pointing me in the direction of this source.



