Defau HART and Robert Svoboda
Although objectively the Earth moves around the Sun in an elliptical orbit, the earth think that the Sun moves around our planet. The line on which it is made, the apparent movement of the Sun, called the Ecliptic. The movement of the nine planets of Jyotish, as it is seen by us from earth, occurs within a "ribbon" that encircles the sky extending for about eight degrees to the North and the same South of the Ecliptic. This limited zone of the sky within which the "wandering" of the planet, is called the zodiac. The zodiac is divided lengthwise into 360 degrees (°). Each degree is divided into 60 arc minutes ( ' ) and each minute into 60 seconds.
Although there is only one Ecliptic and, therefore, only one zodiac, the latter bears two different names in order to distinguish between two ways of counting using one and the same celestial circle: the tropical zodiac and the sidereal zodiac. He and the other zodiac circles consist of 360 degrees, and the other is divided by astrologers into twelve equal parts of 30 degrees each. Twelve parts of the tropical zodiac known as zodiac signs, and twelve segments of the sidereal zodiac — as the zodiacal constellations.
The majority of non-Asian countries astrologers use the vernal equinox (the apparent location of the Sun in the sky on the first day of spring) as the starting point for all your measurements along the zodiac. Due to some rather complex astronomical phenomena of the vernal equinox each year a little "floats", moving in a constant direction on the background of the sidereal zodiac at the rate of about one degree for every 72 This means that if at the beginning of spring in 1938, the Sun was in a certain relative point x relative to the sidereal zodiac, in 2010, it is the same day of the year will be at the point one degree lower than point x, as the vernal equinox point will move just 1 degree in the direction opposite to the normal motion of the planets. This phenomenon is called precession of the equinoxes.
And since the vernal equinox point moves constantly in the background of the so-called fixed, or "fixed", stars in the zodiac, the astrologers of the most ancient cultures, including India, preferred to carry out the measurements along the zodiac from a starting point that is more constant relative to the zodiac constellations, what's the point of the vernal equinox. This method of counting within the zodiacal circle known as fixed or sidereal zodiac (Latin for "sidereal" means "star"), while the zodiac in which the measurements are conducted from the vernal equinox, constantly sliding relative to the fixed stars, called the movable or tropical zodiac ("tropical" means "pertaining to a turn or transition" refers to "rotation" of the Sun, occurring at the equinoxes). Jyotishi call the former "nirayana chakra" (in approximate translation, "the wheel without movement"), and the second — "Sayana chakra" ("rolling wheel").
Although these two ways of measuring the zodiac has been known since ancient times, most of the Jyotisha traditions stubbornly refuse to use the tropical zodiac for astrological purposes, although occasionally I use it for some calendar calculations.
In the sidereal zodiac, certain stars are single or in groups — are used as markers (sample points) for recognition of the various sections of the zodiac, so that within each 30-degree longitudinal cut of the zodiac, or in the immediate vicinity, was one such star or group of stars, i.e. a constellation. In tropical zodiac, the twelve parts (the zodiac signs) are counted from the vernal equinox, which every year changes its position relative to the constellations. These twelve zodiac signs have nothing to do with the twelve zodiacal constellations, the names of which they were once called — not counting the names themselves. In view of the foregoing, persons familiar with the astrology of the West must be careful not to confuse the constellations of the sidereal zodiac with the notation adopted in the West of the tropical zodiac. List the names of the zodiac signs and the twelve currently used zodiacal constellations, ranging from conventional point — the origin of the zodiac-1 — Aries; 2 Taurus; 3 Twin; 4 — Cancer; 5 — Leo; 6 — Virgo; 7 — Libra; 8 — Scorpio; 9, Sagittarius; 10 Capricorn; 11, Aquarius; 12 — Pisces.
Based on the fact that some of the stars comprising the constellations of the zodiac, beyond the 30-degree sectors allotted to each of the twelve constellations, we can conclude that at first the constellations had standard sizes, so that, apparently, the selection of the twelve constellations was preceded by a time selection of the twelve signs of the tropical zodiac. On similar grounds we can conclude that 27 or 28 "Vedic constellations", known as the nakshatras, in the same way once appeared as irregular clusters of stars, and later turned into symbols equal sections of the sky. In the old days, before there was evenly divided the zodiac circle and is the same standard the circle of the nakshatras, jyotishi probably conducted their observations using a separate constellation, and one of the reference elements, the reference system is the zodiac. If this assumption is correct, it should confirm the common idea that astronomical observations in the Vedic period was based not so much on the accuracy of the calculations as at a General acquaintance with the peculiarities of the behavior of astronomical bodies as they pass through a particular portion of the sky.
--HART Defao and Robert Svoboda, Jyotish. Introduction to Indian astrology. The tropical or the sidereal zodiac.