Zodiac

Zodiac (Gr. zodiakos, commonly derived from zoon, 'an animal'), the name given by the ancients to an imaginary band extending round the celestial sphere, having as its mesial line the ecliptic or apparent path of the sun. It was fixed at about $16^\{\backslash circ\}$ in width, for the purpose of comprehending the paths of the sun and of the five planets (Mercury, Venus, Mars, Jupiter, and Saturn) which were then known; and as of these planets Mercury has by far the greatest inclination of orbit to the ecliptic, and the value of that element in his case is only $7^\{\backslash circ\}\; 0\text{'}\; 9\text{'}\text{'}$, the width given to the zodiac was amply sufficient for the required purpose. But when the career of planetary discovery commenced in the beginning of the 19th century the first three which were discovered (Ceres, Pallas, and Juno) at once destroyed the idea which had been long seated in men's minds, that no planets existed beyond the limits of the zodiac, by exhibiting orbits inclined to the ecliptic at no less angles than $10^\{\backslash circ\}\; 36\backslash frac\{1\}\{2\}\text{'}$, $34^\{\backslash circ\}\; 42\backslash frac\{1\}\{2\}\text{'}$, and $13^\{\backslash circ\}\; 3\backslash frac\{1\}\{2\}\text{'}$; and a large number since observed have been found to wander from $0^\{\backslash circ\}$ to $18^\{\backslash circ\}$ beyond the zodiac, from which circumstance they have, along with the three above mentioned, been denominated ultra-zodiacal planets. The stars in the zodiac were grouped into twelve constellations, to each of which $30^\{\backslash circ\}$ or $\backslash frac\{1\}\{12\}$th of the whole circle was assigned, though it often did not fill up that space, but was only situated in it; and this equable division into signs was of great advantage in defining the positions of the sun and planets at any epoch.

The constellations, with the appropriate symbols of the corresponding signs, are as follows: Aries (Ram) ♈; Taurus (Bull) ♉; Gemini (Twins) ♊; Cancer (Crab) ♋; Leo (Lion) ♌; Virgo (Virgin) ♍; Libra (Balance) ♎; Scorpio (Scorpion) ♏; Sagittarius (Archer) ♐; Capricornus (Goat) ♑; Aquarius (Water-bearer) ♒; Pisces (Fishes) ♓. As one half of the ecliptic is to the north, and the other to the south of the equator, the line of intersection of their planes is a diameter of each, and the two points in which this line meets the celestial sphere are known as the equinoctial points. The comparative immobility, with respect to the ecliptic, of these points suggested at once the employment of one or other of them as a point from which to reckon, and accordingly that point at which the sun crosses the equinoctial from south to north was fixed upon, and called the first point (or commencement) of Aries. After the sun had advanced eastward through this sign—i.e. $30^\{\backslash circ\}$ along the ecliptic—he entered the sign of Taurus, continuing his course onward through the others in the order in which they are given above, again crossing the equinoctial southwards at the point where he emerged from Virgo and entered Libra. This was the case with the sun during the time of Hipparchus (q.v.); but though the equinoctial points move very slowly, yet they do so continuously, and the westerly motion of $50\text{'}\text{'}$ annually which they have along the ecliptic has at the present time separated the sign Aries from the constellation Aries, and caused the former to correspond almost to the constellation Pisces. This gradual retrogression of the signs through the constellations of the zodiac will continue till they accomplish, in about 25,868 years, a complete circuit; after which period the sign and constellation of Aries will coincide, as they did in the time of Hipparchus. Neither the zodiac nor its constellations are of much use now in astronomy, except as, like the other constellations, affording an easy though somewhat fantastic nomenclature for the stars, and a rude but sometimes convenient mode of reference to their positions.