Zodiac

For the East Asian zodiac, see Chinese zodiac. For other uses, see Zodiac (disambiguation).

The Earth's orbit around the Sun causes the apparent motion of the latter along the ecliptic (red). Earth is axially tilted 23.4° relative to this plane; its equator, extended to the stars, is shown in light blue.
Astrophotos of the twelve zodiac constellations

The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. Also within this zodiac belt appear the Moon and the brightest planets, along their orbital planes.[1] The zodiac is divided along the ecliptic into 12 equal parts ("signs"), each occupying 30° of celestial longitude. These signs roughly correspond to the astronomical constellations with the following modern names:[2][3] Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.

The signs have been used to determine the time of the year by identifying each sign with the days of the year the Sun is in the respective sign. In Western astrology, and formerly astronomy, the time of each sign is associated with different attributes. The zodiacal system and its angular measurement in 360 sexagesimal degree (°) originated with Babylonian astronomy during the 1st millennium BC. It was communicated into Greek astronomy by the 2nd century BC, as well as into developing the Hindu zodiac. Due to the precession of the equinoxes, the time of year that the Sun is in a given constellation has changed since Babylonian times, and the point of March equinox has moved from Aries into Pisces.

The zodiac forms a celestial coordinate system, or more specifically an ecliptic coordinate system, which takes the ecliptic as the origin of latitude and the Sun's position at vernal equinox as the origin of longitude.[4] In modern astronomy, the ecliptic coordinate system is still used for tracking Solar System objects.

  1. ^ "zodiac". Oxford Dictionaries. Oxford University Press. Archived from the original on 26 September 2016. Retrieved 19 September 2017.
  2. ^ Because the signs are each 30° in longitude but constellations have irregular shapes, and because of precession, they do not correspond exactly to the boundaries of the constellations after which they are named.
  3. ^ Noble, William (1902). "Papers communicated to the Association. The Signs of the Zodiac". Journal of the British Astronomical Association. 12: 242–244. Bibcode:1902JBAA...12..242N.
  4. ^ Leadbetter, Charles (1742). A Compleat System of Astronomy. J. Wilcox, London. p. 94.; numerous examples of this notation appear throughout the book.