The Zodiac

As the Earth travels around the Sun, the Sun appears to pass in front of a succession of constellations. The linear path that the Sun describes across the sky is called the ecliptic. The constellations on that path are collectively called the zodiac and extend a few degrees above and below the ecliptic line.

The ecliptic is a line where the plane described by the Earth's orbit around the Sun crosses the celestial sphere. The plane is close to those described by many other bodies of the solar system. That is why the zodiac is the region of the sky where we find the planets and the asteroid belt, i.e. the disk of our solar system.

The zodiac (circle of animals) is the oldest known celestial coordinate system, probably dating back to about 5000 BC in ancient Babylon. It remained the principle method of describing points in the sky until the use of telescopes required a more precise system.

Different cultures have different definitions of the characters and borders in the sky. The Babylonian zodiac described 18 constellations instead of 12.

The newest member of the modern set is Libra, which the Romans added about 2000 years ago. Libra use to be part of the scorpion; his claws. The arabic names of the two brightest stars in Libra reflect the older description. Zuben el genubi means "the southern claw" and Zuben eschemali means "the northern claw."

The constellations of the zodiac, and the dates that the Sun passes through them, do not correspond to the ones listed in astrological horoscopes. The Earth's orientation in space has changed and those descriptions are antiquated; see precession of the equinox. The table above describes the current path of the Sun around the celestial sphere. Notice that although there are 12 "signs of the zodiac," there are actually 13 constellations on the Sun's path.



Capricornus
© University of Hong Kong Dept of Physics


Constellation
Sun Appears There
Total Days
Aries
The Ram
Apr 19 to May 13
25
Taurus
The Bull
May 14 to Jun 19
37
Gemini
The Twins
Jun 20 to Jul 20
31
Cancer
The Crab
Jul 21 to Aug 9
20
Leo
The Lion
Aug 10 to Sep 15
37
Virgo
The Maiden
Sep 16 to Oct 30
45
Libra
The Scales
Oct 31 to Nov 22
23
Scorpius
The Scorpion
Nov 23 to Nov 29
7
Ophiuchus
The Serpent Holder
Nov 30 to Dec 17
18
Sagittarius
The Archer
Dec 18 to Jan 18
32
Capricornus
The Sea-Goat
Jan 19 to Feb 15
28
Aquarius
The Water Bringer
Feb 16 to Mar 11
24 (25 leap)
Pisces
The Fishes
Mar 12 to Apr 18
38