Western Washington University   College of Sciences and Technology   Physics/Astronomy Dept.   Bellingham, WA USA
 
ASTRO 101

Analemma
Asteroids
Aurora
Big Bang
Black Holes
Bode Titius
Brightest Stars
Comets
Constellations
Coordinates
Cosmology
Cruithne
Dark Matter
Eclipses
Galaxies
Historical
HR Diagram
Hubble's Law
Intelligent Life
Kepler's Laws
Leap Year
Light Waves
Lunar Libration
Messier Objects
Meteors
Milky Way
Moon
Moon Phases
Planets
Precession
Rainbows
Redshift
Seasons
Stellar Evolution
Stardust
Sun & Fusion
Telescopes
Tides
Time of Day
Twilight
Zodiac
 
The Zodiac and the Ecliptic

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."

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

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.