Astronomical Basis of the Zodiac Signs
- Kishore S Kumar
This animation explains why the Zodiac is divided into 12 segments (and not 10 or 15 or any other number). The key is the recognition that Jupiter takes 12 times as long to complete a sidereal revolution as the Sun. The traditional definition of the Zodiac is worded with reference to the Sun's apparent motion around the earth relative to the fixed stars (i.e. sidereal motion). This definition given no clue as to why this sidereal orbit of the Sun is divided into 12 segments and named Aries, Taurus, etc. If the definition of the Zodiac is worded with reference to the sidereal motion of Jupiter around the Sun (and the Earth), then the Sun's sidereal motion acts as "markers" that divide Jupiter's sidereal orbit into 12 segments. This explains why the Zodiac is divided into 12 segments. Play around with the slider in the animation and observe that every full (apparent) revolution of the Sun around the Earth (i.e. one sidereal solar year) sees Jupiter traversing one twelfth (30 degrees) of its orbit.