Tropical zodiac
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The tropical zodiac is based upon tropical time, or the apparent motion of the sun across the sky over the course of a year. The tropical zodiac fixes the position of the sun in the heavens at the northern hemisphere vernal equinox as the first point of the sign of Aries. The signs of the zodiac in tropical astrology are therefore currently divorced from the actual constellations of the zodiac. This is due to the Precession of the equinoxes. The tropical zodiac therefore uses a view of the heavens as seen from Earth approximately 2600 years ago when the northern hemisphere vernal equinox did occur within the constelation of Aries.
This is in contrast to the sidereal zodiac, which is based on the apparent motion of the sun against the actual constellations of the zodiac in the sky.
The tropical zodiac is largely exclusive to The Americas and western European astrologers.
Some western astrologers choose to use the sidereal zodiac and criticise the tropical approach as erroneous. This is a long running dispute within western astrology.
The 13 Astronomical Constellations of the Ecliptic
The dates the sun passed in front of the thirteen astronomical constellations of the ecliptic in 1977 are as follows. The dates will increment by one day every 70½ years. Already several have changed. For example, the sun in now in Taurus at the summer solstice (June 21).
The 21 Astronomical Constellations of the Zodiac
Because of their inclination from the ecliptic, the planets are not restricted to the thirteen constellation of the ecliptic. The seven planets (excluding Pluto, which ranges more widely due to its high inclination) pass through 21 constellations. Thus there are 21 astronomical constellations of the zodiac. These are Aquarius, Aries, Cancer, Capricornus, Cetus, Corvus, Crater, Gemini, Hydra, Leo, Libra, Ophiuchus, Orion, Pegasus, Pisces, Sagittarius, Scorpius, Scutum, Sextans, Taurus, Virgo.
References
"The Real Constellations of the Zodiac". Dr. Lee T. Shapiro, Planetarian, Vol 6, #1, Spring 1977. [1] (http://www.griffithobs.org/IPSRealConst.html)
"The Real, Real Constellations of the Zodiac". John Mosley, Planetarian, Vol. 28, #4, December 1999. [2] (http://www.griffithobs.org/IPSrealrealconstellations.html)