The tropical zodiac defines the vernal point (the first day of spring in the northern hemisphere) as the first degree of Aries, but the sidereal zodiac allows it to process. Many people are confused regarding the difference between the sidereal zodiac and the tropical zodiac signs. Because of a “wobble” in the Earth’s axis of rotation over a period of about 26,000 years (often called a “great year“), the rate at which the vernal equinox precesses in the heavens is approximately 0 deg, 0 min, 50.23 seconds a year, drifting by one degree every 72 years. Precession of the equinoxes thus occurs at a rate of roughly 5 arc minutes of a degree every 6 years. The tropical signs relate to the seasons and not the stars. Here is an example: a person born on, say August 28, 2002 would come to understand that his/her Sun sign was in Virgo according to western astrology (conventional Sun sign dates August 23, to September 22, of every year), but Sun on that same calendar date of the year 2002 was in the constellation Leo (where it had been since August 10, 2002 and would remain until September 15, when it would then finally cross into Virgo).
It is worth pointing out that the sidereal signs and the tropical signs are both geometrical conventions of 30° each, whereas the zodiacal constellations are pictorial representations of mythological figures projected onto the celestial sphere based on patterns of visible star groupings, none of which occupy precisely 30° of the ecliptic. So constellations and signs are not the same, although for historical reasons they might have the same names.
It is also worth noting that some astrologers don’t use the signs of the zodiac at all, focusing more instead on the astrological aspects and other features of the horoscope.
- The sun sign is the sign of the zodiac in which the sun is located for the native. This is the single astrological fact familiar to most people. If an event occurs at sunrise the ascendant and sun sign will be the same; other rising signs can then be estimated at approximately two-hour intervals from there.
- A cusp is the boundary between two signs or houses. For some the cusp includes a small portion of the two signs or houses under consideration.