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Tropical Astrology

The tropical zodiac is the zodiac mostly used by astrologers in the West. Every sign is thirty degrees wide. Constellations, however, may differ very much in size and, again, move around the zodiac continuously. The difference between Sun signs and constellations is inherent in Western astrology—tropical astrology—since at least two thousand years.

’Constellation astrology’ is not completely unknown though. One interesting perspective on the matter is put forth under the soubriquet of visual astrology. Learn more about visual astrology on this website (link opens in a new window).

So, the name for the kind of astrology most common in the Western hemisphere is tropical astrology. This is opposed to the sidereal astrology of India, which is something in-between tropical astrology and ’constellation astrology’. It shares with tropical astrology that all signs are equally large, thirty degrees, but it depends upon the fixed stars for its placement versus Earth. 

The main difference between the two can be said to be where the starting point for the twelve signs falls. In tropical astrology, this starting point is defined by the Sun’s position at the vernal equinox. In sidereal astrology it is determined by the fixed stars.

The effect of this is that in sidereal astrology the zodiac is slowly moving (from a geocentric perspective), whereas in tropical astrology, it is fixed. This is connected with the so-called precession of the equinoxes (the link opens in a new window). 
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To summarize: the tropical zodiac is fixed from a geocentric perspective whereas the sidereal zodiac seems to move. The seeming movement is because Earth’s relationship with the Sun and the solar system is a fixed one, but the relationship between Earth and the fixed stars is not.
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