Sidereal and tropical are two different forms of astrology that are based on two different ways in which a year can be measured.
Simply put, a sidereal year is the time that the Earth takes to travel around the Sun while referencing the fixed stars. These are the celestial bodies that do not appear to move at any point throughout the year, perhaps the best known of these fixed stars is Polaris, the pole star, but in actuality there are hundreds, upon hundreds of fixed stars in the heavens.
The word ‘Tropical’ has become synonymous with meaning hot – people dream of going to a tropical island and sipping exotic drinks, but the etymology of the word actually comes from the Greek word tropikos which means ‘Turn’. So when you read about or hear someone talking about the Tropics of Cancer and Capricorn, what they are referring to are the areas on the globe that are the extreme northern and southern latitudes where the Sun can be directly overhead and where it appears to ‘Turn’ during the course of an annual revolution of the planet Earth. The time that the Sun takes to travel from one point in the sky to the same point using the tropics, is known as a tropical year.
There are subtle differences between a sidereal year and a tropical year, a tropical year is about nine minutes shorter than a sidereal year, now nine minutes does not seem like a great deal in the course of a year (especially when you realise that there are almost 526,000 minutes in an average year)
So, how does these different ways of measuring a year affect the zodiac? astrology
Well, if you think of the sky as a circle which is never ending, has no beginning and no ending, then in order to map or measure the cycle of the zodiac you need to establish a point of reference from which to begin. Sidereal astrology (which is an older practice than tropical astrology), is based on a starting point that was established by the simple fact that the Ancient Greeks (Hellenistic Astrology) saw the Spring Equinox as the start of their year and therefore it was a logical choice that this would be used as the starting point of the zodiac. From that point the ancient Greek astrologers divided the sky into 12 segments with a 30 degree division, starting from the constellation of Aries and then naming each of the subsequent segments after an easily recognisable and well known constellation that corresponded with each division. It is also worth noting that the early Greek astrologers did not use the constellations as a reference point when measuring the position of the planets, as they did not divide the sky equally – they were simply a reference point.
This early type of astrology was based exclusively on cyclical movements, associations were drawn between the energies and qualities associated with a specific time of the year and the signs of the zodiac. There was absolutely no attention paid to the equinoxes as they were only concerned with seasonal influences in their interpretation of the stars. This remains the basis for sidereal astrology, sidereal meaning star.
It was at least 250 years later that Claudius Ptolemy, a Roman citizen who lived in Egypt, but wrote his works in Greek began to examine and advocate a system of astrology that incorporated the vernal equinoxes and the duration of a tropical year. It is from his advocacy that this began to gain popularity to the point that it became the system that most modern Western astrologers use. Ptolemy did not discover this type of astrology, he merely reported on it in his work the ‘Tetrabiblos‘, a four volume treatise on astrology. It is also worth noting that Hindu astrology (Vedic astrology) remains based on the sidereal astrological movements.