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 areas in 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 realize that there are almost 526,000 minutes in an average year)
So, how does these different ways of measuring a year affect the zodiac?
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 included 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.
So, that's a brief history but how does it affect modern astrology?
Well, have you ever noticed in a daily newspaper when you glance at the horoscopes that your date of birth is assigned. Taurus when you are 100% sure that you are in fact an Aries? It has happened to me a few times and that will only happen when you understand that the horoscope that you are looking at is based on sidereal astrology, rather than the more common place tropical astrology.
One thing in life is certain and that is that things change and if you think of astrology dating back to around 1645 BC, when it was first practiced in ancient Babylon, then you should also be aware that very little in life is a fixed point. For example, by the time of Ptolemy, the constellation of Aries had shifted in the night sky so much in just under 2000 years that it was almost in the middle of the section of sky associated with Pisces.
So while sidereal and tropical astrologies are based on the same 12 sections and zodiac signs, there are quite significant differences between dates.
Aries - Tropical Astrology Mar 21 - Apr 20 Sidereal Astrology Apr 14 - May 14
Taurus - Tropical Astrology Apr 21 - May 21 Sidereal Astrology May 15 - Jun 14
Gemini - Tropical Astrology May 22 - Jun 21 Sidereal Astrology Jun 15 - Jul 15
Cancer - Tropical Astrology Jun 22- Jul 22 Sidereal Astrology Jul 16 - Aug 16
Leo - Tropical Astrology Jul 23 - Aug 22 Sidereal Astrology Aug 17 - Sep 16
Virgo - Tropical Astrology Aug 23 - Sep 23 Sidereal Astrology Sep 17 - Oct 16
Libra - Tropical Astrology Sep 24 - Oct 23 Sidereal Astrology Oct 17 - Nov 15
Scorpio - Tropical Astrology Oct 24 - Nov 22 Sidereal Astrology Nov 16 - Dec 15
Sagittarius - Tropical Astrology Nov 23 - Dec 21 Sidereal Astrology Dec 16 - Jan 13
Capricorn - Tropical Astrology Dec 22 - Jan 20 Sidereal Astrology Jan 14 - Feb 12
Aquarius - Tropical Astrology Jan 21 - Feb 19 Sidereal Astrology Feb 13 - Mar 12
Pisces - Tropical Astrology Feb 20 - Mar 20 Sidereal Astrology Mar 13 - Apr 13
These discrepancies of dates sometimes gives skeptics ammunition with which to attack astrology as a concept - if the heavens are not a constant, then surely astrology cannot use them to accurately predict the future? Well, this argument is usually based on a fallacy and that fallacy is that the zodiac signs are the same as the constellations that they are named after. There is a fixed point in astrology and that is the individual who is seeking a reading.
Thousands of years ago, astrology was based on observations on the heavens, however in the 21st century, the art has a much more scientific approach - we are all fixed points based on how our birth date and time and the many other factors that make up who we are The stars in the skies influence us all at any given time and it does not matter whether the references that were first used by the ancient Greeks are no longer what they once were, we are not living in ancient Greece! Astrologers do not use the stars as they were 2000 years ago to ascertain how they are influencing you now. It is also worth noting that while sidereal and tropical astrology differ in zodiac, there are two very important constants - the planets and their aspects. So, while you maybe a Capricorn in tropical astrology and a Sagittarian in Sidereal astrology, the influences that work on you remain the same - it is merely the star signs that have moved.
Astrologers who practice either form of astrology while working on their readings and astrological charts are invariably versed and educated in the path they have chosen. It is wrong to say that tropical astrology is any more valid than sidereal astrology or vice versa - if you are seeing an experienced astrologer then it should not matter what system they use, the astrology charts and predictions that they prepare for you will be based on constants that are within an ever changing, never ending cosmic ballet.
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