Why is the Zodiac measured in two ways?

The Zodiac is the belt of sky which we perceive as encircling the Earth and it intersects the celestial equator (the Earth’s equator projected onto space) at an angle of 23½°.  The Sun, Moon and planets appear to go round the Zodiac as we see from Earth at varying speeds, but it is the Earth and planets that orbit the Sun and the Moon that orbits the Earth.  The Zodiac belt is the main reference point for measuring the positions of the Sun, Moon and planets.

Over some 25,800 years, the Earth wobbles on its axis much like a spinning top slowing down a little.  This has the effect of a changing view of the sky over this extremely long period known as a Great Year.  We currently see the constellations of Gemini and Orion high in the sky at night in December.  In about 12,000 to 13,000 years time (and during the same period in the past), these two constellations would have been high in the sky in June.

Over the Great Year, the pole stars change in the northern hemisphere (there aren’t any in the southern hemisphere), as wobbling Earth points to different pole stars.  At the moment the Pole Star is Polaris.  The same phenomena happens to the Zodiacal constellations in that they very slowly shift backwards (against their order, i.e. from Pisces to Aquarius to Capricorn) at a rate of about 1° every 72 years – an imperceptible motion in one life time.  On about 21st March and 21st September each year (the equinox points where the ecliptic and the celestial equator intersect), the Sun is overhead the celestial equator). The 21st March equinox point is called the First Point of Aries by astronomers. The backdrop of stars behind this point is currently towards the beginning of the constellation of Pisces gradually moving towards the constellation of Aquarius.  This phenomenon is known as the Precession of the Equinoxes.

When astronomers talk about a planet’s location, they refer to its position amongst the constellations, e.g. Jupiter can be seen in the constellation of Pisces.  When astrologers, following the western tradition, refer to the positions of the Sun, Moon and planets, they use the First Point of Aries as the starting point and measure the Zodiac from that point dividing it into 12 sections of 30° each (Aries, Taurus, Gemini etc.), but the backdrop of stars at this point is no longer the constellation of Aries, although it was roughly 2000 years ago.  Therefore, the Zodiac used by astrologers following the western tradition is based on the cycle of the Sun and the seasons (0° Aries being the spring equinox in the northern hemisphere, 0° Cancer being the summer solstice, 0° Libra being the autumn equinox and 0° Capricorn being the winter solstice – reverse seasons for the southern hemisphere).  This is known as the Tropical Zodiac.

Astrologers following the eastern tradition, particularly Vedic astrologers, and a few others including those influenced by the western Rudolf Steiner’s Anthroposophic movement, measure the Zodiac starting at the constellation of Aries and then in order of the rest of the Zodiacal constellations – Taurus, Gemini etc.  This is known as the Sidereal Zodiac and it marks the positions of the Sun, Moon and planets against the constellations.  Even so, users of the Sidereal Zodiac divide the sky into equal sections of 30° each while some constellations take up much more space than others.  Biodynamic farmers and gardeners use the Sidereal Zodiac.

The Zodiac signs on the outer circle represent the Tropical Zodiac.  Just inside these are the constellations.  You will see that Tropical Aries more or less lines up with the constellation of Pisces and that Tropical Taurus more or less falls between the constellation of Aries and Taurus.  Tropical Cancer is in line with the constellation of Gemini.  The circle of Zodiac sign symbols, nearer the centre of the diagram, represents the Sidereal Zodiac.   Just to be confusing, there is some disagreement as to precisely where the Sidereal Zodiac should begin at the start of the constellation of Aries.  The diagram uses the Lahiri system in which roughly 24° is deducted from the Tropical Zodiac to find the position in the Sidereal Zodiac.

Jean Elliott is a director of astrologycollege.com which teaches astrology from beginner to advanced level for those who wish to learn for fun, for personal development or for professional training 


Precession of the equinoxes

Click the diagram for a larger version of precession-of-the-equinoxes


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