Tropical and sidereal
The most well-known and, to many people, challenging difference between Indian and western astrology as commonly understood is that Indian tradition uses a zodiac that is sidereal, or fixed with respect to the constellations for which the signs were named, whereas western tradition for many centuries has favoured one that is tropical, or fixed with regard to the seasons. The twelve zodiac signs and their extensions are the same; the crucial question is how to define the starting point. Because the difference between the two systems increases over time – a phenomenon known as the precession of the equinox – and is currently some 24°–25°, most points in a horoscope now fall in different signs depending on which system is used. This can trigger an identity crisis in someone who identifies strongly with particular zodiac signs! But as it happens, the sidereal zodiac is not peculiar to India.
The tropical zodiac defines 0° Aries as the position of the sun at the spring equinox, around 20 March each year. It is a historical fact that this definition was not used for casting birth charts by the Mesopotamian or early Hellenistic astrologers, nor by the Persians and Indians. (Tropical divisions were used for other purposes, such as calendars, which has confused some modern proponents of ‘tropical Vedic astrology’.) The earliest known author to advocate the use of the tropical zodiac for horoscopes was Ptolemy, who, in the second century CE, wanted to align astrology with the science of his time by anchoring it firmly to seasonal changes and the fourfold qualities of hot, cold, moist and dry. Ptolemy’s ‘scientific’ model – which breaks down in southern-hemisphere countries such as New Zealand or Argentina, where the coldest months are those that Ptolemy associates with heat and vice versa – did not catch on until several centuries after his death. Even early Arabic-language astrologers such as Māshāʾallāh and Sahl ibn Bishr, whose astrology was chiefly derived from Persian sources, used sidereal tables for casting their charts. Following the adoption of Arabic astrology, the use of sidereal values survived in the Latin west into the 12th and 13th centuries (as recently explored by C. Philipp E. Nothaft in this paper, a pre-publication version of which is freely available here); but by the early modern period, tables based on Ptolemy’s tropical values had become standard.
The astrological consultation work offered on this website is all done from a sidereal perspective. To take the Primary Directions Diploma Course, however, you do not have to be a siderealist: the core technique of directions is zodiac-independent, and students are free to work in the zodiac of their choice.