[original post 2005] To make the difference between tropical and sidereal clearer, as related to the positions of the actual stars, here is a chart.
On the outside edge are written the names of the actual constellations. They are shown in their real positions in space related to the time of year as the earth orbits the sun. The zodiac in green is the tropical zodiac, which starts wherever the vernal equinox points to on March 20th/21st. This is currently a point in the constellation of Pisces. Each astrological zodiac systen uses a “normalized” or regularized set of zodiacs of thirty degrees each, so once you determine the starting point you just count around 30 degrees to find the next “zodiac sign”.
The tropical zodiac doesn’t correspond to the stars. Most tropical zodiac signs are not located in the constellation whose name they borrow; at most they touch by a few degrees.
The sidereal zodiac (which Jyotish uses) is shown in blue. It attempts to make its regularized zodiac signs line up with the real stars as much as is consistent with equal sign sizes. It’s more accurate, tho the need to create 30 degree signs means it’s off a bit in some places from the constellations. As you can see, even in the sidereal zodiac, zero Aries is actually at the end of the constellation Pisces!
There are a number of different ideas of exactly where sidereal zero degrees Aries should start in order to have the least amount of inaccuracy.
The definition of where zero Aries is placed is called the Ayanamsha (a Sanskrit word). Many ayanamshas take the fixed star Spica as a starting point for zero Libra (since it’s very close anyway). A sidereal zero Aries point never moves; it is in the same place every year. But the ayanamsha is often stated mathematically as the number of degrees from the tropical Aries point on a specific date. Defined this way an ayanamsha changes its value slightly every year, since the tropical zodiac Aries point moves constantly with reference to the stars.
A sidereal ephemeris will give the ayanamsha value for each year, which enables us to easily calculate the offset between the two zodiacs. So it’s useful to have it stated in this form.
The most commonly used ayanamsha is called Lahiri. The offset of the tropical zodiac from the zero Aries point of the Lahiri sidereal zodiac is currently almost 24 degrees. Today is 31st March. Western astrology says the sun is rising in Aries; Jyotish says the sun is rising in Pisces.