# Sidereal and tropical astrology

In astrology, sidereal and tropical are terms that refer to two different systems of ecliptic coordinates used to divide the ecliptic into twelve "signs". Each sign is divided into 30 degrees, making a total of 360 degrees. The terms sidereal and tropical may also refer to two different definitions of a year, applied in sidereal solar calendars or tropical solar calendars.
While sidereal systems of astrology calculate twelve zodiac signs based on the observable sky and thus account for the apparent backwards movement of fixed stars of about 1 degree every 72 years from the perspective of the Earth due to the Earth's axial precession, tropical systems consider 0 degrees of Aries as always coinciding with the March equinox (known as the spring equinox in the Northern Hemisphere) and define twelve zodiac signs from this starting point, basing their definitions upon the seasons and not upon the observable sky wherein the March equinox currently falls in Pisces due to the Earth's axial precession. These differences have caused sidereal and tropical zodiac systems, which were aligned around 2,000 years ago when the March equinox coincided with Aries in the observable sky, to drift apart over the centuries.
Sidereal astrology accounts for the Earth's axial precession and maintains the alignment between signs and constellations via corrective systems known as ayanamsas (Sanskrit: 'ayana' "movement" + 'aṃśa' "component"), whereas tropical astrology, to reiterate, is based upon the seasonal cycle of the Northern hemisphere and does not take axial precession into consideration. Though tropical astrology typically considers the zodiac of the Northern Hemisphere to be applicable without change to the Southern hemisphere, a small number of tropical astrologers modify the zodiac to reflect seasons in the Southern hemisphere, taking Libra as the sign that coincides with the spring equinox instead of Aries.
Ayanamsa systems used in Hindu astrology (also known as Vedic astrology) include the Lahiri ayanamsa and the Raman ayanamsa, of which the Lahiri ayanamsa is the most widely used. The Fagan-Bradley ayanamsa is an example of an ayanamsa system used in Western sidereal astrology. As of 2020, sun signs calculated using the Sri Yukteswar ayanamsa were around 23 degrees behind tropical sun signs. Per these calculations, persons born between March 12 – April 12, for instance, would have the sun sign of Pisces. Per tropical calculations, in contrast, persons born between March 21 – April 19 would have the sun sign of Aries.

## Astronomic zodiac

A small number of sidereal astrologers do not take the astrological signs as an equal division of the ecliptic but define their signs based on the actual width of the individual constellations. They also include constellations that are disregarded by the traditional zodiac but are still in contact with the ecliptic.
For the purpose of determining the constellations in contact with the ecliptic, the constellation boundaries as defined by the International Astronomical Union in 1930 are used. For example, the Sun enters the IAU boundary of Aries on April 19 at the lower right corner, a position that is still rather closer to the "body" of Pisces, as the first sign rather than of Aries. The IAU defined the constellation boundaries without consideration of astrological purposes.
The dates the Sun passes through the 12 astronomical constellations of the ecliptic are listed below, accurate to the year 2011. The dates will progress by an increment of one day every 70.5 years. The corresponding tropical and sidereal dates are given as well.
| Constellation | Tropical zodiac dates | Sidereal zodiac dates (Lahiri ayanamsa) | Dates based on 14 equal length sign zodiac used by Schmidt | Based on IAU boundaries |
| Aries | Mar 21 – Apr 19 | April 14 – May 14 | April 16 – May 11 | Apr 18 – May 13 |
| Cetus | — | — | May 12 – June 6 | — |
| Taurus | Apr 20 – May 20 | May 15 – Jun 15 | June 7 – July 2 | May 13 – Jun 21 |
| Gemini | May 21 – Jun 20 | Jun 16 – Jul 16 | July 3 – July 28 | Jun 21 – Jul 20 |
| Cancer | Jun 21 – Jul 22 | Jul 17 – Aug 16 | July 29 – August 23 | Jul 20 – Aug 10 |
| Leo | Jul 23 – Aug 22 | Aug 17 – Sep 16 | August 24 – September 18 | Aug 10 – Sep 16 |
| Virgo | Aug 23 – Sep 22 | Sep 17 – Oct 16 | September 19 – October 14 | Sep 16 – Oct 30 |
| Libra | Sep 23 – Oct 22 | Oct 17 – Nov 15 | October 15 – November 9 | Oct 30 – Nov 23 |
| Scorpio | Oct 23 – Nov 21 | Nov 16 – Dec 15 | November 10 – December 5 | Nov 23 – Nov 29 |
| Ophiuchus | — | — | December 6 – December 31 | Nov 29 – Dec 17 |
| Sagittarius | Nov 22– Dec 21 | Dec 16 – Jan 14 | January 1 – January 26 | Dec 17 – Jan 20 |
| Capricorn | Dec 22 – Jan 19 | Jan 15 – Feb 12 | January 27 – February 21 | Jan 20 – Feb 16 |
| Aquarius | Jan 20 – Feb 18 | Feb 13 – Mar 14 | February 22 – March 20 | Feb 16 – Mar 11 |
| Pisces | Feb 19– Mar 20 | Mar 15 – Apr 13 | March 21 – April 15 | Mar 11 – Apr 18 |
* 1 2 3 Schmidt (1970) added a Sun sign for Cetus, falling between Aries and Taurus from May 12 – June 6, as well as the more often seen addition of Ophiuchus, q.v.. However, his symbols for Cetus and Ophiuchus are not the same as the symbols used here. Under the IAU constellation boundaries, the sign of Cetus corresponds to the half of Taurus mostly west of Aldebaran and the Hyades, while the Taurus sign corresponds to the half of Taurus east of Aldebaran and the Hyades.
* 1 2 Ophiuchus is not a sign in traditional forms of tropical and sidereal astrology, and only appears in a few forms of astrology which use the nominal constellation boundaries adopted by the IAU.