Vedic vs Western astrology for developers

Many of your users already know their Western Sun sign, the sign the Sun was in at birth. The astrology API returns Vedic positions, and for most people the Vedic signs are different. This guide explains why and shows a real example. It also suggests what to tell users, so the difference does not look like a bug.

Two zodiacs

The zodiac is the circle of 12 signs. Western astrology uses the tropical zodiac, which is tied to the seasons: Aries always starts at the March (spring) equinox. Vedic astrology uses the sidereal zodiac, which is tied to the stars. The Earth wobbles slowly on its axis. Because of this, the equinox drifts backward against the stars by about 50 arc seconds a year. So the two zodiacs move apart over time.

The gap between them is called the ayanamsa. By default the API uses the Lahiri ayanamsa, the one used most in India. It is named after N. C. Lahiri, who was member secretary of India’s Calendar Reform Committee, set up in 1952. To use one of the others, set ayanamsa to kp or raman. The ayanamsa guide compares all three. The gap is a little over 24 degrees today. In 1990, the year of the example below, it was 23.7 degrees. Each sign is 30 degrees wide. Take a planet in the first 24 degrees or so of a Western sign. In a Vedic chart, it falls one sign earlier. That is about four positions in five.

A real example

Here is the sample chart the docs use: John Doe, born Aug 15, 1990, 10:30 AM, New York. The table shows the rising sign (the sign coming up in the east at birth), the Sun and the Moon. The Vedic column is what POST /v1/planets returns. The Western column adds the ayanamsa from the same response (23.73 degrees for that date) to each position.

• Rising sign

  Vedic (from the API): Virgo 19°

  Western: Libra 13°

• Sun

  Vedic (from the API): Cancer 28°

  Western: Leo 22°

• Moon

  Vedic (from the API): Taurus 24°

  Western: Gemini 18°

Every point is one sign earlier in the Vedic chart. None of the positions is wrong: they are the same sky, measured from two different starting points.

What to tell users

• Label signs clearly, for example “Vedic Sun sign”. Then users will not compare them with a Western horoscope by mistake.
• Add a one-line note near the chart. For example: Vedic astrology measures signs against the stars, so most signs are one earlier than in Western astrology.
• Lead with the Moon sign (rashi) and the rising sign, not the Sun sign. The Moon sign is the sign the Moon was in at birth. Vedic astrology reads the Moon first. Many answers are counted from it. They include the daily horoscope, dashas (planet periods), transits (where the planets are on a given day) and matching. Our post on the Moon sign, Sun sign and rising sign shows which birth details each one needs.
• Do not convert the API's positions to Western signs and mix the two in one reading. The rules behind each answer assume sidereal positions.

Houses and aspects

Houses split a chart into 12 parts. Many Western apps use Placidus houses, which have unequal sizes and need an exact birth time to draw. The API uses whole-sign houses: the rising sign is house 1, the next sign is house 2, and so on (CH-06). There are no house cusps (exact points where a house starts). Two endpoints add other houses beside these: POST /v1/kp uses Placidus cusps for KP astrology (KP-04), and POST /v1/chart/chalit gives the bhava chalit (CH-16). Aspects are a planet's influence on other houses. They are counted in whole houses too, not in degrees with an orb, an allowed margin in degrees (AS-01).

Planets

Vedic astrology uses nine grahas (planets): the Sun, the Moon, Mars, Mercury, Jupiter, Venus, Saturn, Rahu and Ketu (CH-11). Rahu and Ketu are the two lunar nodes, the points where the Moon's path crosses the Sun's path. Uranus, Neptune and Pluto are not used. The API returns only Vedic positions. It has no tropical (Western) output. The ayanamsa is Lahiri unless you choose another of the eight, such as KP or Raman. Read more in the conventions. For the other changes a Vedic chart brings to your code, such as dashas and divisional charts, see Vedic vs Western astrology: what changes in the code.