How charts are computed

If your answer differs from another tool's, it is nearly always a setting. This guide explains the settings.

PositionsCopy Anchor Link

Positions come from the Swiss Ephemeris, under its Professional licence. They are apparent and geocentric, as classical practice expects. We compared them with NASA JPL Horizons at 201 moments from 1900 to 2100, and published the result and the script on the accuracy page.

AyanamshaCopy Anchor Link

A sidereal position is the tropical position minus the ayanamsha. Systems differ by up to a degree or more, which can change a Moon sign or a nakshatra near a boundary. The default is Lahiri, KP uses Krishnamurti, and chart endpoints accept 51 systems. ayanamsha-list compares seven of them. We checked all 51 against the Swiss Ephemeris at four dates from 1900 to 2100, and every one matches.

Every chart's ayanamsha block gives two values. degrees is the mean ayanamsha, the value the dasha and other rules were built on. degrees_true adds nutation (up to about 17 seconds of arc either way). That is the offset the sidereal planet positions actually use and the one ephemerides print. For Lahiri at 1 January 2000, degrees is 23.8571 and degrees_true is 23.8532 (23°51'11"). To check a position by hand, subtract degrees_true from the tropical longitude. Our blog post on ayanamshas goes further.

Rahu and KetuCopy Anchor Link

Rahu is the true node by default. Send "node": "mean" for the mean node, which is what some printed almanacs use. The two can differ by more than a degree. Ketu is always opposite Rahu.

HousesCopy Anchor Link

Vedic charts count whole signs from the ascendant. bhav-chalit gives houses from cusps. KP uses Placidus. Western charts default to Placidus, with Koch, whole sign and equal houses as options.

ConventionsCopy Anchor Link

Traditions differ on many classical calculations. Classical methods states the convention of each endpoint where it matters, and which figures are our own.

If you still disagreeCopy Anchor Link

Check these, in order:

1. The time zone and offset. See Birth times and time zones.
2. The ayanamsha.
3. True or mean node.
4. The house system.

Each answer echoes the settings it used, so you can compare them directly.