Methodology: The Numbers Behind Every Chart

Which ephemeris computes each position

The Sun and the seven planets from Mercury through Neptune come from a truncated VSOP87D series: a published planetary theory, kept to the roughly 1,500 terms that matter across this site's 1800-2050 date range and verified against the full, untruncated series before shipping. The Moon comes from the abbreviated ELP-2000/82 lunar series as tabulated by Jean Meeus in Astronomical Algorithms. Pluto is the one exception: no VSOP87 series covers it, so it uses the Keplerian orbital elements published by the Jet Propulsion Laboratory for approximate positions of the major planets, which makes it the least precise of the ten bodies computed here. Every position is then corrected for light travel time, annual aberration, the Earth's offset from the Earth-Moon barycentre, and nutation, because the ecliptic longitude astrology reads is measured from the equinox of the birth moment itself, not from a fixed reference frame.

Expected precision

Checked against independent implementations of different underlying theories (the full VSOP87 series, the full ELP/MPP02 lunar theory, and a separate published Pluto theory from Meeus, chapter 37), every one of the ten bodies agrees with those references to within about five arcseconds on ordinary test dates, and the worst disagreement found anywhere across the full 1800-2050 range is close to one arcminute, on Pluto specifically. An arcminute is a sixtieth of a degree: for context, the whole zodiac is 360 degrees, so this margin is far finer than any interpretive distinction a reading draws, and finer than the positional shift caused by a birth time that is uncertain by even a minute.

Ayanamsa used for sidereal (Vedic) mode

The Vedic calculator uses the Lahiri ayanamsa, the offset between the tropical and sidereal zodiacs adopted as the official Indian government standard, currently a little over 24 degrees and widening by roughly 50 arcseconds a year as the Earth's axis slowly precesses. Other ayanamsas exist (Raman and Krishnamurti among them) and produce slightly different sidereal positions; this site uses Lahiri specifically and does not currently offer the alternatives.

Default house system, and the alternatives

The homepage and Vedic calculators default to Placidus and Whole Sign respectively. Placidus because it is the most common default in modern Western practice, Whole Sign because it is the system most Vedic practice actually uses. Both calculators also offer Koch, Equal and Porphyry (and each other) for direct comparison. Switching between them changes only the twelve house cusps; it never changes a single planet's position or the ascendant itself, which is identical under all five systems. Placidus and Koch are both undefined inside the polar circles, where some degrees of the ecliptic never rise or set. If a birthplace falls there, this site substitutes Porphyry automatically and says so on the page, rather than printing a number that does not actually mean anything.

Time zones, daylight saving, and historical offsets

A birth's local clock time is converted to Universal Time using the birthplace's IANA time zone identifier, which correctly accounts for the specific daylight-saving rules in force in that place on that exact date - including historical rules that no longer apply today, such as Britain's continuous-summer-time experiment in the late 1960s and early 1970s. The difference between Universal Time and Terrestrial Time (the small, slowly-growing gap caused by the Earth's rotation being slightly irregular) is applied before any planetary position is computed, using the standard published delta-T model.

Sources

• Bretagnon, P. and Francou, G., "Planetary theories in rectangular and spherical variables: VSOP87 solutions," Astronomy & Astrophysics, 1988.
• Meeus, Jean, Astronomical Algorithms, 2nd edition, Willmann-Bell, 1998 (ELP-2000/82 lunar series, Pluto positions, delta-T model).
• Standish, E.M. and Williams, J.G., "Approximate Positions of the Planets" (Keplerian elements and rates), Jet Propulsion Laboratory / Solar System Dynamics Group.
• Lahiri, N.C., ayanamsa tables, as adopted by the Indian Astronomical Ephemeris.
• IANA Time Zone Database, for historical and current time-zone and daylight-saving rules.

Two honest limits

This site's series and elements are fitted for the years 1800 to 2050; every calculator here refuses dates outside that window rather than extrapolating past where the underlying data is trustworthy. And, as noted above, Pluto's independent verification only reaches back to 1885, as far as the published Pluto theory used to check it goes, so a Pluto placement before that year carries more uncertainty than the other nine bodies, though still nowhere near enough to change which sign or house it falls in for any birth this site accepts.

Explore further

• The calculator these figures describeSee this methodology applied to a real, worked chart.
• About this siteWho runs it and why, rather than how the numbers are computed.
• Astrology glossaryPlain definitions for any term used on this page.