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In short
An ephemeris is a table, or a program, that gives the positions of the Sun, Moon and planets for any date. This app does not read one: it computes each position from published astronomical formulas, Meeus for the Sun and Moon and JPL Keplerian elements for the planets. Its own rules document calls this accurate to well inside a nakshatra, but not a full ephemeris.
What the term means
An ephemeris lists where each body of the sky stands at regular dates. A chart needs that information for one moment: the longitude of every graha. An astrologer can read it from a printed ephemeris, or a program can compute it. Pages on this site also use the word for the computing method behind a program ("an app on a full ephemeris"), and two programs on different methods can disagree slightly about the same moment.
Where the term comes from
The classical texts start with positions already in hand. BPHS Ch. 3 Sl. 8-9 say only that the positions of the grahas are to be taken "as per Drikganit"; they give no method. The word itself appears in the translator's notes: in Ch. 27 the birth weekday can be found "from ephemeris", and the declinations Ayan Bal needs from "a standard modern ephemeris" (Sl. 15-17). The case book Advanced Predictive Astrology carries notices for printed daily ephemerides on the Lahiri ayanamsa.
How this app computes positions
- There is no stored table. The Sun and Moon come from formulas in Meeus's Astronomical Algorithms (2nd ed.), the Moon from the 35 principal terms of its lunar series. Mercury to Saturn come from JPL's published Keplerian elements, which the code labels valid for 1800 to 2050, moved to the equinox of the date. Rahu is the Moon's mean node.
- The rules document gives the accuracy: the Sun to about 0.001 degree at the equinoxes and solstices, the Moon to about 0.1 to 0.3 degree, and, checked against the VSOP87 theory for 1950 to 2030, Mercury, Venus and Mars within 0.02 degree and Jupiter and Saturn within 0.16.
- It calls that "accurate enough" for nakshatras and dasha balances but "not a full ephemeris", and names Swiss Ephemeris as the intended source of truth where more precision is needed. The app does not use Swiss Ephemeris today.
- A point within those margins of a sign, nakshatra or divisional boundary can come out differently in another program.
Pages that use this term
- Why apps disagree on your nakshatra - what a full ephemeris would change.
- Navamsa: the second chart - why precise divisional charts need more than this.
- Kaal Sarp Dosha explained - a close call that another ephemeris could move.
- When astrology and numerology disagree - computed positions beside numerology.
- Methodology - the formulas compared with a printed chart.
Related terms
- Julian Day - the time input of every formula.
- Mean node - how Rahu is placed.
- Lahiri ayanamsa - applied after the positions are computed.
- Graha - the bodies being placed.