Accuracy

Your sky, to the arcsecond.

Most astrology apps never tell you where their planets come from. Ours come from NASA JPL's planetary ephemeris, the kind used to navigate spacecraft, and we check them against NASA directly. The results are below, recomputed by the server that reads your chart.

Figures as of 27 October 2026

One arcsecond is 1/3600 of a degree, about the width of a dime seen from two miles away. A zodiac sign is 30 degrees wide, so our worst error is less than a hundred-thousandth of a sign.

Every planet, against NASA

The worst disagreement for each body, in arcseconds, across every date in the reference set. The dashed line is our one-arcsecond limit.

Reference: NASA JPL Horizons, apparent geocentric ecliptic longitude. The Moon moves fastest, so it shows the largest error.

The AI never makes up your chart

A chatbot asked about your Venus will happily make up an answer. Ours can't. The sky is computed first, and the AI writes only from those computed facts.

Then we check its work. Every reply is scored before it is counted. Each planet, sign and house it names has to appear in the computed chart, and so does each pairing: "Venus in Scorpio" only passes if your Venus is actually in Scorpio. A compatibility reading that fails is never shown; you get the itemized report instead.

What else we check

• Equinoxes and solstices. The Sun crosses 0° Aries, Cancer and Capricorn at the minute NASA and timeanddate.com publish. That pins the whole zodiac.
• Eclipses, to the second. Your reading names the eclipse before you were born. We check our eclipse engine against NASA's Five Millennium Canon of Eclipses: 16 solar and lunar eclipses from 1985–2009, every one within 4.5 seconds of NASA's time of greatest eclipse. The 2026 total solar eclipse lines up too.
• Your rising sign, from first principles. At sunrise the Sun sits on the Ascendant, by definition. We confirm that in Chicago, London, Sydney and Reykjavik, which checks time zones, houses and angles together.
• The night the clocks change. When the clocks go back, 1:30 a.m. happens twice. That hour can change your rising sign. Most apps quietly pick one. We flag it, chart the most likely one, and tell you what changes if your birth record meant the other.
• Birthplaces. 34,106 places, each with coordinates and its time zone's full history from the IANA time zone database, including past daylight-saving rules.
• Building or fading. Every transit is marked applying or separating from the planet's real speed, including retrograde planets backing onto a point.
• Pattern claims about your life. Before the app says "you feel this way around the full moon," the pattern has to pass a statistical test that guards against coincidence. It usually takes about three months of check-ins.

Which astrology this is

Astrologers disagree about some of this, so we state our choices:

Zodiac

Tropical: 0° Aries is the March equinox

Houses

Placidus, the modern Western default. Whole sign, equal and Koch are built in for astrologers who prefer them.

Rulerships

Traditional

Aspects

Conjunction, sextile, square, trine and opposition, with a 3° orb for transits

Points

Sun to Pluto, Chiron, the true North Node, Ascendant and Midheaven

No birth time

No rising sign and no houses: we leave them out rather than guess. If your Moon could be in either of two signs that day, we tell you.

Where the old astrology comes from

Your reading includes a section called "What the old astrologers would say": day and night births, the planetary hours, temperament, the Lot of Fortune, the bright stars. Every item names its source, and every source is a public-domain text you can read for free:

• Ptolemy, Tetrabiblos (2nd century), tr. J. M. Ashmand, 1822. Read it
• William Lilly, Christian Astrology (1647, first edition). Read it
• Cassius Dio, Roman History 37.18–19, on how the planetary days and hours work. Read it
• Vivian Robson, The Fixed Stars and Constellations in Astrology (1923). Read it

The wording in the app is ours; the doctrines are theirs. Where the old authors disagree, we follow the older text and keep a note. If we couldn't trace something to a primary source, it isn't in the app.