Planet positions
Planet positions come from the Swiss Ephemeris, the astronomical calculation library from Astrodienst, sampled in advance for every day from 1950 to 2075 (the Moon every six hours) and read by your browser. Between samples the tool interpolates, which agrees with the Swiss Ephemeris to within a few arcseconds. Positions are geocentric and in the tropical zodiac. The sidereal values in the two-systems tool subtract the Lahiri ayanamsa. The lunar node is the mean node.
Houses, the Ascendant and the Midheaven
The Ascendant, Midheaven and house cusps are calculated in the browser from the local sidereal time, the true obliquity of the ecliptic (with nutation) and your latitude. Placidus is the default, with Whole Sign as an option. Above the polar circle Placidus cannot be defined, and the tool says so and uses Porphyry houses.
We check this code against the Swiss Ephemeris. For 200 random births between 1950 and 2075 at latitudes up to 60 degrees, the largest difference in any house cusp, the Ascendant or the Midheaven is under 0.1 arcminute, and Whole Sign houses match exactly.
Time at the birth place
The clock time you enter is converted to universal time using the time zone rules in force at your birth place on your birth date, including daylight saving and historical changes, so a chart for an old birth in a place whose rules changed still uses the right offset. You must choose the place from the list so the coordinates are exact.
Aspects
We show the five major aspects. Orbs are 8 degrees for the conjunction, opposition and trine, 7 for the square and 5 for the sextile when the Sun or Moon is involved, and 6, 6, 6, 5 and 4 otherwise. Aspects to the angles or the node are held to 5 degrees at most. Astrologers use different orbs, so another site may show a slightly different list.
Numerology
The Pythagorean table gives letters 1 to 9 in alphabetical order. The Chaldean table (as popularised by Cheiro) assigns 1 to 8 by sound: 1 for A, I, J, Q, Y; 2 for B, K, R; 3 for C, G, L, S; 4 for D, M, T; 5 for E, H, N, X; 6 for U, V, W; 7 for O, Z; 8 for F, P. The life path is found by reducing the month, day and year separately, adding them and reducing again, keeping 11, 22 and 33; we tell you when adding every digit at once would give a different answer. Both the numerology code and the chart code have automated tests.
Birthstones
The modern list follows the American Gem Society. The traditional list follows the 1937 list of the National Association of Goldsmiths of Great Britain and Ireland, now the National Association of Jewellers. Both are summarised in Wikipedia, Birthstone.
What the tools cannot do
They cannot predict events, health, money or relationships, and nothing here should be read as if they could. They describe where bodies were in the sky, and what traditions have said about them.
Corrections
If you find an error, write to [email protected]. We will check it, fix it and record what changed.