How we calculate your birth chart
Ephemeris
Positions of the Sun, Moon and planets come from astronomy-engine 2.1.19 (an open-source library by Don Cross, MIT licence), which computes the planets from VSOP87 series. We use apparent geocentric positions on the tropical zodiac, measured from the true equinox of the date. The lunar North Node is the mean node.
Houses
The site uses Placidus houses. Above 66° latitude, where Placidus cusps cannot be formed, it falls back to Equal houses counted from the Ascendant. The engine also implements Whole Sign and Equal houses, but the site does not offer a choice of house system yet. The Ascendant and Midheaven are calculated from the apparent sidereal time at your birthplace's longitude and the obliquity of the ecliptic.
Time zones and daylight saving
Your birth time is converted to UTC with the IANA time-zone database built into your browser, including historical daylight saving rules and local mean time before standard time zones. A wall-clock time that occurs twice when clocks go back is read as the first occurrence; a time skipped when clocks go forward is moved one hour earlier. The birth place search uses a list of cities with at least 30,000 inhabitants, each with its IANA time zone.
Missing time or place
Without a birth time we use 12:00 local time and leave out the rising sign and houses; the Moon moves about 13° a day, so its position can then be off by up to about 6–7°. Without a birth place we use your own time zone and also leave out the rising sign and houses.
Aspects
Aspects use these orbs: conjunction and opposition 8°, square and trine 7°, sextile 4°, quincunx 3°; 2° more when the Sun or Moon is involved (not for the quincunx).
Accuracy
Our test suite checks the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn and Pluto against published reference positions for 1 January 2000 to within 0.05°, the March 2024 equinox to within 5 minutes, and Placidus cusps for consistency. Times on the calendar pages are rounded to the minute; retrograde stations can differ by a few minutes from other almanacs because a planet turns very slowly.
Horoscopes
Sign horoscopes use the sky at 12:00 UTC of each day (weekly and monthly ones the whole span) and count houses from your sign (whole-sign houses). The text is assembled from fixed interpretations of those facts, so everyone of the same sign reads the same page; the lucky number and time are a playful daily rotation, not a calculation.