How the chart is computed

The positions

The planets, the Sun and the Moon are computed by Astronomy Engine, an open source astronomical library by Don Cross that fits the same JPL planetary models professional astronomers use. It gives geocentric apparent positions, which is the technical name for where a planet looked to be from Earth at that instant, corrected for the time its light took to arrive. Those positions are expressed as longitudes along the true ecliptic of date, and that is, by definition, the tropical zodiac: 0 degrees Aries is the point of the spring equinox for that year.

We checked the engine against the Swiss Ephemeris, the standard reference for professional chart software, across births in eight places from Reykjavik to Sydney between 1954 and 2015. Every planet agreed to within 0.005 degrees, which is a third of an arc minute. The Ascendant, Midheaven and all twelve Placidus cusps agreed to within 0.0001 degrees. The lunar node is the mean node, computed from the standard polynomial, which is what most printed charts use.

The clock

A birth certificate records wall clock time, and wall clocks have been changed by law thousands of times. Your browser carries the full IANA time zone history, so when you pick a town the chart knows which offset and which daylight rule was in force on that date. If the time you give fell in the hour that a spring clock change skipped, the chart keeps the offset from before the change and tells you so. If it fell in the hour an autumn change repeated, it takes the first occurrence and tells you that too.

The horizon

The Ascendant is the degree of the zodiac rising on the eastern horizon at your birthplace, and the Midheaven is the degree culminating overhead. Both come from your latitude, your longitude and the sidereal time, computed with the true obliquity of the ecliptic for the date. They need an accurate time: the Ascendant moves about one degree every four minutes, so an hour's error moves it half a sign.

The houses

Placidus is the default because it is the system most charts you have seen were drawn with. It divides the time a point spends above and below the horizon, which is why it is computed by iteration rather than a formula, and why it fails north of about 66 degrees. Inside the polar circles the chart falls back to whole sign houses and says so. Whole sign is also offered on its own: the oldest system, in which the sign that holds your Ascendant is your whole first house. Both are legitimate, and astrologers have been arguing about them since Ptolemy.

Aspects and orbs

An aspect is an angle between two planets close to one of the classical values: 0 degrees (conjunction), 60 (sextile), 90 (square), 120 (trine), 180 (opposition), with the 150 degree quincunx included at a tight orb. The allowances are 8 degrees for conjunctions and oppositions, 7 for trines and squares, 5 for sextiles and 2.5 for the quincunx, each widened by one degree when the Sun or Moon is involved. The list is sorted by exactness, so the aspects at the top are the ones that matter most.

Decans and numbers

Each sign is split into three decans of ten degrees. We use the triplicity system, in which the first decan belongs to the sign's own ruler and the second and third to the rulers of the next two signs of the same element. Life path numbers reduce the month, day and year separately and keep 11, 22 and 33 as master numbers, which is the Pythagorean method used in most Western numerology.

What is measured and what is lore

The positions are astronomy and they are correct. The meanings are a tradition: twenty-five centuries of people writing down what they thought those positions said about a life. We report the tradition straight and keep the humor, and we do not pretend it is physics. If you want to know why anyone ever thought the sky could describe a person, that argument, from Pythagoras to Jung and on into modern physics, is the whole purpose of The Ordered Cosmos.

Privacy

The chart is computed on your device. No birth data, name or location is transmitted or stored by this site. The link made by the copy button carries the data in the address itself, so share it only with people you want to have it.