Accurate birth chart calculation: how the site does it

Astronomical ephemeris, to the arc minute

Planet positions come from astronomical ephemerides built on the VSOP87 and ELP models, standard references in celestial mechanics. The site computes them itself, without calling any outside service, to within about one arc minute, a sixtieth of a degree. That is far finer than interpretation requires, since a sign spans thirty degrees.

The engine is tested against facts that do not depend on the site: at the moment of the March 2024 equinox it puts the Sun at 0° Aries, and at the full moon of 23 April 2024 it puts the Moon opposite the Sun. Positions are tropical and geocentric, the zodiac of Western astrology seen from the Earth, as the caption under the wheel says: “Tropical zodiac”.

The same engine calculates everything else on the site: today’s sky, the dates of planetary transits, the lunar calendar with the time of each Moon sign change. To see what the sky page shows, read the guide to today’s sky.

Chiron, the Nodes and Lilith: the site’s conventions

Three points in the chart are not planets, and each site makes its own choices. Here are this site’s, so you can compare like with like.

• Chiron: its position comes from data published by JPL, NASA’s Jet Propulsion Laboratory, one point every two days from 1900 to 2100.
• The lunar Nodes: the site uses the true node, which follows the real wobble of the Moon’s orbit, and places the South Node exactly opposite the North Node. The mean node, chosen by other sites, can sit about a degree and a half away.
• Lilith: this is the mean Black Moon, the mean apogee of the Moon’s orbit. The “true” or “osculating” Lilith of other sites can lie several degrees away, sometimes much more.
• Aspects: each one comes with its orb, the gap from the exact angle. Orbs vary from site to site: a wide aspect can appear here and not elsewhere, or the other way round.

Time zones and daylight saving: converting your birth time

You enter the local time, the one on your birth record. The site converts it to universal time with the IANA time zone database, which keeps the history of each place: time zone changes, each era’s daylight saving time, wartime clock changes. On the chart page, the line under the title shows the time zone used and its offset, for example “Europe/London (UTC+1)” for a summer birth in London in recent decades.

Take a birth in Paris on 12 July 1975 at 9:10 am. France had no daylight saving time yet in 1975: it was 8:10 in universal time, the line shows “Europe/Paris (UTC+1)”, and the Ascendant is in Virgo. A calculation applying today’s summer time would start an hour too early: the Ascendant would move into Leo, taking every house with it.

Never adjust the time yourself for daylight saving: enter the time on the record and let the site convert it. And if you have no time at all, see what to do without a birth time.

The place: 99,000 places, or your coordinates

The place gives the latitude and longitude, which set the Ascendant and the houses, as well as the time zone. The city search draws on a database of about 99,000 places built into the site, including every French municipality; it tolerates typos and suggests “Did you mean…” when unsure. Once you pick a city, its time zone appears under the field, for example “time zone Europe/London”.

Your place is not listed? The form widens the search to places worldwide, neighbourhoods and hamlets included. You can also tap “My city isn’t listed / enter coordinates” and type the latitude and longitude; the time zone is worked out automatically if you leave its box empty.

No need for the exact village: the planets sit at the same place in the zodiac for a whole region, and within 30 km the Ascendant and the houses move by less than a degree. The nearest large town gives the same chart.

Houses: Placidus, and the exceptions

The twelve houses are calculated in Placidus, a widely used system in Western astrology, from your exact time and place. The caption under the wheel says so: “Tropical zodiac · Placidus houses · major aspects”. Above 66° of latitude, Placidus can no longer build some of the houses: the site then switches to equal houses and says so under the title, “Polar latitude: equal houses used instead of Placidus.”

The form does not offer another system, but the chart page accepts two more through its address: add “&houses=equal” at the end for equal houses, or “&houses=whole” for whole signs. The caption under the wheel changes with it, so you can compare with another site on equal settings.

Why two sites give a different Ascendant

A gap of one or two arc minutes between two sites is normal: every engine rounds in its own way. A whole sign apart almost always comes from the data or the conventions, not the ephemeris. If it is your Ascendant that differs, go through these causes in this order.

• The time: the Ascendant moves about one degree every four minutes, and half an hour is sometimes enough to change its sign. Make sure you used the time on the record, in local time.
• Time zone and daylight saving: a tool that gets the clock rules of the period wrong shifts everything by an hour, as in the 1975 example.
• The place: a town with the same name on the other side of the world changes everything; a neighbouring town, almost nothing.
• The zodiac: Vedic astrology sites use the sidereal zodiac, about 24 degrees apart; this site works in the tropical zodiac of Western astrology.
• The house system: it does not change the Ascendant itself, but it moves the other cusps, and so the house a planet falls in.

Checking your chart yourself

A few minutes are enough to check your chart, and it is the best way to trust it. Calculate it with the free birth chart, then follow these steps.

1. Reread the line under the title: date, time, place, time zone and offset must match your birth.
2. Open the positions table (the “Placements and dominants” fold on a phone): each planet is given to the degree and the minute.
3. To copy everything at once, open “The text summary” in “Your chart in detail”: the whole chart as text, with the time zone, the houses and the orbs.
4. Compare with another source set up the same way: tropical zodiac, true node, mean Lilith, Placidus houses. A few arc minutes apart is normal; beyond that, go back to the data you entered.

To understand every symbol of the wheel and the table, follow the guide to the chart wheel, then the step-by-step calculation guide.