Why charts differ between services

The positions of the planets in the sky are a matter of astronomy, and with the same data any careful program gives almost the same degrees. The differences begin before the program has calculated anything.

• Time. The Ascendant passes through all twelve signs in a day, that is, one sign every two hours on average. An error of one hour shifts it by about 15°, half a sign, and an error of ten minutes by about 2.5°.
• Time zone and daylight saving time. The time on a document is local, but positions have to be calculated for universal time. If a service applies today’s time-zone rules instead of those in force in the year of birth, or forgets about daylight saving time, the result is a shift of an hour or more. We convert local time to universal time by the historical rules of the time zone of that period, including daylight saving time.
• Place. Cities with the same name, the wrong village or region chosen, a mistake in the country. The place affects the Ascendant and the houses most of all, and the Moon sign almost never.
• Date. Day and month mixed up, the wrong year, a birth close to midnight: when the local day has not yet changed, the universal day may already be different.

The time in a birth certificate or a record is often written rounded: to five minutes, to half an hour, sometimes to an hour. One service takes it as exact, another asks you to enter an “approximate” time. For the Sun and the distant planets the difference is unnoticeable, while the Ascendant and the houses may change. The angles move by about 1° every 4 minutes; more on this in the article on the angles of the natal chart.

We do not guess and we do not hide the uncertainty. An exact time is checked within ±2 minutes: if anything changes over that span (the sign on the Ascendant, a house), it is treated as unknown. With an approximate time the chart is calculated at both ends of the span, and whatever differs at the ends stays unknown. The article on a chart without an exact time covers this in detail.

Even with identical data two charts can look different, because the astrological tradition has no single standard. Here are the main forks.

None of these options is an “error” in itself. Astrology is an interpretive tradition, not an exact science, and different schools choose different tools. What matters is only not to mix them up: a Sun sign in the tropical zodiac and a description of a sign in the sidereal one are different things.

We calculate in the tropical zodiac, geocentrically, with houses in the Placidus system. We take the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, and of the angles the Ascendant and the Midheaven (MC). There are five aspects: conjunction, sextile, square, trine and opposition, with orbs set by a table. We do not interpret the lunar nodes, Lilith or Chiron. The full description is in the methodology.

So if your chart with us did not match another one, look at the table above: most often the cause is found in one or two rows.

The example is fictional: the positions were invented for explanation and are not calculated from anyone’s data.

Suppose the same person entered their data into two services and got the following differences.

Here there is no “right” or “wrong” chart. The first two rows show different input data or a different zodiac, and the last two show different rules applied to the same chart. Only charts built by the same rules should be compared.

1. Make sure the time comes from a reliable source, and remember how accurate it is: to the minute, to five minutes, to an hour. The article how to find your birth time explains where to look for it.
2. Check the date against the document: the order of day and month, the year.
3. Choose the birth place from the list rather than going by how you remember the name: make sure the country and region are correct.
4. Enter the time as it is written in the document, that is, local time. You do not need to convert it yourself to another zone or to daylight saving time: we convert it to universal time ourselves, by the rules of the time zone of that period.
5. If you are unsure about the minutes, enter an approximate time with a margin: that is more honest than an exact figure you are not certain of.
6. When comparing two services, first match up their settings: zodiac, house system, set of points.

The basics of reading a chart do not depend on which service you are looking at. If you are only starting out, read what a natal chart is and how to read it.

To see what a chart looks like by our rules, build a chart: the site will show which factors are known exactly and which stay unknown. For more on how houses behave in different systems, see the article on houses in the natal chart.